WARNING in xfs_lwr.c, xfs_write()
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From: Dave Chinner on 13 Jun 2010 18:50 On Sat, Jun 12, 2010 at 01:00:52AM -0400, Ilia Mirkin wrote: > Sorry to pick up an old-ish thread, but I have a similar situation: > > On Sun, May 23, 2010 at 9:19 PM, Dave Chinner <david(a)fromorbit.com> wrote: > > On Sun, May 23, 2010 at 09:23:44AM -0500, Roman Kononov wrote: > >> On 2010-05-23, 20:18:56 +1000, Dave Chinner <david(a)fromorbit.com> wrote: > >> > Can you find out what the application is triggering this? > > I noticed this happening with mysql and xtrabackup -- the latter opens > up mysql's files while mysql is still running (and modifying its own > files) and backs them up in a (hopefully) safe way. That's not safe at all - there's no guarantee you'll end up with a consistent database image doing backups like this. Have you ever tried to restore and use one of these backups? > mysql had been > running on the machine without any such warnings for a while before we > ran the backup, so I'm pretty sure that the backup is involved, > although its process is never listed. Specifically the warning is: > > [2584257.839386] ------------[ cut here ]------------ > [2584257.839395] WARNING: at fs/xfs/linux-2.6/xfs_lrw.c:651 > xfs_write+0x3dc/0x784() > [2584257.839398] Hardware name: PowerEdge R710 > [2584257.839399] Modules linked in: nfsd cifs iTCO_wdt iTCO_vendor_support > [2584257.839406] Pid: 7761, comm: mysqld Not tainted 2.6.33-gentoo-r2 #1 > [2584257.839407] Call Trace: > [2584257.839411] [<ffffffff8120da46>] ? xfs_write+0x3dc/0x784 > [2584257.839415] [<ffffffff81038733>] warn_slowpath_common+0x77/0xa4 > [2584257.839417] [<ffffffff8103876f>] warn_slowpath_null+0xf/0x11 > [2584257.839419] [<ffffffff8120da46>] xfs_write+0x3dc/0x784 > [2584257.839424] [<ffffffff810033ce>] ? apic_timer_interrupt+0xe/0x20 > [2584257.839427] [<ffffffff8120a51a>] xfs_file_aio_write+0x5a/0x5c > [2584257.839430] [<ffffffff810d7cbe>] do_sync_write+0xc0/0x106 > [2584257.839435] [<ffffffff810ff862>] ? __fsnotify_parent+0xc7/0xd3 > [2584257.839437] [<ffffffff810d8624>] vfs_write+0xab/0x105 > [2584257.839439] [<ffffffff810d86da>] sys_pwrite64+0x5c/0x7d > [2584257.839442] [<ffffffff81002a6b>] system_call_fastpath+0x16/0x1b > [2584257.839444] ---[ end trace 8b0c2a6e5e86745f ]--- > > > Yes, it should be safe, but the kernel code can't know whether this > > is true or not - there are no specific interlocks with direct IO to > > prevent concurrent buffered IO to the same region while a direct IO > > is in progress. XFS does best effort attempts to maintain coherency > > does not provide any guarantees, hence the warning when known race > > conditions are tripped. > > Would it be safe to remove the warning at > fs/xfs/linux-2.6/xfs_lrw.c:651 (which looks like it has moved to > xfs_file.c in 2.6.34)? It seems undesirable to get a long stream of > these (51 in this particular instance) every time we run a backup... You can if you want, but then you won't know when your backup or database might have been corrupted, right? > IOW, is the warning purely something along the lines of "Userspace is > doing something wonky, but the underlying FS will still be fine no > matter what" kind of deal, or could there be an actual problem with > the XFS metadata itself? Nothing wrong with the filesystem metadata will occur - as I said eariler in the thread that this is a warning to tell us that data corruption is possible due to userspace doing something stupid, not a filesystem bug. Cheers, Dave. -- Dave Chinner david(a)fromorbit.com -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
From: Ilia Mirkin on 13 Jun 2010 19:20 On Sun, Jun 13, 2010 at 6:47 PM, Dave Chinner <david(a)fromorbit.com> wrote: > On Sat, Jun 12, 2010 at 01:00:52AM -0400, Ilia Mirkin wrote: >> Sorry to pick up an old-ish thread, but I have a similar situation: >> >> On Sun, May 23, 2010 at 9:19 PM, Dave Chinner <david(a)fromorbit.com> wrote: >> > On Sun, May 23, 2010 at 09:23:44AM -0500, Roman Kononov wrote: >> >> On 2010-05-23, 20:18:56 +1000, Dave Chinner <david(a)fromorbit.com> wrote: >> >> > Can you find out what the application is triggering this? >> >> I noticed this happening with mysql and xtrabackup -- the latter opens >> up mysql's files while mysql is still running (and modifying its own >> files) and backs them up in a (hopefully) safe way. > > That's not safe at all - there's no guarantee you'll end up with a > consistent database image doing backups like this. Have you ever > tried to restore and use one of these backups? Yep, works great. [Used it to initialize a slave, did the full checksums, so it's unlikely to have randomly corrupt data.] It's the only credible way to backup a sizeable mysql db, since it works online with InnoDB; the other options involve either only using MyISAM (non-transactional) or locking the db for the duration (we couldn't wait that long, but attempting to do it on a backup machine looked like it was going to take somewhere between 3 and 7 days, although we gave up after 24 hours... not something we can afford to do with any kind of regularity). >> >> Would it be safe to remove the warning at >> fs/xfs/linux-2.6/xfs_lrw.c:651 (which looks like it has moved to >> xfs_file.c in 2.6.34)? It seems undesirable to get a long stream of >> these (51 in this particular instance) every time we run a backup... > > You can if you want, but then you won't know when your backup or > database might have been corrupted, right? No, but I wouldn't know that without the warnings either -- for all I know xtrabackup could be buggy in all kinds of ways. The only real way to check is to use the backup data in some way. > >> IOW, is the warning purely something along the lines of "Userspace is >> doing something wonky, but the underlying FS will still be fine no >> matter what" kind of deal, or could there be an actual problem with >> the XFS metadata itself? > > Nothing wrong with the filesystem metadata will occur - as I said > eariler in the thread that this is a warning to tell us that data > corruption is possible due to userspace doing something stupid, not > a filesystem bug. OK, thanks for the clarification. Ideally these wouldn't taint the kernel either -- perhaps these can be downgraded to a message that explicitly suggests that nothing is wrong with kernel-space things, only user-space? The backtrace doesn't really get you much, so really all you want to show is the offending process... Thanks, Ilia Mirkin imirkin(a)alum.mit.edu -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
From: Dave Chinner on 13 Jun 2010 21:30 On Sun, Jun 13, 2010 at 07:10:30PM -0400, Ilia Mirkin wrote: > On Sun, Jun 13, 2010 at 6:47 PM, Dave Chinner <david(a)fromorbit.com> wrote: > > On Sat, Jun 12, 2010 at 01:00:52AM -0400, Ilia Mirkin wrote: > >> Sorry to pick up an old-ish thread, but I have a similar situation: > >> > >> On Sun, May 23, 2010 at 9:19 PM, Dave Chinner <david(a)fromorbit.com> wrote: > >> > On Sun, May 23, 2010 at 09:23:44AM -0500, Roman Kononov wrote: > >> >> On 2010-05-23, 20:18:56 +1000, Dave Chinner <david(a)fromorbit.com> wrote: > >> >> > Can you find out what the application is triggering this? > >> > >> I noticed this happening with mysql and xtrabackup -- the latter opens > >> up mysql's files while mysql is still running (and modifying its own > >> files) and backs them up in a (hopefully) safe way. > > > > That's not safe at all - there's no guarantee you'll end up with a > > consistent database image doing backups like this. Have you ever > > tried to restore and use one of these backups? > > Yep, works great. [Used it to initialize a slave, did the full > checksums, so it's unlikely to have randomly corrupt data.] You were lucky, I'd say. xtrabackup is supposed to be tightly integrated with mysql, so perhaps it should be using the same IO methods that the admin has selected for their database. Maybe you need to talk to the xtrabackup folks to get them to add a "backup via direct IO" method if the mysql database is using direct IO so that other uses don't have the same issues. > >> Would it be safe to remove the warning at > >> fs/xfs/linux-2.6/xfs_lrw.c:651 (which looks like it has moved to > >> xfs_file.c in 2.6.34)? It seems undesirable to get a long stream of > >> these (51 in this particular instance) every time we run a backup... > > > > You can if you want, but then you won't know when your backup or > > database might have been corrupted, right? > > No, but I wouldn't know that without the warnings either -- for all I > know xtrabackup could be buggy in all kinds of ways. The only real way > to check is to use the backup data in some way. Yup, but you still can't rely on the backup for disaster recovery without first doing a full application level consistency check it if one of these warnings was generated while it was being taken. > >> IOW, is the warning purely something along the lines of "Userspace is > >> doing something wonky, but the underlying FS will still be fine no > >> matter what" kind of deal, or could there be an actual problem with > >> the XFS metadata itself? > > > > Nothing wrong with the filesystem metadata will occur - as I said > > eariler in the thread that this is a warning to tell us that data > > corruption is possible due to userspace doing something stupid, not > > a filesystem bug. > > OK, thanks for the clarification. Ideally these wouldn't taint the > kernel either Why not? Something has potentially compromised the integrity of the system and that's exactly what the taint flag is there for. > -- perhaps these can be downgraded to a message that > explicitly suggests that nothing is wrong with kernel-space things, > only user-space? The backtrace doesn't really get you much, so really > all you want to show is the offending process... They are there to be meaningful to the XFS developer, not the user, and it conveys all the information we need to start a deeper investigation. IOWs, it's a defensive mechanism that we have in place because direct IO is effectively handing responsibility for data integrity to userspace. Hence when userspace is doing something obviously dangerous to data integrity we want loud, noticable warnings so that the filesystem is not blamed for the data corruption that will inevitably occur. And from a "I read it on the interwebs so it must be true" perspective, without a loud obnoxious warning we'll never hear about problems until someone flames us about silent data corruption on a random blog that gets slashdotted and then referenced for the next 10 years as the next canonical "XFS eats my data!" reference for the clueless.... Cheers, Dave. -- Dave Chinner david(a)fromorbit.com -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
