| From | "Aneesh Kumar K.V" <> | Subject | Re: [PATCHv4 12/24] thp: PMD splitting without splitting compound page | Date | Wed, 01 Apr 2015 12:08:35 +0530 |
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"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> 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
gup_pte_range() 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. ??
-aneesh
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