Re: [PATCHv5 06/28] mm: handle PTE-mapped tail pages in gerneric fast gup implementaiton

[Vlastimil Babka]

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: Vlastimil Babka <vbabka@xxxxxxx>

> ---
>   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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