From: Ilia Mirkin on 13 Jun 2010 23:30 On Sun, Jun 13, 2010 at 9:29 PM, Dave Chinner <david(a)fromorbit.com> wrote: > On Sun, Jun 13, 2010 at 07:10:30PM -0400, Ilia Mirkin wrote: >> On Sun, Jun 13, 2010 at 6:47 PM, Dave Chinner <david(a)fromorbit.com> wrote: >> > On Sat, Jun 12, 2010 at 01:00:52AM -0400, Ilia Mirkin wrote: >> >> Sorry to pick up an old-ish thread, but I have a similar situation: >> >> >> >> On Sun, May 23, 2010 at 9:19 PM, Dave Chinner <david(a)fromorbit.com> wrote: >> >> > On Sun, May 23, 2010 at 09:23:44AM -0500, Roman Kononov wrote: >> >> >> On 2010-05-23, 20:18:56 +1000, Dave Chinner <david(a)fromorbit.com> wrote: >> >> >> > Can you find out what the application is triggering this? >> >> >> >> I noticed this happening with mysql and xtrabackup -- the latter opens >> >> up mysql's files while mysql is still running (and modifying its own >> >> files) and backs them up in a (hopefully) safe way. >> > >> > That's not safe at all - there's no guarantee you'll end up with a >> > consistent database image doing backups like this. Have you ever >> > tried to restore and use one of these backups? >> >> Yep, works great. [Used it to initialize a slave, did the full >> checksums, so it's unlikely to have randomly corrupt data.] > > You were lucky, I'd say. �xtrabackup is supposed to be tightly > integrated with mysql, so perhaps it should be using the same IO > methods that the admin has selected for their database. Maybe you > need to talk to the xtrabackup folks to get them to add a "backup > via direct IO" method if the mysql database is using direct IO so > that other uses don't have the same issues. Maybe. We've been using this technique, although on a different physical machine and with ext3, for quite some time (and we verify all backups). I did notice that there is a minor difference in configuration, esp wrt direct IO, so I'll check it out in more detail. [We're now setting innodb_flush_method to O_DIRECT whereas we weren't before... although based on the documentation and a cursory understanding of how xtrabackup works, this shouldn't be harmful.] > And from a "I read it on the interwebs so it must be true" > perspective, without a loud obnoxious warning we'll never hear about > problems until someone flames us about silent data corruption on a > random blog that gets slashdotted and then referenced for the next > 10 years as the next canonical "XFS eats my data!" reference for the > clueless.... Instead it will be "mysql works fine on ext3, but with xfs it spams the logs with warnings, therefore xfs must be broken". I don't think there's anything realistically that you can do about uninformed users and FUD. Although I wasn't suggesting to get rid of the warning, rather to make it more explicit as to what it's warning about. I interpret a WARN as a BUG that can be recovered but where the underlying system needs a careful look; my first inclination after seeing a fs-related WARN would be to take the system down and run an fsck. What's happening here seems more akin to getting a WARN when calling an ioctl with invalid parameters. --- Ilia Mirkin imirkin(a)alum.mit.edu -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
From: Roman Kononov on 14 Jun 2010 11:20
2010-06-13 23:27 CDT, Ilia Mirkin <imirkin(a)alum.mit.edu> said: >Instead it will be "mysql works fine on ext3, but with xfs it spams >the logs with warnings, therefore xfs must be broken". I don't think >there's anything realistically that you can do about uninformed users >and FUD. Although I wasn't suggesting to get rid of the warning, >rather to make it more explicit as to what it's warning about. I >interpret a WARN as a BUG that can be recovered but where the >underlying system needs a careful look; my first inclination after >seeing a fs-related WARN would be to take the system down and run an >fsck. What's happening here seems more akin to getting a WARN when >calling an ioctl with invalid parameters. I agree. My reaction to this WARN was horrible: I brought the system down, started fsck-ing and re-installing older kernels, with all kinds of FUD, which took me considerable time. The message was not well explained on the Internet, nor was it clear reading the source code. After talking to the mailing list and investigation of my S/W, I've realized that the system works fine, and the warning now sounds to me as useless and unwanted noise of quite high volume. I am suggesting to issue a notice once per filesytem/mount without taint. The notice could be as such: "WARNING: Userspace issues direct IO which races with buffered or mmap IO on the same file (inode <number>, device <name>). File data corruption is possible. This message is issued only once per mount". Thanks. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/ |