linux-ext4 - Re: [PATCH] MM: introduce memalloc_retry

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Date:   Wed, 17 Nov 2021 16:53:11 +1100
From:   Dave Chinner <david@...morbit.com>
To:     NeilBrown <neilb@...e.de>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>, Theodore Ts'o <tytso@....edu>,
        linux-ext4@...r.kernel.org, Jaegeuk Kim <jaegeuk@...nel.org>,
        Chao Yu <chao@...nel.org>,
        "Darrick J. Wong" <djwong@...nel.org>, linux-xfs@...r.kernel.org,
        Chuck Lever <chuck.lever@...cle.com>,
        linux-f2fs-devel@...ts.sourceforge.net, linux-nfs@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] MM: introduce memalloc_retry_wait()

On Wed, Nov 17, 2021 at 03:28:10PM +1100, NeilBrown wrote:
> 
> Various places in the kernel - largely in filesystems - respond to a
> memory allocation failure by looping around and re-trying.
.....
> diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
> index aca874d33fe6..f2f2a5b28808 100644
> --- a/include/linux/sched/mm.h
> +++ b/include/linux/sched/mm.h
> @@ -214,6 +214,27 @@ static inline void fs_reclaim_acquire(gfp_t gfp_mask) { }
>  static inline void fs_reclaim_release(gfp_t gfp_mask) { }
>  #endif
>  
> +/* Any memory-allocation retry loop should use
> + * memalloc_retry_wait(), and pass the flags for the most
> + * constrained allocation attempt that might have failed.
> + * This provides useful documentation of where loops are,
> + * and a central place to fine tune the waiting as the MM
> + * implementation changes.
> + */
> +static inline void memalloc_retry_wait(gfp_t gfp_flags)
> +{
> +	gfp_flags = current_gfp_context(gfp_flags);
> +	if ((gfp_flags & __GFP_DIRECT_RECLAIM) &&
> +	    !(gfp_flags & __GFP_NORETRY))
> +		/* Probably waited already, no need for much more */
> +		schedule_timeout_uninterruptible(1);
> +	else
> +		/* Probably didn't wait, and has now released a lock,
> +		 * so now is a good time to wait
> +		 */
> +		schedule_timeout_uninterruptible(HZ/50);
> +}

The existing congestion_wait() calls io_schedule_timeout() under
TASK_UNINTERRUPTIBLE conditions.

Does changing all these calls just to a plain
schedule_timeout_uninterruptible() make any difference to behaviour?
At least process accounting will appear different (uninterruptible
sleep instead of IO wait), and I suspect that the block plug
flushing in io_schedule() might be a good idea to retain for all the
filesystems that call this function from IO-related routines.

Cheers,

Dave.

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