Submitted this bug to Red Hat Bugzilla for RHEL 4.5. Not sure if Samba code is implicated, but submitting it here too just in case. Feel free to close it if it doesn't belong here. https://bugzilla.redhat.com/show_bug.cgi?id=396281 Description of problem: Reading data from files over Samba share causes them to temporarily appear as modified when same files are examined via NFS share. Does not happen if cached 'ls' data is present. Results in spurious rebuilding of output files when 'make' is run concurrently from Samba and NFS shares. Note that in our environment objects for different platforms are kept in separate directories in a manner similar to the way 'gcc' builds work. The fictitious source file timestamp modifications are what cause the problem. Version-Release number of selected component (if applicable): Samba+NFS server, x86_64 samba-3.0.10-1.4E.12.2 kernel-smp-2.6.9-55.EL Samba client Windows 2003 R2 SP2 X64, current patches NFS client, i686 kernel-2.6.9-55.EL How reproducible: 1) place a bunch of files in a directory (verified this with 'gzip' source) 2) issue 'touch time_mark' 3) create Samba share of directory 4) create NFSv3 share of directory 5) issue 'find . -type f -print >/dev/null' in directory via Samba client (alternately 'make -n' could be run) 6) issue 'find . -type f -newer time_mark -ls' in directory via NFS client 7) observe that all files are reported as modified!!! 8) repeat (6) after 'acregmax' interval and observe that files are no longer reported as modified Actual results: In practice this has been causing 'make' to rebuild things that it should not. Putting it nicely, it's been driving us nuts for many months. Expected results: Obviously, the files were never modified and should not be reported as such. Additional info: If you run 'ls -l' in the directory via the NFS share, the problem is cured. You may have to unmount and remount the share to bring back the bug. Before we figured this out, tried NFSv4 for awhile. Seemed like it fixed the above issue but it's not certain. NFSv4 + PoPToP started hosing our system so it's gone and I have no further patience or time for more experiments. Determined the problem appears for both 'i686' and 'x86_64' servers, so it's a portable bug. Further details: does not happen at top-level of NFSv3 mount, must be in subdirectory if any directories are touched by the 'find' on Samba share, problem is suppressed does not happen when files reside on 'ext2' file system does happen when files reside on 'ext3' file system have 'noreservation' option active too lazy to try it with 'noreservation' turned off tried mounting Samba share remotely with CIFS problem does not happen with 'find' or 'cat' run on CIFS mount files accessed via Windows UNC path, not mapped drive obviously CYGWIN is in use on the Windows system can produce problem with MKS, but a 'cat' is also required: find . -type f -print | xargs cat >/dev/null can also produce the problem using CMD shell and 'type \\server\...\file' command CMD 'dir' doesn't work, probably because directory file is examined does happen with even tiny number of files does happen with WXP SP2 32-bit Samba client
So if I read this correctly, using a Windows client and using a Cygwin mount to read files on a Samba share modifies the time ? I'm guessing that we're only obeying the modification requests from the client, as we don't arbitrarily modify file timestamps on our own. To debug - firstly use the latest released Samba (3.0.27a) so we know the codebase you're using, then run the command in a directory with only one file (to minimize the log info) with Samba running with debug level 10, and also attach a wireshark trace of this activity. Jeremy.
Not exactly. What's happening is that reading a file over a Samba share from a Windows system (with or without CYGWIN), via a UNC path will cause the file to *appear* to have been modified from the POV of a NFS mount, but only for a few seconds until the NFS cache entry times-out. It's convoluted--took months to finally figure out what was causing the spurious 'make' target building. Built and installed 3.0.27a, reproduced the problem and attached the requested trace information. The file of interest is 'djl0'. First a Windows CMD 'type' command is used to access the file at around packet 26. Then a NFS remote 'find' is run on the directory where 'djl0' is located. The 'find' shows 'djl0' with modify time of "Nov 24 02:46" even though the actual modify time of the file is "Nov 24 02:20".
Created attachment 2994 [details] tar file with traces
Looking at NFS trace, can't see the file names from the 'find' command. Perhaps NFS has some special differential encoding or similar approach. However the first "Regular File" packet is the one that shows the target. Packet 68 in the trace. The 'mtime' shown is incorrect.
Created attachment 2995 [details] tar file with traces Better trace here. Remounted the NFS file share so now the names appear instead of just the NFS handles. # Windows 'type' access of file 04:00:41 SMB access of 'djl0' at packet 21 Last Write: Nov 24, 2007 02:20:44.000000000 # first 'find' invocation 04:00:45 NFS access of 'djl0' at packet 97 mtime: Nov 24, 2007 04:00:45.000000000 04:00:45 NFS access of 'djl0' at packet 107 mtime: Nov 24, 2007 04:00:45.000000000 # second 'find' invocation 04:00:54 NFS access of 'djl0' at packet 176 mtime: Nov 24, 2007 02:20:44.000000000
Created attachment 2996 [details] file lease test program Examined Samba traces and discerned that the cause of the problem: Samba issues fcntl(fd, F_SETLEASE, F_WRLCK); on files that are opened for read access. The Linux kernel set the modify time as current when the activity trace is put in place and then restores the modify time once it's released. Reproduced this with the attached test case. Seems to me this is a kernel bug. File modify time should not be altered until an actual modify occurs.
