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@xxxxxxxxxxxxxxx>
> Tested-by: Sasha Levin <sasha.levin@xxxxxxxxxx>
Acked-by: Jerome Marchand <jmarchan@xxxxxxxxxx>
> ---
> 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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