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 12:07:56 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Pekka Enberg <penberg@...helsinki.fi>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org, npiggin@...e.de,
	benh@...nel.crashing.org, 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


* Pekka Enberg <penberg@...helsinki.fi> wrote:

> On Fri, Jun 12, 2009 at 12:10 PM, Ingo Molnar<mingo@...e.hu> wrote:
> >> @@ -1548,6 +1548,20 @@ new_slab:
> >>               goto load_freelist;
> >>       }
> >>
> >> +     /*
> >> +      * Lets not wait if we're booting up or suspending even if the user
> >> +      * asks for it.
> >> +      */
> >> +     if (system_state != SYSTEM_RUNNING)
> >> +             gfpflags &= ~__GFP_WAIT;
> >
> > Hiding that bug like that is not particularly clean IMO. We should
> > not let system_state hacks spread like that.
> >
> > We emit a debug warning but dont crash, so all should be fine and
> > the culprits can then be fixed, right?
> 
> OK, lets not use system_state then and go with Ben's approach 
> then. Again, neither of the patches are about "hiding buggy 
> callers" but changing allocation policy wrt. gfp flags during boot 
> (and later on during suspend).

IMHO such invisible side-channels modifying the semantics of GFP 
flags is a bit dubious.

We could do GFP_INIT or GFP_BOOT. These can imply other useful 
modifiers as well: panic-on-failure for example. (this would clean 
up a fair amount of init code that currently checks for an panics on 
allocation failure.)

	Ingo
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