linux-kernel - Re: [PATCH 15/28] mm, page_alloc: Move might_sleep

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Message-ID: <571F8645.6060503@suse.cz>
Date:	Tue, 26 Apr 2016 17:16:21 +0200
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Mel Gorman <mgorman@...hsingularity.net>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Jesper Dangaard Brouer <brouer@...hat.com>,
	Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 15/28] mm, page_alloc: Move might_sleep_if check to the
 allocator slowpath

On 04/26/2016 04:50 PM, Mel Gorman wrote:
> On Tue, Apr 26, 2016 at 03:41:22PM +0200, Vlastimil Babka wrote:
>> On 04/15/2016 11:07 AM, Mel Gorman wrote:
>> >There is a debugging check for callers that specify __GFP_DIRECT_RECLAIM
>> >from a context that cannot sleep. Triggering this is almost certainly
>> >a bug but it's also overhead in the fast path.
>>
>> For CONFIG_DEBUG_ATOMIC_SLEEP, enabling is asking for the overhead. But for
>> CONFIG_PREEMPT_VOLUNTARY which turns it into _cond_resched(), I guess it's
>> not.
>>
>
> Either way, it struck me as odd. It does depend on the config and it's
> marginal so if there is a problem then I can drop it.

What I tried to say is that it makes sense, but it's perhaps non-obvious :)

>> >Move the check to the slow
>> >path. It'll be harder to trigger as it'll only be checked when watermarks
>> >are depleted but it'll also only be checked in a path that can sleep.
>>
>> Hmm what about zone_reclaim_mode=1, should the check be also duplicated to
>> that part of get_page_from_freelist()?
>>
>
> zone_reclaim has a !gfpflags_allow_blocking() check, does not call
> cond_resched() before that check so it does not fall into an accidental
> sleep path. I'm not seeing why the check is necessary there.

Hmm I thought the primary purpose of this might_sleep_if() is to catch those 
(via the DEBUG_ATOMIC_SLEEP) that do pass __GFP_DIRECT_RECLAIM (which means 
gfpflags_allow_blocking() will be true and zone_reclaim will proceed), but do so 
from the wrong context. Am I getting that wrong?