[PATCHv7 06/36] mm: handle PTE-mapped tail pages in gerneric fast gup implementaiton

From: Kirill A. Shutemov
Date: Tue Jun 23 2015 - 09:47:27 EST

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>
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 2dd6706ea29f..d975bdc2465f 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,

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;


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