linux-kernel - Re: [PATCH v2 2/2] xfs: Fix false positive lockdep warning with sb_internal & fs

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Message-ID: <20200618004505.GG2005@dread.disaster.area>
Date:   Thu, 18 Jun 2020 10:45:05 +1000
From:   Dave Chinner <david@...morbit.com>
To:     Waiman Long <longman@...hat.com>
Cc:     "Darrick J. Wong" <darrick.wong@...cle.com>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        linux-xfs@...r.kernel.org, linux-kernel@...r.kernel.org,
        Qian Cai <cai@....pw>, Eric Sandeen <sandeen@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v2 2/2] xfs: Fix false positive lockdep warning with
 sb_internal & fs_reclaim

On Wed, Jun 17, 2020 at 01:53:10PM -0400, Waiman Long wrote:
>  fs/xfs/xfs_log.c   |  9 +++++++++
>  fs/xfs/xfs_trans.c | 31 +++++++++++++++++++++++++++----
>  2 files changed, 36 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;

The more I look at this, the more I think Darrick is right and I
somewhat misinterpretted what he meant by "the top of the freeze
path".

i.e. setting PF_MEMALLOC_NOFS here is out of place - only one caller
of xlog_unmount_write requires PF_MEMALLOC_NOFS
context. That context should be set in the caller that requires this
context, and in this case it is xfs_fs_freeze(). This is top of the
final freeze state processing (what I think Darrick meant), not the
top of the freeze syscall call chain (what I thought he meant).

So if set PF_MEMALLOC_NOFS setting in xfs_fs_freeze(), it covers all
the allocations in this problematic path, and it should obliviates
the need for the first patch in the series altogether.

Cheers,

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