Re: [PATCHv4 12/24] thp: PMD splitting without splitting compound page

From: Aneesh Kumar K.V
Date: Wed Apr 01 2015 - 02:38:58 EST

"Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx> writes:

> Current split_huge_page() combines two operations: splitting PMDs into
> tables of PTEs and splitting underlying compound page. This patch
> changes split_huge_pmd() implementation to split the given PMD without
> splitting other PMDs this page mapped with or underlying compound page.
> In order to do this we have to get rid of tail page refcounting, which
> uses _mapcount of tail pages. Tail page refcounting is needed to be able
> to split THP page at any point: we always know which of tail pages is
> pinned (i.e. by get_user_pages()) and can distribute page count
> correctly.
> We can avoid this by allowing split_huge_page() to fail if the compound
> page is pinned. This patch removes all infrastructure for tail page
> refcounting and make split_huge_page() to always return -EBUSY. All
> split_huge_page() users already know how to handle its fail. Proper
> implementation will be added later.
> Without tail page refcounting, implementation of split_huge_pmd() is
> pretty straight-forward.

With this we now have pte mapping part of a compound page(). Now the
gneric gup implementation does

ptem = ptep = pte_offset_map(&pmd, addr);
do {

if (!page_cache_get_speculative(page))
goto pte_unmap;

That page_cache_get_speculative will fail in our case because it does
if (unlikely(!get_page_unless_zero(page))) on a tail page. ??


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