linux-kernel - Re: [PATCH 09/11] mm, compaction: Abstract compaction feedback to helpers

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Message-Id: <20160405165826.012236e79db7f396fda546a8@linux-foundation.org>
Date:	Tue, 5 Apr 2016 16:58:26 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Michal Hocko <mhocko@...nel.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Mel Gorman <mgorman@...e.de>,
	David Rientjes <rientjes@...gle.com>,
	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
	Joonsoo Kim <js1304@...il.com>,
	Hillf Danton <hillf.zj@...baba-inc.com>, <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Michal Hocko <mhocko@...e.com>, Hugh Dickins <hughd@...gle.com>
Subject: Re: [PATCH 09/11] mm, compaction: Abstract compaction feedback to
 helpers

On Tue,  5 Apr 2016 13:25:31 +0200 Michal Hocko <mhocko@...nel.org> wrote:

> From: Michal Hocko <mhocko@...e.com>
> 
> Compaction can provide a wild variation of feedback to the caller. Many
> of them are implementation specific and the caller of the compaction
> (especially the page allocator) shouldn't be bound to specifics of the
> current implementation.
> 
> This patch abstracts the feedback into three basic types:
> 	- compaction_made_progress - compaction was active and made some
> 	  progress.
> 	- compaction_failed - compaction failed and further attempts to
> 	  invoke it would most probably fail and therefore it is not
> 	  worth retrying
> 	- compaction_withdrawn - compaction wasn't invoked for an
>           implementation specific reasons. In the current implementation
>           it means that the compaction was deferred, contended or the
>           page scanners met too early without any progress. Retrying is
>           still worthwhile.
> 
> ...
>
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -3362,25 +3362,12 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
>  	if (page)
>  		goto got_pg;
>  
> -	/* Checks for THP-specific high-order allocations */
> -	if (is_thp_gfp_mask(gfp_mask)) {
> -		/*
> -		 * If compaction is deferred for high-order allocations, it is
> -		 * because sync compaction recently failed. If this is the case
> -		 * and the caller requested a THP allocation, we do not want
> -		 * to heavily disrupt the system, so we fail the allocation
> -		 * instead of entering direct reclaim.
> -		 */
> -		if (compact_result == COMPACT_DEFERRED)
> -			goto nopage;
> -
> -		/*
> -		 * Compaction is contended so rather back off than cause
> -		 * excessive stalls.
> -		 */
> -		if(compact_result == COMPACT_CONTENDED)
> -			goto nopage;
> -	}
> +	/*
> +	 * Checks for THP-specific high-order allocations and back off
> +	 * if the the compaction backed off
> +	 */
> +	if (is_thp_gfp_mask(gfp_mask) && compaction_withdrawn(compact_result))
> +		goto nopage;

This change smashed into Hugh's "huge tmpfs: shmem_huge_gfpmask and
shmem_recovery_gfpmask".

I ended up doing this:

	/* Checks for THP-specific high-order allocations */
	if (!is_thp_allocation(gfp_mask, order))
		migration_mode = MIGRATE_SYNC_LIGHT;

	/*
	 * Checks for THP-specific high-order allocations and back off
	 * if the the compaction backed off
	 */
	if (is_thp_allocation(gfp_mask) && compaction_withdrawn(compact_result))
		goto nopage;