[{"data":1,"prerenderedAt":2208},["ShallowReactive",2],{"/blog/how-we-spent-two-weeks-hunting-an-nfs-bug":3,"navigation-en-us":1422,"banner-en-us":1848,"footer-en-us":1858,"blog-post-authors-en-us-Stan Hu":2103,"blog-related-posts-en-us-how-we-spent-two-weeks-hunting-an-nfs-bug":2118,"blog-promotions-en-us":2144,"next-steps-en-us":2198},{"id":4,"title":5,"authors":6,"body":8,"category":1400,"date":1401,"description":1402,"extension":1403,"externalUrl":1404,"faq":1404,"featured":1405,"heroImage":1406,"meta":1407,"navigation":1136,"path":1408,"seo":1409,"slug":1413,"stem":1414,"tags":1415,"template":1420,"updatedDate":1404,"__hash__":1421},"blogPosts/en-us/blog/how-we-spent-two-weeks-hunting-an-nfs-bug.md","How we spent two weeks hunting an NFS bug in the Linux kernel",[7],"Stan Hu",{"type":9,"value":10,"toc":1386},"minimark",[11,15,48,60,75,84,93,98,106,114,142,153,184,199,207,217,223,229,267,276,290,297,307,313,316,479,488,491,497,500,506,523,529,546,552,561,565,574,579,582,601,609,615,618,622,628,657,661,667,683,689,708,711,715,718,722,725,736,754,757,760,768,776,798,808,900,921,924,987,993,999,1015,1020,1024,1035,1114,1117,1127,1184,1201,1277,1291,1295,1315,1323,1328,1339,1358,1362,1365,1368,1382],[12,13,14],"p",{},"UPDATE 2019-08-06: This bug has now been resolved in the following\ndistributions:",[16,17,18,27,34],"ul",{},[19,20,21],"li",{},[22,23,26],"a",{"href":24,"rel":25},"https://access.redhat.com/errata/RHSA-2019:2029",[],"Red Hat Enterprise Linux 7",[19,28,29],{},[22,30,33],{"href":31,"rel":32},"https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1802585",[],"Ubuntu",[19,35,36,37,42,43],{},"Linux mainline: Backported to ",[22,38,41],{"href":39,"rel":40},"https://lkml.org/lkml/2019/8/2/562",[],"4.14-stable"," and ",[22,44,47],{"href":45,"rel":46},"https://lkml.org/lkml/2019/8/2/639",[],"4.19-stable",[12,49,50,51,55,56,59],{},"On Sep. 14, the GitLab support team escalated a critical\nproblem encountered by one of our customers: GitLab would run fine for a\nwhile, but after some time users encountered errors. When attempting to\nclone certain repositories via Git, users would see an opaque ",[52,53,54],"code",{},"Stale file error"," message. The error message persisted for a long time,\nblocking employees from being able to work, unless a system\nadministrator intervened manually by running ",[52,57,58],{},"ls"," in the directory\nitself.",[12,61,62,63,68,69,74],{},"Thus launched an investigation into the inner workings of Git and the\nNetwork File System (NFS). The investigation uncovered a bug with the\nLinux v4.0 NFS client and culiminated with a ",[22,64,67],{"href":65,"rel":66},"https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?h=be189f7e7f03de35887e5a85ddcf39b91b5d7fc1",[],"kernel patch that was written by\nTrond Myklebust","\nand ",[22,70,73],{"href":71,"rel":72},"https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?h=c7a2c49ea6c9eebbe44ff2c08b663b2905ee2c13",[],"merged in the latest mainline Linux kernel","\non Oct. 26.",[12,76,77,78,83],{},"This post describes the journey of investigating the issue and\ndetails the thought process and tools by which we tracked down the\nbug. It was inspired by the fine detective work in ",[22,79,82],{"href":80,"rel":81},"http://www.be9.io/2015/09/21/memory-leak/",[],"How I spent two\nweeks hunting a memory leak in Ruby","\nby Oleg Dashevskii.",[12,85,86,87,92],{},"More importantly, this experience exemplifies how open source software\ndebugging has become a team sport that involves expertise across\nmultiple people, companies, and locations. The GitLab motto \"",[22,88,91],{"href":89,"rel":90},"https://handbook.gitlab.com/handbook/company/mission/#mission",[],"everyone can\ncontribute","\" applies not only to GitLab itself, but also to other open\nsource projects, such as the Linux kernel.",[94,95,97],"h2",{"id":96},"reproducing-the-bug","Reproducing the bug",[12,99,100,101,105],{},"While we have run NFS on GitLab.com for many years, we have stopped\nusing it to access repository data across our application\nmachines. Instead, we have ",[22,102,104],{"href":103},"/blog/the-road-to-gitaly-1-0/","abstracted all Git calls to\nGitaly",".\nStill, NFS remains a supported configuration for our customers who\nmanage their own installation of GitLab, but we had never seen the exact\nproblem described by the customer before.",[12,107,108,113],{},[22,109,112],{"href":110,"rel":111},"https://gitlab.com/gitlab-org/gitlab-ce/issues/51437",[],"Our customer gave us a few important clues",":",[115,116,117,124,130,136],"ol",{},[19,118,119,120,123],{},"The full error message read, ",[52,121,122],{},"fatal: Couldn't read ./packed-refs: Stale file handle",".",[19,125,126,127,123],{},"The error seemed to start when they started a manual Git garbage\ncollection run via ",[52,128,129],{},"git gc",[19,131,132,133,135],{},"The error would go away if a system administrator ran ",[52,134,58],{}," in the\ndirectory.",[19,137,138,139,141],{},"The error