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Message-ID: <84144f020601270307t7266a4ccs5071d4b288a9257f@mail.gmail.com>
Date: Fri, 27 Jan 2006 13:07:54 +0200
From: Pekka Enberg <penberg@cs.helsinki.fi>
To: Paul Jackson <pj@sgi.com>
Subject: Re: [patch 0/9] Critical Mempools
Cc: colpatch@us.ibm.com, bcrl@kvack.org, clameter@engr.sgi.com,
       linux-kernel@vger.kernel.org, sri@us.ibm.com, andrea@suse.de,
       pavel@suse.cz, linux-mm@kvack.org
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Hi,

Pekka wrote:
> > As as side note, we already have __GFP_NOFAIL. How is it different
> > from GFP_CRITICAL and why aren't we improving that?

On 1/27/06, Paul Jackson <pj@sgi.com> wrote:
> Don't these two flags invoke two different mechanisms.
>   __GFP_NOFAIL can sleep for HZ/50 then retry, rather than return failure.
>   __GFP_CRITICAL can steal from the emergency pool rather than fail.
>
> I would favor renaming at least the __GFP_CRITICAL to something
> like __GFP_EMERGPOOL, to highlight the relevant distinction.

Yeah you're right. __GFP_NOFAIL guarantees to never fail but it
doesn't guarantee to actually succeed either. I think the suggested
semantics for __GFP_EMERGPOOL are that while it can fail, it tries to
avoid that by dipping into page reserves. However, I do still think
it's a bad idea to allow the slab allocator to steal whole pages for
critical allocations because in low-memory condition, it should be
fairly easy to exhaust the reserves and waste most of that memory at
the same time.

                            Pekka
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