On Fri, 6 Apr 2007 10:01:38 -0700 (PDT)
Christoph Lameter <[email protected]> wrote:
> On Thu, 5 Apr 2007, Andrew Morton wrote:
>
> > > We add PageTail as an alias of PageReclaim. Compound pages cannot
> > > currently be reclaimed. Because of the alias one needs to check
> > > PageCompound first.
> >
> > So slub is using compound pages so that it can locate the head page in
> > higher-order pages, whereas slab uses per-object (or per-order-0-page?)
> > metadata for that?
>
> Both SLAB and SLUB use compound pages.
Not really. slab sets the page->lru.prev of each constituent page to point
at the controlling slab.
I assume the PageCompound() handling in (for example) page_get_cache() is
for the NOMMU special case.
> >
> > I see four instances of
> >
> > + page = virt_to_page(p);
> > +
> > + if (unlikely(PageCompound(page)))
> > + page = page->first_page;
> >
> > A new virt_to_head_page() is needed.
>
> Ok.
>
> > Sigh. We're seeing rather a lot of churn to accommodate slub. Do we
> > actually have any justification for all this? If we end up deciding to
> > merge slub and to deprecate then remove slab, what would our reasons have
> > been?
>
> This is not SLUB specific. SLAB does the same. SLAB does special casing
> for page size slabs and does not allow slab debugging because it would
> use page->private if debugging would be enabled. Which would get into
> trouble on i386.
>
> If you want less SLUB churn on other fronts then I can add the special
> casing for PAGE_SIZE slabs back into SLUB. Then we keep all the
> inconsistencies in SLAB use in the code.
>
> I'd rather clean up the stuff and then also remove the special casing from
> SLAB.
>
Will slub handle NOMMU anonymous pages appropriately?
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