also would go away after ",[52,140,129],{}," process ended.",[12,143,144,145,148,149,152],{},"The first two items seemed obviously related. When you push to a branch\nin Git, Git creates a loose reference, a fancy name for a file that\npoints your branch name to the commit. For example, a push to ",[52,146,147],{},"master","\nwill create a file called ",[52,150,151],{},"refs/heads/master"," in the repository:",[154,155,160],"pre",{"className":156,"code":157,"language":158,"meta":159,"style":159},"language-bash shiki shiki-themes github-light","$ cat refs/heads/master\n2e33a554576d06d9e71bfd6814ee9ba3a7838963\n","bash","",[52,161,162,178],{"__ignoreMap":159},[163,164,167,171,175],"span",{"class":165,"line":166},"line",1,[163,168,170],{"class":169},"s7eDp","$",[163,172,174],{"class":173},"sYBdl"," cat",[163,176,177],{"class":173}," refs/heads/master\n",[163,179,181],{"class":165,"line":180},2,[163,182,183],{"class":169},"2e33a554576d06d9e71bfd6814ee9ba3a7838963\n",[12,185,186,188,189,192,193,195,196,198],{},[52,187,129],{}," has several jobs, but one of them is to collect these loose\nreferences (refs) and bundle them up into a single file called\n",[52,190,191],{},"packed-refs",". This makes things a bit faster by eliminating the need to\nread lots of little files in favor of reading one large one. For\nexample, after running ",[52,194,129],{},", an example ",[52,197,191],{}," might look\nlike:",[154,200,205],{"className":201,"code":203,"language":204,"meta":159},[202],"language-text","# pack-refs with: peeled fully-peeled sorted\n564c3424d6f9175cf5f2d522e10d20d781511bf1 refs/heads/10-8-stable\nedb037cbc85225261e8ede5455be4aad771ba3bb refs/heads/11-0-stable\n94b9323033693af247128c8648023fe5b53e80f9 refs/heads/11-1-stable\n2e33a554576d06d9e71bfd6814ee9ba3a7838963 refs/heads/master\n","text",[52,206,203],{"__ignoreMap":159},[12,208,209,210,212,213,216],{},"How exactly is this ",[52,211,191],{}," file created? To answer that, we ran\n",[52,214,215],{},"strace git gc"," with a loose ref present. Here are the pertinent lines\nfrom that:",[154,218,221],{"className":219,"code":220,"language":204,"meta":159},[202],"28705 open(\"/tmp/libgit2/.git/packed-refs.lock\", O_RDWR|O_CREAT|O_EXCL|O_CLOEXEC, 0666) = 3\n28705 open(\".git/packed-refs\", O_RDONLY) = 3\n28705 open(\"/tmp/libgit2/.git/packed-refs.new\", O_RDWR|O_CREAT|O_EXCL|O_CLOEXEC, 0666) = 4\n28705 rename(\"/tmp/libgit2/.git/packed-refs.new\", \"/tmp/libgit2/.git/packed-refs\") = 0\n28705 unlink(\"/tmp/libgit2/.git/packed-refs.lock\") = 0\n",[52,222,220],{"__ignoreMap":159},[12,224,225,226,228],{},"The system calls showed that ",[52,227,129],{}," did the following:",[115,230,231,241,246,251,259,264],{},[19,232,233,234,237,238,240],{},"Open ",[52,235,236],{},"packed-refs.lock",". This tells other processes that ",[52,239,191],{}," is locked and cannot be changed.",[19,242,233,243,123],{},[52,244,245],{},"packed-refs.new",[19,247,248,249,123],{},"Write loose refs to ",[52,250,245],{},[19,252,253,254,256,257,123],{},"Rename ",[52,255,245],{}," to ",[52,258,191],{},[19,260,261,262,123],{},"Remove ",[52,263,236],{},[19,265,266],{},"Remove loose refs.",[12,268,269,270,272,273,275],{},"The fourth step is the key here: the rename where Git puts ",[52,271,191],{},"\ninto action. In addition to collecting loose refs, ",[52,274,129],{}," also\nperforms a more expensive task of scanning for unused objects and\nremoving them. This task can take over an hour for large\nrepositories.",[12,277,278,279,281,282,285,286,289],{},"That made us wonder: for a large repository, does ",[52,280,129],{}," keep the file\nopen while it's running this sweep? Looking at the ",[52,283,284],{},"strace"," logs and\nprobing the process with ",[52,287,288],{},"lsof",", we found that it did the following:",[12,291,292],{},[293,294],"img",{"alt":295,"src":296},"Git Garbage Collection","https://res.cloudinary.com/about-gitlab-com/image/upload/v1782398757/blog/Content%20Images/nfs-debug/git-gc-diagram.svg",[12,298,299,300,302,303,306],{},"Notice that ",[52,301,191],{}," is closed only at the end, after the potentially\nlong ",[52,304,305],{},"Garbage collect objects"," step takes place.",[12,308,309,310,312],{},"That made us wonder: how does NFS behave when one node has ",[52,311,191],{},"\nopen while another renames over that file?",[12,314,315],{},"To experiment, we asked the customer to run the following experiment on\ntwo different machines (Alice and Bob):",[115,317,318,372,412,452],{},[19,319,320,321,324,325,328,329],{},"On the shared NFS volume, create two files: ",[52,322,323],{},"test1.txt"," and\n",[52,326,327],{},"test2.txt"," with different contents to make it easy to distinguish them:",[154,330,332],{"className":156,"code":331,"language":158,"meta":159,"style":159},"alice $ echo \"1 - Old file\" > /path/to/nfs/test1.txt\nalice $ echo \"2 - New file\" > /path/to/nfs/test2.txt\n",[52,333,334,356],{"__ignoreMap":159},[163,335,336,339,343,346,349,353],{"class":165,"line":166},[163,337,338],{"class":169},"alice",[163,340,342],{"class":341},"sgsFI"," $ ",[163,344,345],{"class":173},"echo",[163,347,348],{"class":173}," \"1 - Old file\"",[163,350,352],{"class":351},"sD7c4"," >",[163,354,355],{"class":173}," /path/to/nfs/test1.txt\n",[163,357,358,360,362,364,367,369],{"class":165,"line":180},[163,359,338],{"class":169},[163,361,342],{"class":341},[163,363,345],{"class":173},[163,365,366],{"class":173}," \"2 - New file\"",[163,368,352],{"class":351},[163,370,371],{"class":173}," /path/to/nfs/test2.txt\n",[19,373,374,375,113,377],{},"On machine Alice, keep a file open to ",[52,376,323],{},[154,378,380],{"className":156,"code":379,"language":158,"meta":159,"style":159}," alice $ irb\n irb(main):001:0> File.open('/path/to/nfs/test1.txt')\n",[52,381,382,392],{"__ignoreMap":159},[163,383,384,387,389],{"class":165,"line":166},[163,385,386],{"class":169}," alice",[163,388,342],{"class":341},[163,390,391],{"class":173},"irb\n",[163,393,394,397,400,403,406,409],{"class":165,"line":180},[163,395,396],{"class":169}," irb(main",[163,398,399],{"class":341},"):001:",[163,401,402],{"class":351},"0>",[163,404,405],{"class":341}," File.open(",[163,407,408],{"class":169},"'/path/to/nfs/test1.txt'",[163,410,411],{"class":341},")\n",[19,413,414,415,417,418],{},"On machine Alice, show the contents of ",[52,416,323],{}," continuously:",[154,419,421],{"className":156,"code":420,"language":158,"meta":159,"style":159},"alice $ while true; do cat test1.txt; done\n",[52,422,423],{"__ignoreMap":159},[163,424,425,427,429,432,436,439,442,444,447,449],{"class":165,"line":166},[163,426,338],{"class":169},[163,428,342],{"class":341},[163,430,431],{"class":173},"while",[163,433,435],{"class":434},"sYu0t"," true",[163,437,438],{"class":341},"; ",[163,440,441],{"class":351},"do",[163,443,174],{"class":169},[163,445,446],{"class":173}," test1.txt",[163,448,438],{"class":341},[163,450,451],{"class":351},"done\n",[19,453,454,455],{},"Then on machine Bob, run:",[154,456,458],{"className":156,"code":457,"language":158,"meta":159,"style":159},"bob $ mv -f test2.txt test1.txt\n",[52,459,460],{"__ignoreMap":159},[163,461,462,465,467,470,473,476],{"class":165,"line":166},[163,463,464],{"class":169},"bob",[163,466,342],{"class":341},[163,468,469],{"class":173},"mv",[163,471,472],{"class":434}," -f",[163,474,475],{"class":173}," test2.txt",[163,477,478],{"class":173}," test1.txt\n",[12,480,481,482,484,485,487],{},"This last step emulates what ",[52,483,129],{}," does with ",[52,486,191],{}," by\noverwriting the existing file.",[12,489,490],{},"On the customer's machine, the result looked something like:",[154,492,495],{"className":493,"code":494,"language":204,"meta":159},[202],"1 - Old file\n1 - Old file\n1 - Old file\ncat: test1.txt: Stale file handle\n",[52,496,494],{"__ignoreMap":159},[12,498,499],{},"Bingo! We seemed to reproduce the problem in a controlled way. However,\nthe same experiment using a Linux NFS server did not have this\nproblem. The result was what you would expect: the new contents were\npicked up after the rename:",[154,501,504],{"className":502,"code":503,"language":204,"meta":159},[202],"1 - Old file\n1 - Old file\n1 - Old file\n2 - New file  \u003C--- RENAME HAPPENED\n2 - New file\n2 - New file\n",[52,505,503],{"__ignoreMap":159},[12,507,508,509,514,515,518,519,522],{},"Why the difference in behavior? It turns out that the customer was using\nan ",[22,510,513],{"href":511,"rel":512},"https://www.dellemc.com/en-us/storage/isilon/index.htm",[],"Isilon NFS\nappliance"," that\nonly supported NFS v4.0. By switching the mount parameters to v4.0 via\nthe ",[52,516,517],{},"vers=4.0"," parameter in ",[52,520,521],{},"/etc/fstab",", the test revealed a different\nresult with the Linux NFS server:",[154,524,527],{"className":525,"code":526,"language":204,"meta":159},[202],"1 - Old file\n1 - Old file\n1 - Old file\n1 - Old file \u003C--- RENAME HAPPENED\n1 - Old file\n1 - Old file\n",[52,528,526],{"__ignoreMap":159},[12,530,531,532,535,536,540,541,113],{},"Instead of a ",[52,533,534],{},"Stale file handle",", the Linux NFS v4.0 server showed stale\n",[537,538,539],"em",{},"contents",". It turns out this difference in behavior can be explained by\nthe NFS spec. From ",[22,542,545],{"href":543,"rel":544},"https://tools.ietf.org/html/rfc3010#page-153",[],"RFC\n3010",[547,548,549],"blockquote",{},[12,550,551],{},"A filehandle may or may not become stale or expire on a rename.