Wow. That's thorough analysis. Volker
Thank you. The interesting bit is that the altered modify time is visible *only* via NFS. On the local system the modify time of the file remains as the correct value for the duration the the lease is held. This definitely is a NFS/kernel bug, so I'm closing the report--Samba clearly is not the direct cause of the issue.
The Plot Thickens ================= It turns out the observed behavior is by design; full details at http://bugzilla.kernel.org/show_bug.cgi?id=9454 I don't fully understand all the interactions, so I'm pasting comments from Bruce Fields, a 'nfsd' maintainer. Also leaving the bug closed, as it seems more appropriate for the Samba team to determine the future status of this issue. >------- Comment #5 From bfields@fieldses.org 2007-11-26 08:57:56 [reply] ------- > >This is by design--see fs/locks.c:lease_get_mtime(). > >The argument is: if somebody has a write lease on the file (are >you exporting this via Samba? That'd be the typical user), then >they're caching writes--they're explicitly telling us that we do >not know whether the file is still the same, because we may have >modified it on a remote client and not told us about it. So >lease_get_mtime() reports the current time as the mtime, >prompting you to actually try opening the file, at which point >the write lease gets broken and any cached writes get flushed >out. > >It's a terrible kludge, I agree, and maybe we should remove it. >But I'd first like to understand what circumstances prompted >smoebody to add it originally, and talk to the Samba people >about how they're using these write leases. > >(In NFSv4, by the way, there's a callback to the client to allow >the server to find out the attributes of a file that the client >holding a write lease is caching, which solves this problem. We >haven't implemented that yet; it seems likely to be an enormous >pain. I wonder if Samba would want something similar?) > > >------- Comment #6 From starlight@binnacle.cx 2007-11-26 10:17:08 [reply] ------- > >Yes, using a Samba mount along with NFS mounts to 'make' build >an application on multiple platforms simultaneously. When >re-building existing trees the result is spurious target >building, a real problem. Took months of frustration to >figure out what was going on. > >Actually reported this as a bug to Samba along with RH, and a >request for info from the Samba team provoked the analysis that >isolated the problem. Closed the Samba bug but it's all still >there: > >https://bugzilla.samba.org/show_bug.cgi?id=5103 > >My impression from looking at the Samba code is that the leases >are put in place so that files can be monitored for changes, >but that's just a guess. Perhaps Samba can use a read lease >instead of a write lease. > > >------- Comment #7 From bfields@fieldses.org 2007-11-26 10:55:17 [reply] ------- > >Thanks for the pointer to the samba bug. I understand the >frustration. > >My feeling is that: yes, it's suboptimal (but perhaps not a bug) >for samba to be requesting a write lease when a read lease >(sufficient to alert it to modifications of the file) would be >enough. > >It seems to me that the real bug is the incomplete lease >implementation--if the purpose of a lease is to allow a remote >client to cache writes to the file, then there's no way for us >to give a sensible answer to a stat call, unless we break the >lease first. > >Perhaps we could find out (in the samba case) what the >consequences would be of not updating mtime in this case? I >suppose the worst case would be that a modification to a file >made on a samba client could be indefinitely delayed from being >flushed to the server. > >If we agree that that would be less of a problem than these >spurious bumps in the mtime, then the best solution for now may >just be to rip out lease_get_mtime(). I'll cook up a straw-man >patch.... > > >------- Comment #8 From bfields@fieldses.org 2007-11-26 10:56:16 [reply] ------- > >Created an attachment (id=13763) [details] >remove lease_get_mtime > >Not sure if this is what we want to do, but it's at least >something you could test to confirm the source of the problem. > > >------- Comment #9 From starlight@binnacle.cx 2007-11-26 >11:13:01 [reply] ------- > >Thanks! Happen to have a kernel source tree set up for the >Centos/RH images running on the server, so I can try this out in >the next week or so. Seems certain to work. > >In the interim perhaps you could start a dialog with Samba about >the reasoning behind the application of the F_WRLCK? Or if you >wish I could do so using the Samaba bug report cited above. >
Created attachment 3008 [details] kernel-2.6.9-55_remove_lease_get_mtime.patch Adapted patch for RHEL 4.5 2.6.9-55 kernel (attached). Works as expected and eliminates the spurious target rebuilding when a concurrent Samaba and NFS 'make' is run on same source tree. Test case also shows expected behavior. Obviously did not do any regression testing with other Samba file scenarios. In our build trees the same files and directories are never written by more than one server.
