Re: [PATCH 3/3] mm: tail page refcounting optimization for slab andhugetlbfs

[Khalid Aziz]

On 11/15/2013 10:47 AM, Andrea Arcangeli wrote:
> This skips the _mapcount mangling for slab and hugetlbfs pages.
>
> The main trouble in doing this is to guarantee that PageSlab and
> PageHeadHuge remains constant for all get_page/put_page run on the
> tail of slab or hugetlbfs compound pages. Otherwise if they're set
> during get_page but not set during put_page, the _mapcount of the tail
> page would underflow.
>
> PageHeadHuge will remain true until the compound page is released and
> enters the buddy allocator so it won't risk to change even if the tail
> page is the last reference left on the page.
>
> PG_slab instead is cleared before the slab frees the head page with
> put_page, so if the tail pin is released after the slab freed the
> page, we would have a problem. But in the slab case the tail pin
> cannot be the last reference left on the page. This is because the
> slab code is free to reuse the compound page after a
> kfree/kmem_cache_free without having to check if there's any tail pin
> left. In turn all tail pins must be always released while the head is
> still pinned by the slab code and so we know PG_slab will be still set
> too.
>
> Signed-off-by: Andrea Arcangeli <aarcange@xxxxxxxxxx>

Reviewed-by: Khalid Aziz <khalid.aziz@xxxxxxxxxx>

--
Khalid
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