linux-kernel - Re: [PATCH v6] xfs: Fix false positive lockdep warning with sb_internal & fs

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Message-ID: <20200709223802.GY2005@dread.disaster.area>
Date:   Fri, 10 Jul 2020 08:38:02 +1000
From:   Dave Chinner <david@...morbit.com>
To:     Waiman Long <longman@...hat.com>
Cc:     "Darrick J. Wong" <darrick.wong@...cle.com>,
        linux-xfs@...r.kernel.org, linux-kernel@...r.kernel.org,
        Qian Cai <cai@....pw>, Eric Sandeen <sandeen@...hat.com>
Subject: Re: [PATCH v6] xfs: Fix false positive lockdep warning with
 sb_internal & fs_reclaim

On Tue, Jul 07, 2020 at 03:16:29PM -0400, Waiman Long wrote:
> One way to avoid this splat is to add GFP_NOFS to the affected allocation
> calls by using the memalloc_nofs_save()/memalloc_nofs_restore() pair.
> This shouldn't matter unless the system is really running out of memory.
> In that particular case, the filesystem freeze operation may fail while
> it was succeeding previously.
> 
> 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@...morbit.com>
> Signed-off-by: Waiman Long <longman@...hat.com>

Looks good. Thanks for working through this, Waiman.

Reviewed-by: Dave Chinner <dchinner@...hat.com>

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com