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    SubjectRe: [PATCHv5 06/28] mm: handle PTE-mapped tail pages in gerneric fast gup implementaiton
    On 04/23/2015 11:03 PM, Kirill A. Shutemov wrote:
    > With new refcounting we are going to see THP tail pages mapped with PTE.
    > Generic fast GUP rely on page_cache_get_speculative() to obtain
    > reference on page. page_cache_get_speculative() always fails on tail
    > pages, because ->_count on tail pages is always zero.
    >
    > Let's handle tail pages in gup_pte_range().
    >
    > New split_huge_page() will rely on migration entries to freeze page's
    > counts. Recheck PTE value after page_cache_get_speculative() on head
    > page should be enough to serialize against split.
    >
    > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
    > Tested-by: Sasha Levin <sasha.levin@oracle.com>

    Acked-by: Vlastimil Babka <vbabka@suse.cz>

    > ---
    > mm/gup.c | 8 +++++---
    > 1 file changed, 5 insertions(+), 3 deletions(-)
    >
    > diff --git a/mm/gup.c b/mm/gup.c
    > index ebdb39b3e820..eaeeae15006b 100644
    > --- a/mm/gup.c
    > +++ b/mm/gup.c
    > @@ -1051,7 +1051,7 @@ static int gup_pte_range(pmd_t pmd, unsigned long addr, unsigned long end,
    > * for an example see gup_get_pte in arch/x86/mm/gup.c
    > */
    > pte_t pte = READ_ONCE(*ptep);
    > - struct page *page;
    > + struct page *head, *page;
    >
    > /*
    > * Similar to the PMD case below, NUMA hinting must take slow
    > @@ -1063,15 +1063,17 @@ static int gup_pte_range(pmd_t pmd, unsigned long addr, unsigned long end,
    >
    > VM_BUG_ON(!pfn_valid(pte_pfn(pte)));
    > page = pte_page(pte);
    > + head = compound_head(page);
    >
    > - if (!page_cache_get_speculative(page))
    > + if (!page_cache_get_speculative(head))
    > goto pte_unmap;
    >
    > if (unlikely(pte_val(pte) != pte_val(*ptep))) {
    > - put_page(page);
    > + put_page(head);
    > goto pte_unmap;
    > }
    >
    > + VM_BUG_ON_PAGE(compound_head(page) != head, page);
    > pages[*nr] = page;
    > (*nr)++;
    >
    >



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