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SubjectRe: [RFC PATCH 2/4] mm: make consistent use of PG_slab flag
On Fri, 6 Jul 2012, Jiang Liu wrote:

> On 07/05/2012 10:47 PM, Christoph Lameter wrote:
> > On Tue, 3 Jul 2012, Jiang Liu wrote:
> >
> >> PG_slabobject: mark whether a (compound) page hosts SLUB/SLOB objects.
> >
> > Any subsystem may allocate a compound page to store metadata.
> >
> > The compound pages used by SLOB and SLUB are not managed in any way but
> > the calls to kfree and kmalloc are converted to calls to the page
> > allocator. There is no "management" by the slab allocators for these
> > cases and its inaccurate to say that these are SLUB/SLOB objects since the
> > allocators never deal with these objects.
> >
> Hi Chris,
> I think there's a little difference with SLUB and SLOB for compound page.
> For SLOB, it relies on the page allocator to allocate compound page to fulfill
> request bigger than one page. For SLUB, it relies on the page allocator if the
> request is bigger than two pages. So SLUB may allocate a 2-pages compound page
> to host SLUB managed objects.
> My proposal may be summarized as below:
> 1) PG_slab flag marks a memory object is allocated from slab allocator.
> 2) PG_slabobject marks a (compound) page hosts SLUB/SLOB managed objects.
> 3) Only set PG_slab/PG_slabobject on the head page of compound pages.
> 4) For SLAB, PG_slabobject is redundant and so not used.
> A summary of proposed usage of PG_slab(S) and PG_slabobject(O) with
> SLAB/SLUB/SLOB allocators as below:
> pagesize SLAB SLUB SLOB
> 1page S S,O S,O
> 2page S S,O S
> >=4page S S S

There is no point of recognizing such objects because those will be
kmalloc objects and they can only be freed in a subsystem specific way.
There is no standard way to even figure out which subsystem allocated
them. So for all practical purposes those are unrecoverable.

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