linux-kernel - Re: [PATCH v2] slab,slub: ignore __GFP_WAIT if we're booting or suspending

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Message-ID: <alpine.LFD.2.01.0906120800450.3237@localhost.localdomain>
Date:	Fri, 12 Jun 2009 08:04:49 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Pekka J Enberg <penberg@...helsinki.fi>
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

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.

			Linus
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