linux-kernel - Re: [PATCH 08/11] mm, compaction: Simplify __alloc_pages_direct

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Message-ID: <570BAB3E.1060601@suse.cz>
Date:	Mon, 11 Apr 2016 15:48:46 +0200
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Michal Hocko <mhocko@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.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>
Subject: Re: [PATCH 08/11] mm, compaction: Simplify
 __alloc_pages_direct_compact feedback interface

On 04/05/2016 01:25 PM, Michal Hocko wrote:
> From: Michal Hocko <mhocko@...e.com>
>
> __alloc_pages_direct_compact communicates potential back off by two
> variables:
> 	- deferred_compaction tells that the compaction returned
> 	  COMPACT_DEFERRED
> 	- contended_compaction is set when there is a contention on
> 	  zone->lock resp. zone->lru_lock locks
>
> __alloc_pages_slowpath then backs of for THP allocation requests to
> prevent from long stalls. This is rather messy and it would be much
> cleaner to return a single compact result value and hide all the nasty
> details into __alloc_pages_direct_compact.

On the other hand, the nasty subtle details of THP allocation handling 
are now split between __alloc_pages_direct_compact and 
__alloc_pages_slowpath(). Lesser evil, I guess. I wish we could get rid 
of these special cases, now that latency of THP direct allocations is 
reduced by Mel's new defaults.

> This patch shouldn't introduce any functional changes.
>
> Signed-off-by: Michal Hocko <mhocko@...e.com>

Acked-by: Vlastimil Babka <vbabka@...e.cz>