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Subject: Re: [PATCH 08/11] mm, compaction: Simplify
 __alloc_pages_direct_compact feedback interface
References: <1459855533-4600-1-git-send-email-mhocko@kernel.org>
 <1459855533-4600-9-git-send-email-mhocko@kernel.org>
From: Vlastimil Babka <vbabka@suse.cz>
Message-ID: <570BAB3E.1060601@suse.cz>
Date: Mon, 11 Apr 2016 15:48:46 +0200
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To: Michal Hocko <mhocko@kernel.org>, Andrew Morton <akpm@linux-foundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>, Johannes Weiner <hannes@cmpxchg.org>, Mel Gorman <mgorman@suse.de>, David Rientjes <rientjes@google.com>, Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>, Joonsoo Kim <js1304@gmail.com>, Hillf Danton <hillf.zj@alibaba-inc.com>, linux-mm@kvack.org, LKML <linux-kernel@vger.kernel.org>, Michal Hocko <mhocko@suse.com>

On 04/05/2016 01:25 PM, Michal Hocko wrote:
> From: Michal Hocko <mhocko@suse.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@suse.com>

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

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