\nHowever, server implementors are strongly encouraged to attempt to keep\nfile handles from becoming stale or expiring in this fashion.",[12,553,554,555,557,558,560],{},"In other words, NFS servers can choose how to behave if a file is\nrenamed; it's perfectly valid for any NFS server to return a ",[52,556,54],{}," when that happens. We surmised that even though the results were\ndifferent, the problem was likely related to the same issue. We\nsuspected some cache validation issue because running ",[52,559,58],{}," in the\ndirectory would \"clear\" the error. Now that we had a reproducible test\ncase, we asked the experts: the Linux NFS maintainers.",[94,562,564],{"id":563},"false-path-nfs-server-delegations","False path: NFS server delegations",[12,566,567,568,573],{},"With a clear set of reproduction steps, I ",[22,569,572],{"href":570,"rel":571},"https://marc.info/?l=linux-nfs&m=153721785231614&w=2",[],"sent an email to the Linux\nNFS mailing list","\ndescribing what we had found. Over the week, I went back and forth with\nBruce Fields, the Linux NFS server maintainer, who suggested this was a\nNFS bug and that it would be useful to look at the network traffic. He\nthought there might be an issue with NFS server delegations.",[575,576,578],"h3",{"id":577},"what-is-an-nfs-server-delegation","What is an NFS server delegation?",[12,580,581],{},"In a nutshell, NFS v4 introduced server delegations as a way to speed up file access. A server can\ndelegate read or write access to a client so that the client doesn't\nhave to keep asking the server whether that file has changed by another\nclient. In simpler terms, a write delegation is akin to someone lending\nyou a notebook and saying, \"Go ahead and write in here, and I'll take it\nback when I'm ready.\" Instead of having to ask to borrow the notebook\nevery time you want to write a new paragraph, you have free rein until\nthe owner reclaims the notebook. In NFS terms, this reclamation process\nis called a delegation recall.",[12,583,584,585,587,588,590,591,593,594,596,597,600],{},"Indeed, a bug in the NFS delegation recall might explain the ",[52,586,534],{}," problem. Remember that in the earlier experiment, Alice had\nan open file to ",[52,589,323],{}," when it was replaced by ",[52,592,327],{}," later.\nIt's possible that the server failed to recall the delegation on\n",[52,595,323],{},", resulting in an incorrect state. To check whether this was\nan issue, we turned to ",[52,598,599],{},"tcpdump"," to capture NFS traffic and used\nWireshark to visualize it.",[12,602,603,608],{},[22,604,607],{"href":605,"rel":606},"https://www.wireshark.org/",[],"Wireshark"," is a wonderful open source tool\nfor analyzing network traffic, and it's especially good for viewing NFS\nin action. We captured a trace using the following command on the NFS server:",[154,610,613],{"className":611,"code":612,"language":204,"meta":159},[202],"tcpdump -s 0 -w /tmp/nfs.pcap port 2049\n",[52,614,612],{"__ignoreMap":159},[12,616,617],{},"This command captures all NFS traffic, which typically is on TCP port 2049.\nBecause our experiment worked properly with NFS v4.1 but did not\nwith NFS v4.0, we could compare and contrast how NFS behaved\nin a non-working and a working case. With Wireshark, we saw the\nfollowing behavior:",[575,619,621],{"id":620},"nfs-v40-stale-file-case","NFS v4.0 (stale file case)",[12,623,624],{},[293,625],{"alt":626,"src":627},"NFS v4.0 flow","https://res.cloudinary.com/about-gitlab-com/image/upload/v1782398758/blog/Content%20Images/nfs-debug/nfs-4.0-flow.svg",[12,629,630,631,633,634,637,638,641,642,645,646,649,650,653,654,656],{},"In this diagram, we can see in step 1 Alice opens ",[52,632,323],{}," and gets\nback an NFS file handle along with a ",[52,635,636],{},"stateid"," of 0x3000. When Bob\nattempts to rename the file, the NFS server tells to Bob to retry via\nthe ",[52,639,640],{},"NFS4ERR_DELAY"," message while it recalls the delegation from Alice\nvia the ",[52,643,644],{},"CB_RECALL"," message (step 3). Alice then returns her delegation\nvia ",[52,647,648],{},"DELEGRETURN"," (step 4), and then Bob attempts to send another\n",[52,651,652],{},"RENAME"," message (step 5). The ",[52,655,652],{}," completes in both cases, but\nAlice continues to read using the same file handle.",[575,658,660],{"id":659},"nfs-v41-working-case","NFS v4.1 (working case)",[12,662,663],{},[293,664],{"alt":665,"src":666},"NFS v4.1 flow","https://res.cloudinary.com/about-gitlab-com/image/upload/v1782398759/blog/Content%20Images/nfs-debug/nfs-4.1-flow.svg",[12,668,669,670,672,673,324,676,679,680,682],{},"The main difference happens at the bottom at step 6. Notice in NFS v4.0\n(the stale file case), Alice attempts to reuse the same ",[52,671,636],{},". In\nNFS v4.1 (working case), Alice performs an additional ",[52,674,675],{},"LOOKUP",[52,677,678],{},"OPEN",", which causes the server to return a different ",[52,681,636],{},". In v4.0,\nthese extra messages are never sent. This explains why Alice continues\nto see stale content because she uses the old file handle.",[12,684,685,686,688],{},"What makes Alice decide to do the extra ",[52,687,675],{},"? The delegation recall\nseemed to work fine, but perhaps there was still an issue, such as a\nmissing invalidation step. To rule that out, we disabled NFS delegations\nby issuing this command on the NFS server itself:",[154,690,694],{"className":691,"code":692,"language":693,"meta":159,"style":159},"language-sh shiki shiki-themes github-light","echo 0 > /proc/sys/fs/leases-enable\n","sh",[52,695,696],{"__ignoreMap":159},[163,697,698,700,703,705],{"class":165,"line":166},[163,699,345],{"class":434},[163,701,702],{"class":434}," 0",[163,704,352],{"class":351},[163,706,707],{"class":173}," /proc/sys/fs/leases-enable\n",[12,709,710],{},"We repeated the experiment, but the problem persisted. All this\nconvinced us this wasn't a NFS server issue or a problem with NFS\ndelegations; it was a problem that led us to look into the NFS client\nwithin the kernel.",[94,712,714],{"id":713},"digging-deeper-the-linux-nfs-client","Digging deeper: the Linux NFS client",[12,716,717],{},"The first question we had to answer for the NFS maintainers:",[575,719,721],{"id":720},"was-this-problem-still-in-the-latest-upstream-kernel","Was this problem still in the latest upstream kernel?",[12,723,724],{},"The issue occurred with both CentOS 7.2 and Ubuntu 16.04 kernels, which\nused versions 3.10.0-862.11.6 and 4.4.0-130, respectively. However, both\nthose kernels lagged the most recent kernel, which was 4.19-rc2 at the\ntime.",[12,726,727,728,731,732,735],{},"We deployed a new Ubuntu 16.04 virtual machine on Google Cloud Platform\n(GCP), cloned the latest Linux kernel, and set up a kernel development\nenvironment. After generating a ",[52,729,730],{},".config"," file via ",[52,733,734],{},"make menuconfig",", we\nchecked two items:",[115,737,738,745],{},[19,739,740,741,744],{},"The NFS driver was compiled as a module (",[52,742,743],{},"CONFIG_NFSD=m",").",[19,746,747,748,753],{},"The ",[22,749,752],{"href":750,"rel":751},"https://cloud.google.com/compute/docs/images/building-custom-os",[],"required GCP kernel settings","\nwere set properly.",[12,755,756],{},"Just as a geneticist would use fruit flies to study evolution in\nreal time, the first item allowed us to make quick changes in the NFS\nclient without having to reboot the kernel. The second item was required\nto ensure that the kernel would actually boot after it was\ninstalled. Fortunately, the default kernel settings had all the settings\nright out of the box.",[12,758,759],{},"With our custom kernel, we verified that the stale file problem still\nexisted in the latest version. That begged a number of questions:",[115,761,762,765],{},[19,763,764],{},"Where exactly was this problem happening?",[19,766,767],{},"Why was this problem happening with NFS v4.0 but not in v4.1?",[12,769,770,771,775],{},"To answer these questions, we began to investigate the NFS ",[22,772,774],{"href":773},"/solutions/source-code-management/","source\ncode",". Since we didn't have a kernel debugger available, we sprinkled the\nsource code with two main types of calls:",[115,777,778,792],{},[19,779,780,783,784,744],{},[52,781,782],{},"pr_info()"," (",[22,785,788,789],{"href":786,"rel":787},"https://lwn.net/Articles/487437/",[],"what used to be ",[52,790,791],{},"printk",[19,793,794,797],{},[52,795,796],{},"dump_stack()",": This would show the stack trace of the current function call.",[12,799,800,801,804,805,113],{},"For example, one of the first things we did was hook into the\n",[52,802,803],{},"nfs4_file_open()"," function in ",[52,806,807],{},"fs/nfs/nfs4file.c",[154,809,813],{"className":810,"code":811,"language":812,"meta":159,"style":159},"language-c shiki shiki-themes github-light","static int\nnfs4_file_open(struct inode *inode, struct file *filp)\n{\n...\n        pr_info(\"nfs4_file_open start\\n\");\n        dump_stack();\n\n","c",[52,814,815,823,859,865,871,891],{"__ignoreMap":159},[163,816,817,820],{"class":165,"line":166},[163,818,819],{"class":351},"static",[163,821,822],{"class":351}," int\n",[163,824,825,828,831,834,837,840,844,847,849,852,854,857],{"class":165,"line":180},[163,826,827],{"class":169},"nfs4_file_open",[163,829,830],{"class":341},"(",[163,832,833],{"class":351},"struct",[163,835,836],{"class":341}," inode ",[163,838,839],{"class":351},"*",[163,841,843],{"class":842},"sqxcx","inode",[163,845,846],{"class":341},", ",[163,848,833],{"class":351},[163,850,851],{"class":341}," file ",[163,853,839],{"class":351},[163,855,856],{"class":842},"filp",[163,858,411],{"class":341},[163,860,862],{"class":165,"line":861},3,[163,863,864],{"class":341},"{\n",[163,866,868],{"class":165,"line":867},4,[163,869,870],{"class":341},"...