On Wed, 2008-02-27 at 09:14 +0100, Peter Zijlstra wrote:

On Wed, 2008-02-27 at 10:05 +0200, Pekka Enberg wrote:

quoted

Hi Peter,

On Wed, Feb 27, 2008 at 9:58 AM, Peter Zijlstra [off-list ref] wrote:

quoted

 > 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.

Humm, and here I sit staring at the screen. Perhaps I should go get my
morning juice, but...

  if (mem_reserve_kmalloc_charge(my_res, sizeof(*foo), 0)) {
    foo = kmalloc(sizeof(*foo), gfp|__GFP_MEMALLOC)
    if (!kmem_is_emergency(foo))
      mem_reserve_kmalloc_charge(my_res, -sizeof(*foo), 0)
  } else
    foo = kmalloc(sizeof(*foo), gfp);

Just doesn't look too pretty..

And needing to always account the allocation seems wrong.. but I'll take
poison and see if that wakes up my mind.