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

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Date:	Fri, 12 Jun 2009 19:21:55 +1000
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Pekka J Enberg <penberg@...helsinki.fi>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, npiggin@...e.de,
	akpm@...ux-foundation.org, cl@...ux-foundation.org,
	torvalds@...ux-foundation.org
Subject: Re: [PATCH v2] slab,slub: ignore __GFP_WAIT if we're booting or
 suspending

> We emit a debug warning but dont crash, so all should be fine and 
> the culprits can then be fixed, right?

 ... rewind ... :-)

Ok so, no, the culprit cannot be all fixed in a satifactory way.

The main reason is that I believe it's not "right" to have every caller
of slab around know whether GFP_KERNEL is good to go or it should get
into GFP_NOWAIT. This depends on many factors (among others us moving
things around more), and is not actually a good solution for thing that
can be called both at boot and later, such as get_vm_area().

I really think we are looking for trouble (and a lot of hidden bugs) by
trying to "fix" all callers, in addition to making some code like
vmalloc() more failure prone because it's unconditionally changed from
GFP_KERNEL to GFP_NOWAIT.

It seems a lot more reasonably to me to have sl*b naturally degrade to
NOWAIT when it's too early to enable interrupts.

In addition, my proposal of having bits to mask off gfp will also be
useful in fixing similar issues with suspend/resume vs. GFP_NOIO which
should really become implicit when devices start becoming suspended.

Cheers,
Ben.

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