\n",[163,872,874,877,879,882,885,888],{"class":165,"line":873},5,[163,875,876],{"class":169},"        pr_info",[163,878,830],{"class":341},[163,880,881],{"class":173},"\"nfs4_file_open start",[163,883,884],{"class":434},"\\n",[163,886,887],{"class":173},"\"",[163,889,890],{"class":341},");\n",[163,892,894,897],{"class":165,"line":893},6,[163,895,896],{"class":169},"        dump_stack",[163,898,899],{"class":341},"();\n",[12,901,902,903,912,913,920],{},"Admittedly, we could have ",[22,904,907,908,911],{"href":905,"rel":906},"https://www.kernel.org/doc/html/v4.15/admin-guide/dynamic-debug-howto.html",[],"activated the ",[52,909,910],{},"dprintk"," messages with the\nLinux dynamic\ndebug","\nor used\n",[22,914,917],{"href":915,"rel":916},"https://www.thegeekdiary.com/how-to-enable-nfs-debug-logging-using-rpcdebug/",[],[52,918,919],{},"rpcdebug",",\nbut it was nice to be able to add our own messages to verify changes\nwere being made.",[12,922,923],{},"Every time we made changes, we recompiled the module and reinstalled it\ninto the kernel via the commands:",[154,925,927],{"className":691,"code":926,"language":693,"meta":159,"style":159},"make modules\nsudo umount /mnt/nfs-test\nsudo rmmod nfsv4\nsudo rmmod nfs\nsudo insmod fs/nfs/nfs.ko\nsudo mount -a\n",[52,928,929,937,948,958,967,977],{"__ignoreMap":159},[163,930,931,934],{"class":165,"line":166},[163,932,933],{"class":169},"make",[163,935,936],{"class":173}," modules\n",[163,938,939,942,945],{"class":165,"line":180},[163,940,941],{"class":169},"sudo",[163,943,944],{"class":173}," umount",[163,946,947],{"class":173}," /mnt/nfs-test\n",[163,949,950,952,955],{"class":165,"line":861},[163,951,941],{"class":169},[163,953,954],{"class":173}," rmmod",[163,956,957],{"class":173}," nfsv4\n",[163,959,960,962,964],{"class":165,"line":867},[163,961,941],{"class":169},[163,963,954],{"class":173},[163,965,966],{"class":173}," nfs\n",[163,968,969,971,974],{"class":165,"line":873},[163,970,941],{"class":169},[163,972,973],{"class":173}," insmod",[163,975,976],{"class":173}," fs/nfs/nfs.ko\n",[163,978,979,981,984],{"class":165,"line":893},[163,980,941],{"class":169},[163,982,983],{"class":173}," mount",[163,985,986],{"class":434}," -a\n",[12,988,989,990,113],{},"With our NFS module installed, repeating the experiments would print\nmessages that would help us understand the NFS code a bit more. For\nexample, you can see exactly what happens when an application calls ",[52,991,992],{},"open()",[154,994,997],{"className":995,"code":996,"language":204,"meta":159},[202],"Sep 24 20:20:38 test-kernel kernel: [ 1145.233460] Call Trace:\nSep 24 20:20:38 test-kernel kernel: [ 1145.233462]  dump_stack+0x8e/0xd5\nSep 24 20:20:38 test-kernel kernel: [ 1145.233480] nfs4_file_open+0x56/0x2a0 [nfsv4]\nSep 24 20:20:38 test-kernel kernel: [ 1145.233488]  ? nfs42_clone_file_range+0x1c0/0x1c0 [nfsv4]\nSep 24 20:20:38 test-kernel kernel: [ 1145.233490] do_dentry_open+0x1f6/0x360\nSep 24 20:20:38 test-kernel kernel: [ 1145.233492]  vfs_open+0x2f/0x40\nSep 24 20:20:38 test-kernel kernel: [ 1145.233493]  path_openat+0x2e8/0x1690\nSep 24 20:20:38 test-kernel kernel: [ 1145.233496]  ? mem_cgroup_try_charge+0x8b/0x190\nSep 24 20:20:38 test-kernel kernel: [ 1145.233497]  do_filp_open+0x9b/0x110\nSep 24 20:20:38 test-kernel kernel: [ 1145.233499]  ? __check_object_size+0xb8/0x1b0\nSep 24 20:20:38 test-kernel kernel: [ 1145.233501]  ? __alloc_fd+0x46/0x170\nSep 24 20:20:38 test-kernel kernel: [ 1145.233503]  do_sys_open+0x1ba/0x250\nSep 24 20:20:38 test-kernel kernel: [ 1145.233505]  ? do_sys_open+0x1ba/0x250\nSep 24 20:20:38 test-kernel kernel: [ 1145.233507] __x64_sys_openat+0x20/0x30\nSep 24 20:20:38 test-kernel kernel: [ 1145.233508]  do_syscall_64+0x65/0x130\n",[52,998,996],{"__ignoreMap":159},[12,1000,1001,1002,42,1005,1008,1009,1014],{},"What are the ",[52,1003,1004],{},"do_dentry_open",[52,1006,1007],{},"vfs_open"," calls above? Linux has a\n",[22,1010,1013],{"href":1011,"rel":1012},"https://www.kernel.org/doc/Documentation/filesystems/vfs.txt",[],"virtual filesystem\n(VFS)",", an\nabstraction layer which provides a common interface for all\nfilesystems. The VFS documentation explains:",[547,1016,1017],{},[12,1018,1019],{},"The VFS implements the open(2), stat(2), chmod(2), and similar system\ncalls. The pathname argument that is passed to them is used by the VFS\nto search through the directory entry cache (also known as the dentry\ncache or dcache). This provides a very fast look-up mechanism to\ntranslate a pathname (filename) into a specific dentry. Dentries live\nin RAM and are never saved to disc: they exist only for performance.",[575,1021,1023],{"id":1022},"this-gave-us-a-clue-what-if-this-was-a-problem-with-the-dentry-cache","This gave us a clue: what if this was a problem with the dentry cache?",[12,1025,1026,1027,1030,1031,1034],{},"We noticed a lot of dentry cache validation was done in\n",[52,1028,1029],{},"fs/nfs/dir.c",". In particular, ",[52,1032,1033],{},"nfs4_lookup_revalidate()"," sounded\npromising. As an experiment, we hacked that function to bail\nout early:",[154,1036,1040],{"className":1037,"code":1038,"language":1039,"meta":159,"style":159},"language-diff shiki shiki-themes github-light","diff --git a/fs/nfs/dir.c b/fs/nfs/dir.c\nindex 8bfaa658b2c1..ad479bfeb669 100644\n--- a/fs/nfs/dir.c\n+++ b/fs/nfs/dir.c\n@@ -1159,6 +1159,7 @@ static int nfs_lookup_revalidate(struct dentry *dentry, unsigned int flags)\n        trace_nfs_lookup_revalidate_enter(dir, dentry, flags);\n        error = NFS_PROTO(dir)->lookup(dir, &dentry->d_name, fhandle, fattr, label);\n        trace_nfs_lookup_revalidate_exit(dir, dentry, flags, error);\n+       goto out_bad;\n        if (error == -ESTALE || error == -ENOENT)\n                goto out_bad;\n        if (error)\n\n","diff",[52,1041,1042,1047,1052,1058,1064,1073,1078,1084,1090,1096,1102,1108],{"__ignoreMap":159},[163,1043,1044],{"class":165,"line":166},[163,1045,1046],{"class":434},"diff --git a/fs/nfs/dir.c b/fs/nfs/dir.c\n",[163,1048,1049],{"class":165,"line":180},[163,1050,1051],{"class":341},"index 8bfaa658b2c1..ad479bfeb669 100644\n",[163,1053,1054],{"class":165,"line":861},[163,1055,1057],{"class":1056},"s9hmT","--- a/fs/nfs/dir.c\n",[163,1059,1060],{"class":165,"line":867},[163,1061,1063],{"class":1062},"shJU0","+++ b/fs/nfs/dir.c\n",[163,1065,1066,1070],{"class":165,"line":873},[163,1067,1069],{"class":1068},"soKTg","@@ -1159,6 +1159,7 @@",[163,1071,1072],{"class":341}," static int nfs_lookup_revalidate(struct dentry *dentry, unsigned int flags)\n",[163,1074,1075],{"class":165,"line":893},[163,1076,1077],{"class":341},"        trace_nfs_lookup_revalidate_enter(dir, dentry, flags);\n",[163,1079,1081],{"class":165,"line":1080},7,[163,1082,1083],{"class":341},"        error = NFS_PROTO(dir)->lookup(dir, &dentry->d_name, fhandle, fattr, label);\n",[163,1085,1087],{"class":165,"line":1086},8,[163,1088,1089],{"class":341},"        trace_nfs_lookup_revalidate_exit(dir, dentry, flags, error);\n",[163,1091,1093],{"class":165,"line":1092},9,[163,1094,1095],{"class":1062},"+       goto out_bad;\n",[163,1097,1099],{"class":165,"line":1098},10,[163,1100,1101],{"class":341},"        if (error == -ESTALE || error == -ENOENT)\n",[163,1103,1105],{"class":165,"line":1104},11,[163,1106,1107],{"class":341},"                goto out_bad;\n",[163,1109,1111],{"class":165,"line":1110},12,[163,1112,1113],{"class":341},"        if (error)\n",[12,1115,1116],{},"That made the stale file problem in our experiment go away! Now we were onto something.",[12,1118,1119,1120,1122,1123,1126],{},"To answer, \"Why does this problem not happen in NFS v4.1?\", we added\n",[52,1121,782],{}," calls to every ",[52,1124,1125],{},"if"," block in that function. After running our\nexperiments with NFS v4.0 and v4.1, we found this special condition being run\nin the v4.1 case:",[154,1128,1130],{"className":810,"code":1129,"language":812,"meta":159,"style":159},"\n        if (NFS_SB(dentry->d_sb)->caps & NFS_CAP_ATOMIC_OPEN_V1) {\n          goto no_open;\n        }\n\n",[52,1131,1132,1138,1171,1179],{"__ignoreMap":159},[163,1133,1134],{"class":165,"line":166},[163,1135,1137],{"emptyLinePlaceholder":1136},true,"\n",[163,1139,1140,1143,1145,1148,1151,1154,1157,1160,1162,1165,1168],{"class":165,"line":180},[163,1141,1142],{"class":351},"        if",[163,1144,783],{"class":341},[163,1146,1147],{"class":169},"NFS_SB",[163,1149,1150],{"class":341},"(dentry",[163,1152,1153],{"class":351},"->",[163,1155,1156],{"class":842},"d_sb",[163,1158,1159],{"class":341},")",[163,1161,1153],{"class":351},[163,1163,1164],{"class":341},"caps ",[163,1166,1167],{"class":351},"&",[163,1169,1170],{"class":341}," NFS_CAP_ATOMIC_OPEN_V1) {\n",[163,1172,1173,1176],{"class":165,"line":861},[163,1174,1175],{"class":351},"          goto",[163,1177,1178],{"class":341}," no_open;\n",[163,1180,1181],{"class":165,"line":867},[163,1182,1183],{"class":341},"        }\n",[12,1185,1186,1187,1190,1191,1196,1197,1200],{},"What is ",[52,1188,1189],{},"NFS_CAP_ATOMIC_OPEN_V1","? We saw ",[22,1192,1195],{"href":1193,"rel":1194},"https://patchwork.kernel.org/patch/2300511/",[],"this kernel\npatch"," mentioned this was\nan NFS v4.1-specific feature, and the code in ",[52,1198,1199],{},"fs/nfs/nfs4proc.c","\nconfirmed that this flag was a capability present in v4.1 but not in v4.0:",[154,1202,1204],{"className":810,"code":1203,"language":812,"meta":159,"style":159},"static const struct nfs4_minor_version_ops nfs_v4_1_minor_ops = {\n        .minor_version = 1,\n        .init_caps = NFS_CAP_READDIRPLUS\n                | NFS_CAP_ATOMIC_OPEN\n                | NFS_CAP_POSIX_LOCK\n                | NFS_CAP_STATEID_NFSV41\n                | NFS_CAP_ATOMIC_OPEN_V1\n\n",[52,1205,1206,1225,1238,1248,1256,1263,1270],{"__ignoreMap":159},[163,1207,1208,1210,1213,1216,1219,1222],{"class":165,"line":166},[163,1209,819],{"class":351},[163,1211,1212],{"class":351}," const",[163,1214,1215],{"class":351}," struct",[163,1217,1218],{"class":341}," nfs4_minor_version_ops nfs_v4_1_minor_ops ",[163,1220,1221],{"class":351},"=",[163,1223,1224],{"class":341}," {\n",[163,1226,1227,1230,1232,1235],{"class":165,"line":180},[163,1228,1229],{"class":341},"        .minor_version ",[163,1231,1221],{"class":351},[163,1233,1234],{"class":434}," 1",[163,1236,1237],{"class":341},",\n",[163,1239,1240,1243,1245],{"class":165,"line":861},[163,1241,1242],{"class":341},"        .init_caps ",[163,1244,1221],{"class":351},[163,1246,1247],{"class":341}," NFS_CAP_READDIRPLUS\n",[163,1249,1250,1253],{"class":165,"line":867},[163,1251,1252],{"class":351},"                |",[163,1254,1255],{"class":341}," NFS_CAP_ATOMIC_OPEN\n",[163,1257,1258,1260],{"class":165,"line":873},[163,1259,1252],{"class":351},[163,1261,1262],{"class":341}," NFS_CAP_POSIX_LOCK\n",[163,1264,1265,1267],{"class":165,"line":893},[163,1266,1252],{"class":351},[163,1268,1269],{"class":341}," NFS_CAP_STATEID_NFSV41\n",[163,1271,1272,1274],{"class":165,"line":1080},[163,1273,1252],{"class":351},[163,1275,1276],{"class":341}," NFS_CAP_ATOMIC_OPEN_V1\n",[12,1278,1279,1280,1283,1284,1287,1288,1290],{},"That explained the difference in behavior: in the v4.1 case, the ",[52,1281,1282],{},"goto no_open"," would cause more validation to happen in\n",[52,1285,1286],{},"nfs_lookup_revalidate()",", but in v4.0, the ",[52,1289,1033],{},"\nwould return earlier. Now, how do we actually solve the problem?",[94,1292,1294],{"id":1293},"the-solution","The solution",[12,1296,1297,1298,1303,1304,1309,1310,123],{},"I reported the ",[22,1299,1302],{"href":1300,"rel":1301},"https://marc.info/?l=linux-nfs&m=153782129412452&w=2",[],"findings to the NFS mailing\nlist"," and proposed\n",[22,1305,1308],{"href":1306,"rel":1307},"https://marc.info/?l=linux-nfs&m=153807208928650&w=2",[],"a naive patch",". A\nweek after the report, Trond Myklebust sent a ",[22,1311,1314],{"href":1312,"rel":1313},"https://marc.info/?l=linux-nfs&m=153816500525563&w=2",[],"patch series to the list\nfixing this bug and found another related issue for NFS\nv4.1",[12,1316,1317,1318,113],{},"It turns out the fix for the NFS v4.0 bug was deeper in the code base\nthan we had looked. Trond summarized it well in the\n",[22,1319,1322],{"href":1320,"rel":1321},"https://marc.info/?l=linux-nfs&m=153816500525564&w=2",[],"patch",[547,1324,1325],{},[12,1326,1327],{},"We need to ensure that inode and dentry revalidation occurs correctly\non reopen of a file that is already open. Currently, we can end up not\nrevalidating either in the case of NFSv4.0, due to the 'cached open'\npath.  Let's fix that by ensuring that we only do cached open for the\nspecial cases of open recovery and delegation return.",[12,1329,1330,1331,68,1334,123],{},"We confirmed that this fix made the stale file problem go away and filed\nbug reports with\n",[22,1332,33],{"href":31,"rel":1333},[],[22,1335,1338],{"href":1336,"rel":1337},"https://bugzilla.redhat.com/show_bug.cgi?id=1648482",[],"RedHat",[12,1340,1341,1342,1347,1348,1351,1352,1354,1355,1357],{},"Knowing full well that kernel changes may take a while to make it to\nstable releases, we also added a ",[22,1343,1346],{"href":1344,"rel":1345},"https://gitlab.com/gitlab-org/gitaly/merge_requests/924",[],"workaround in\nGitaly"," to deal\nwith this issue. We did experiments to test that calling ",[52,1349,1350],{},"stat()"," on the\n",[52,1353,191],{}," file appears to cause the kernel to revalidate the dentry\ncache for the renamed file. For simplicity, this is implemented in\nGitaly regardless of whether the filesystem is NFS; we only do this once\nbefore Gitaly \"opens\" a repository, and there are already other ",[52,1356,1350],{},"\ncalls that check for other files.",[94,1359,1361],{"id":1360},"what-we-learned","What we learned",[12,1363,1364],{},"A bug can be anywhere in your software stack, and sometimes you have to\nlook beyond your application to find it. Having helpful partners in the\nopen source world makes that job much easier.",[12,1366,1367],{},"We are extremely grateful to Trond Myklebust for fixing the problem, and\nBruce Fields for responding to questions and helping us understand\nNFS. Their responsiveness and professionalism truly reflects the best of\nthe open source community.",[12,1369,1370,1371,1376,1377],{},"Photo by ",[22,1372,1375],{"href":1373,"rel":1374},"https://www.flickr.com/photos/dynamosquito",[],"dynamosquito"," on ",[22,1378,1381],{"href":1379,"rel":1380},"https://www.flickr.com/photos/dynamosquito/4265771518",[],"Flickr",[1383,1384,1385],"style",{},"html pre.shiki code .s7eDp, html code.shiki .s7eDp{--shiki-default:#6F42C1}html pre.shiki code .sYBdl, html code.shiki .sYBdl{--shiki-default:#032F62}html .default .shiki span {color: var(--shiki-default);background: var(--shiki-default-bg);font-style: var(--shiki-default-font-style);font-weight: var(--shiki-default-font-weight);text-decoration: var(--shiki-default-text-decoration);}html .shiki span {color: var(--shiki-default);background: var(--shiki-default-bg);font-style: var(--shiki-default-font-style);font-weight: var(--shiki-default-font-weight);text-decoration: var(--shiki-default-text-decoration);}html pre.shiki code .sgsFI, html code.shiki .sgsFI{--shiki-default:#24292E}html pre.shiki code .sD7c4, html code.shiki .sD7c4{--shiki-default:#D73A49}html pre.shiki code .sYu0t, html code.shiki .sYu0t{--shiki-default:#005CC5}html pre.shiki code .sqxcx, html code.shiki .sqxcx{--shiki-default:#E36209}html pre.shiki code .s9hmT, html code.shiki .s9hmT{--shiki-default:#B31D28}html pre.shiki code .shJU0, html code.shiki .shJU0{--shiki-default:#22863A}html pre.shiki code .soKTg, html code.shiki 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