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Message-ID: <4A326ECB.3070607@cs.helsinki.fi>
Date: Fri, 12 Jun 2009 18:05:47 +0300
From: Pekka Enberg <penberg@...helsinki.fi>
To: Linus Torvalds <torvalds@...ux-foundation.org>
CC: linux-mm@...ck.org, linux-kernel@...r.kernel.org, mingo@...e.hu,
npiggin@...e.de, benh@...nel.crashing.org,
akpm@...ux-foundation.org, cl@...ux-foundation.org
Subject: Re: [PATCH v2] slab,slub: ignore __GFP_WAIT if we're booting or suspending
Hi Linus,
Linus Torvalds wrote:
>
> On Fri, 12 Jun 2009, Pekka J Enberg wrote:
>>
>> + if (system_state != SYSTEM_RUNNING)
>> + local_flags &= ~__GFP_WAIT;
>> +
>> + might_sleep_if(local_flags & __GFP_WAIT);
>
> This is pointless.
>
> You're doing the "might_sleep_if()" way too late. At that point, you've
> already lost 99% of all coverage, since now none of the cases of just
> finding a free slab entry on the list will ever trigger that
> "might_sleep()" case.
>
> So you need to do this _early_, at the entry-point, not late, at cache
> re-fill time.
>
> So rather than removing the might_sleep_if() at the early point, and then
> moving it to this late stage (because you only do the local_flags fixups
> late), you need to move the local-flags fixup early instead, and do the
> might_sleep_it() there.
>
> The whole point of "might_sleep()" is that it triggers every time if
> something is called in the wrong context - not just for the cases where it
> actually _does_ sleep.
OK, makes sense. So what do you think of this patch then:
http://patchwork.kernel.org/patch/29733/
It's what Ben has been proposing all along in a slightly edited form.
Pekka
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