mm: handle PTE-mapped tail pages in gerneric fast gup implementaiton

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 <[email protected]>
Tested-by: Sasha Levin <[email protected]>
Tested-by: Aneesh Kumar K.V <[email protected]>
Acked-by: Jerome Marchand <[email protected]>
Acked-by: Vlastimil Babka <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: Rik van Riel <[email protected]>
Cc: Naoya Horiguchi <[email protected]>
Cc: Steve Capper <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Christoph Lameter <[email protected]>
Cc: David Rientjes <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

diff --git a/mm/gup.c b/mm/gup.c
index 745a50f2d57d9f06ce9306271729b51fd0f89d31..6880085d37908e9580403d3b66981437d6853c76 100644 (file)
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -1097,7 +1097,7 @@ 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
@@ -1109,15 +1109,17 @@ 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)++;