linux-kernel - Re: [PATCH 2/2] fs: xfs: use BUG_ON if writepage call comes from direct reclaim

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Message-ID: <20180703171920.GC5711@magnolia>
Date:   Tue, 3 Jul 2018 10:19:20 -0700
From:   "Darrick J. Wong" <darrick.wong@...cle.com>
To:     Yang Shi <yang.shi@...ux.alibaba.com>
Cc:     mgorman@...hsingularity.net, tytso@....edu,
        adilger.kernel@...ger.ca, dchinner@...hat.com,
        akpm@...ux-foundation.org, linux-ext4@...r.kernel.org,
        linux-xfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] fs: xfs: use BUG_ON if writepage call comes from
 direct reclaim

On Tue, Jul 03, 2018 at 12:11:19PM +0800, Yang Shi wrote:
> direct reclaim doesn't write out filesystem page, only kswapd could do
> this. So, if it is called from direct relaim, it is definitely a bug.
> 
> And, Mel Gorman mentioned "Ultimately, this will be a BUG_ON." in commit
> 94054fa3fca1fd78db02cb3d68d5627120f0a1d4 ("xfs: warn if direct reclaim
> tries to writeback pages"),
> 
> It has been many years since that commit, so it should be safe to
> elevate WARN_ON to BUG_ON now.
> 
> Cc: Mel Gorman <mgorman@...hsingularity.net>
> Cc: Darrick J. Wong <darrick.wong@...cle.com>
> Cc: Dave Chinner <dchinner@...hat.com>
> Signed-off-by: Yang Shi <yang.shi@...ux.alibaba.com>
> ---
>  fs/xfs/xfs_aops.c | 6 ++----
>  1 file changed, 2 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/xfs/xfs_aops.c b/fs/xfs/xfs_aops.c
> index 8eb3ba3..7efc2d2 100644
> --- a/fs/xfs/xfs_aops.c
> +++ b/fs/xfs/xfs_aops.c
> @@ -1080,11 +1080,9 @@ static inline int xfs_bio_add_buffer(struct bio *bio, struct buffer_head *bh)
>  	 * allow reclaim from kswapd as the stack usage there is relatively low.
>  	 *
>  	 * This should never happen except in the case of a VM regression so
> -	 * warn about it.
> +	 * BUG about it.
>  	 */
> -	if (WARN_ON_ONCE((current->flags & (PF_MEMALLOC|PF_KSWAPD)) ==
> -			PF_MEMALLOC))
> -		goto redirty;
> +	BUG_ON((current->flags & (PF_MEMALLOC|PF_KSWAPD)) == PF_MEMALLOC);

Ugh, please do not increase the BUG() factor.  Even if this happens due
to a regression it's /much/ easier to debug if we don't halt the system.

(IOWs, I decline to take this patch.)

--D

>  
>  	/*
>  	 * Given that we do not allow direct reclaim to call us, we should
> -- 
> 1.8.3.1
> 
> --
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