Concise description of patch rationale from original: >From: J. Bruce Fields <> >Date: Mon, 26 Nov 2007 13:48:34 -0500 >Subject: [PATCH] nfsd: stop incrementing mtime on presence of write lease > >The lease_get_mtime() function has the effect of setting the mtime of a >file (as far as any nfs client is concerned) to the current time, >whenever there is a write lease held on the file. > >The presence of a write lease may mean that some client (almost >certainly a samba client) is caching writes to that file. Thus >increasing the mtime has the effect of making the nfs client read data >from that file, thus opening the file for read, thus breaking the write >lease, thus forcing the samba client to flush any cached writes. > >However Samba seems to be requesting write leases even when a read lease >would do. And unfortunately the consequences of the spurious mtime >updates--make unnecessarily rebuilding, for example--may be worse than >the consequences of no mtime update--modifications from a samba client >taking longer to be noticed on the nfs client. > >So perhaps we should rip out lease_get_mtime().
On suggestion of the RH NFS developer, tried "kernel oplocks = no" in the Samba config and it does work around the issue.
The NFSv3 problem only relates to F_WRLCK leases. Non-exclusive F_RDLCK leases don't interfere with nfs mtime. Samba currently always uses F_WRLCK and has no option for F_RDLCK. The following patch makes samba reject exclusive oplocks, but accept level 2 oplocks and implement them using F_RDLCK. This allows NFSv3 and samba to both share the same filesystem without splurious mtime problems, without performance penalties of completely disabling oplocks and without the data integrity problems of disabling kernel oplocks. Maybe there could be an option along these lines for samba? diff -rup samba-3.2.5-orig/source/smbd/open.c samba-3.2.5/source/smbd/open.c --- samba-3.2.5-orig/source/smbd/open.c 2009-03-10 12:34:51.000000000 +1300 +++ samba-3.2.5/source/smbd/open.c 2009-03-10 13:19:07.000000000 +1300 @@ -1842,7 +1842,16 @@ NTSTATUS open_file_ntcreate(connection_s (fsp->oplock_type != FAKE_LEVEL_II_OPLOCK)) { if (!set_file_oplock(fsp, fsp->oplock_type)) { /* Could not get the kernel oplock */ - fsp->oplock_type = NO_OPLOCK; + if (EXCLUSIVE_OPLOCK_TYPE(fsp->oplock_type)) { + /* try readonly... */ + fsp->oplock_type = LEVEL_II_OPLOCK; + if (!set_file_oplock(fsp, fsp->oplock_type)) { + fsp->oplock_type = NO_OPLOCK; + } + + } else { + fsp->oplock_type = NO_OPLOCK; + } } } diff -rup samba-3.2.5-orig/source/smbd/oplock_linux.c samba-3.2.5/source/smbd/oplock_linux.c --- samba-3.2.5-orig/source/smbd/oplock_linux.c 2009-03-10 12:34:51.000000000 +1300 +++ samba-3.2.5/source/smbd/oplock_linux.c 2009-03-12 08:22:06.000000000 +1300 @@ -130,7 +130,12 @@ static files_struct *linux_oplock_receiv static bool linux_set_kernel_oplock(files_struct *fsp, int oplock_type) { - if ( SMB_VFS_LINUX_SETLEASE(fsp, F_WRLCK) == -1) { + if (EXCLUSIVE_OPLOCK_TYPE(oplock_type)) { + return False; /* don't grant exclusive/write locks as they + interfere with mtime reporting through NFSv3 */ + + } + if ( SMB_VFS_LINUX_SETLEASE(fsp, F_RDLCK) == -1) { DEBUG(3,("linux_set_kernel_oplock: Refused oplock on file %s, " "fd = %d, file_id = %s. (%s)\n", fsp->fsp_name, fsp->fh->fd,
I think this patch should go a bit further: Couldn't we look at whether the client actually wants write access to the file and request a RDLK if that is not the case? Would that make sense? Volker
I have a patch to do that, but it is completely untested and also over 200 lines - I don't know if there would be copyright issues? Ultimately it doesn't solve the splurious mtime nfs issue, as samba will still grant WRLCK when requested by the client, which triggers the problem.
Copyright issues? In what sense do you have issues with the copyright? Do have reasons not to submit patches under your personal copyright? If you can submit patches on your own, it would be best if you sent a git format-patch formatted patch to samba-technical@samba.org, so that it's really publically visible. Second, I would like to see that patch. I think Tim Prouty should also comment here, he has done a lot of oplock work lately. Third, if retrieving RDLK is a workaround for deficient kernel behaviour, I would like to see this in a VFS module and not in the core Samba code. This would require the oplock operations to be passed through the VFS though. But as the VFS has grown like mad lately, probably that doesn't matter anymore :-) Volker