Re: [PATCH 2/2] xfs: Fix false positive lockdep warning with sb_internal & fs_reclaim

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Mon, Jun 15, 2020 at 04:53:38PM -0400, Waiman Long wrote:
> 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.

<nod>

> > 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.

<shrug> It sounds like you & Dave had already worked that out, so we can
leave this as it is.  I saw the untrue claim in the code comment and
started asking more questions. ;)

(Says me who has been checked out the last few days, not following
the various lockdep shuttup patch threads...)

--D

> Cheers,
> Longman
>