Re: [PATCH 00/28] Swap over NFS -v16

Peter Zijlstra
2008-02-27T08:14:45Z

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.