Messages in this thread | | | Subject | Re: [PATCH 2/2] xfs: Fix false positive lockdep warning with sb_internal & fs_reclaim | From | Waiman Long <> | Date | Mon, 15 Jun 2020 16:53:38 -0400 |
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On 6/15/20 12:43 PM, Darrick J. Wong wrote: > On Mon, Jun 15, 2020 at 12:08:30PM -0400, Waiman Long wrote: >> Depending on the workloads, the following circular locking dependency >> warning between sb_internal (a percpu rwsem) and fs_reclaim (a pseudo >> lock) may show up: >> >> ====================================================== >> WARNING: possible circular locking dependency detected >> 5.0.0-rc1+ #60 Tainted: G W >> ------------------------------------------------------ >> fsfreeze/4346 is trying to acquire lock: >> 0000000026f1d784 (fs_reclaim){+.+.}, at: >> fs_reclaim_acquire.part.19+0x5/0x30 >> >> but task is already holding lock: >> 0000000072bfc54b (sb_internal){++++}, at: percpu_down_write+0xb4/0x650 >> >> which lock already depends on the new lock. >> : >> Possible unsafe locking scenario: >> >> CPU0 CPU1 >> ---- ---- >> lock(sb_internal); >> lock(fs_reclaim); >> lock(sb_internal); >> lock(fs_reclaim); >> >> *** DEADLOCK *** >> >> 4 locks held by fsfreeze/4346: >> #0: 00000000b478ef56 (sb_writers#8){++++}, at: percpu_down_write+0xb4/0x650 >> #1: 000000001ec487a9 (&type->s_umount_key#28){++++}, at: freeze_super+0xda/0x290 >> #2: 000000003edbd5a0 (sb_pagefaults){++++}, at: percpu_down_write+0xb4/0x650 >> #3: 0000000072bfc54b (sb_internal){++++}, at: percpu_down_write+0xb4/0x650 >> >> stack backtrace: >> Call Trace: >> dump_stack+0xe0/0x19a >> print_circular_bug.isra.10.cold.34+0x2f4/0x435 >> check_prev_add.constprop.19+0xca1/0x15f0 >> validate_chain.isra.14+0x11af/0x3b50 >> __lock_acquire+0x728/0x1200 >> lock_acquire+0x269/0x5a0 >> fs_reclaim_acquire.part.19+0x29/0x30 >> fs_reclaim_acquire+0x19/0x20 >> kmem_cache_alloc+0x3e/0x3f0 >> kmem_zone_alloc+0x79/0x150 >> xfs_trans_alloc+0xfa/0x9d0 >> xfs_sync_sb+0x86/0x170 >> xfs_log_sbcount+0x10f/0x140 >> xfs_quiesce_attr+0x134/0x270 >> xfs_fs_freeze+0x4a/0x70 >> freeze_super+0x1af/0x290 >> do_vfs_ioctl+0xedc/0x16c0 >> ksys_ioctl+0x41/0x80 >> __x64_sys_ioctl+0x73/0xa9 >> do_syscall_64+0x18f/0xd23 >> entry_SYSCALL_64_after_hwframe+0x49/0xbe >> >> This is a false positive as all the dirty pages are flushed out before >> the filesystem can be frozen. >> >> Perhaps breaking the fs_reclaim pseudo lock into a per filesystem lock >> may fix the issue. However, that will greatly complicate the logic and >> may not be worth it. >> >> Another way to fix it is to disable the taking of the fs_reclaim >> pseudo lock when in the freezing code path as a reclaim on the >> freezed filesystem is not possible. By using the newly introduced >> PF_MEMALLOC_NOLOCKDEP flag, lockdep checking is disabled in >> xfs_trans_alloc() if XFS_TRANS_NO_WRITECOUNT flag is set. >> >> In the freezing path, there is another path where memory allocation >> is being done without the XFS_TRANS_NO_WRITECOUNT flag: >> >> xfs_fs_freeze() >> => xfs_quiesce_attr() >> => xfs_log_quiesce() >> => xfs_log_unmount_write() >> => xlog_unmount_write() >> => xfs_log_reserve() >> => xlog_ticket_alloc() >> >> In this case, we just disable fs reclaim for this particular 600 bytes >> memory allocation. >> >> Without this patch, the command sequence below will show that the lock >> dependency chain sb_internal -> fs_reclaim exists. >> >> # fsfreeze -f /home >> # fsfreeze --unfreeze /home >> # grep -i fs_reclaim -C 3 /proc/lockdep_chains | grep -C 5 sb_internal >> >> After applying the patch, such sb_internal -> fs_reclaim lock dependency >> chain can no longer be found. Because of that, the locking dependency >> warning will not be shown. >> >> Suggested-by: Dave Chinner <david@fromorbit.com> >> Signed-off-by: Waiman Long <longman@redhat.com> >> --- >> fs/xfs/xfs_log.c | 9 +++++++++ >> fs/xfs/xfs_trans.c | 30 ++++++++++++++++++++++++++---- >> 2 files changed, 35 insertions(+), 4 deletions(-) >> >> diff --git a/fs/xfs/xfs_log.c b/fs/xfs/xfs_log.c >> index 00fda2e8e738..33244680d0d4 100644 >> --- a/fs/xfs/xfs_log.c >> +++ b/fs/xfs/xfs_log.c >> @@ -830,8 +830,17 @@ xlog_unmount_write( >> xfs_lsn_t lsn; >> uint flags = XLOG_UNMOUNT_TRANS; >> int error; >> + unsigned long pflags; >> >> + /* >> + * xfs_log_reserve() allocates memory. This can lead to fs reclaim >> + * which may conflicts with the unmount process. To avoid that, >> + * disable fs reclaim for this allocation. >> + */ >> + current_set_flags_nested(&pflags, PF_MEMALLOC_NOFS); >> error = xfs_log_reserve(mp, 600, 1, &tic, XFS_LOG, 0); >> + current_restore_flags_nested(&pflags, PF_MEMALLOC_NOFS); >> + >> if (error) >> goto out_err; >> >> diff --git a/fs/xfs/xfs_trans.c b/fs/xfs/xfs_trans.c >> index 3c94e5ff4316..ddb10ad3f51f 100644 >> --- a/fs/xfs/xfs_trans.c >> +++ b/fs/xfs/xfs_trans.c >> @@ -255,7 +255,27 @@ xfs_trans_alloc( >> struct xfs_trans **tpp) >> { >> struct xfs_trans *tp; >> - int error; >> + int error = 0; >> + unsigned long pflags = -1; >> + >> + /* >> + * When XFS_TRANS_NO_WRITECOUNT is set, it means there are no dirty >> + * data pages in the filesystem at this point. > That's not true. Look at the other callers of xfs_trans_alloc_empty. Yes, I am aware of that. I can change it to check the freeze state. > > Also: Why not set PF_MEMALLOC_NOFS at the start of the freeze call > chain?
I guess we can do that, but it eliminates a potential source for memory reclaim leading to freeze error when not much free memory is left. We can go this route if you think this is not a problem.
Cheers, Longman
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