Re: [PATCH 0/5] make slab gfp fair

Peter Zijlstra
2007-05-14T16:10:29Z

On Mon, 2007-05-14 at 08:53 -0700, Christoph Lameter wrote:
> On Mon, 14 May 2007, Peter Zijlstra wrote:
>
> > In the interest of creating a reserve based allocator; we need to make the slab
> > allocator (*sigh*, all three) fair with respect to GFP flags.
>
> I am not sure what the point of all of this is.
>
> > That is, we need to protect memory from being used by easier gfp flags than it
> > was allocated with. If our reserve is placed below GFP_ATOMIC, we do not want a
> > GFP_KERNEL allocation to walk away with it - a scenario that is perfectly
> > possible with the current allocators.
>
> Why does this have to handled by the slab allocators at all? If you have
> free pages in the page allocator then the slab allocators will be able to
> use that reserve.

Yes, too freely. GFP flags are only ever checked when you allocate a new
page. Hence, if you have a low reaching alloc allocating a slab page;
subsequent non critical GFP_KERNEL allocs can fill up that slab. Hence
you would need to reserve a slab per object instead of the normal
packing.