Subject: Re: [PATCH 00/28] Swap over NFS -v16
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: Pekka Enberg <penberg@cs.helsinki.fi>
Cc: Neil Brown <neilb@suse.de>, Andrew Morton <akpm@linux-foundation.org>,
       Linus Torvalds <torvalds@linux-foundation.org>,
       linux-kernel@vger.kernel.org, linux-mm@kvack.org,
       netdev@vger.kernel.org, trond.myklebust@fys.uio.no
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Date: Wed, 27 Feb 2008 09:14:19 +0100
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On Wed, 2008-02-27 at 10:05 +0200, Pekka Enberg wrote:
> Hi Peter,
> 
> On Wed, Feb 27, 2008 at 9:58 AM, Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:
> >  > 1/ I note there is no way to tell if memory returned by kmalloc is
> >  >   from the emergency reserve - which contrasts with alloc_page
> >  >   which does make that information available through page->reserve.
> >  >   This seems a slightly unfortunate aspect of the interface.
> >
> >  Yes, but alas there is no room to store such information in kmalloc().
> >  That is, in a sane way. I think it was Daniel Phillips who suggested
> >  encoding it in the return pointer by flipping the low bit - but that is
> >  just too ugly and breaks all current kmalloc sites to boot.
> 
> Why can't you add a kmem_is_emergency() to SLUB that looks up the
> cache/slab/page (whatever is the smallest unit of the emergency pool
> here) for the object and use that?

There is an idea.. :-) It would mean preserving page->reserved, but SLUB
has plenty of page flags to pick from. Or maybe I should move the thing
to a page flag anyway. If we do that SLAB would allow something similar,
just look up the page for whatever address you get and look at PG_emerg
or something.

Having this would clean things up. I'll go work on this.

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