mm/thp: refix __split_huge_pmd_locked() for migration PMD

Migration entries do not contribute to a page's reference count: move
__split_huge_pmd_locked()'s page_ref_add() into pmd_migration's else
block (along with the page_count() check - a page is quite likely to
have reference count frozen to 0 when a migration entry is found).

This will fix a very rare anonymous memory leak, after a
split_huge_pmd() raced with an anon split_huge_page() or an anon THP
migrate_pages(): since the wrongly raised refcount stopped the page
(perhaps small, perhaps huge, depending on when the race hit) from ever
being freed.

At first I thought there were worse risks, from prematurely unfreezing a
frozen page: but now think that would only affect page cache pages,
which do not come this way (except for anonymous pages in swap cache,
perhaps).

Link: https://lkml.kernel.org/r/84792468-f512-e48f-378c-e34c3641e97@google.com
Fixes: ec0abae6dcdf ("mm/thp: fix __split_huge_pmd_locked() for migration PMD")
Signed-off-by: Hugh Dickins <hughd@google.com>
Reviewed-by: Yang Shi <shy828301@gmail.com>
Cc: Ralph Campbell <rcampbell@nvidia.com>
Cc: Zi Yan <ziy@nvidia.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 50a9a2946e58ba7aceaa7db7d8a98c0963161f72..9c52b3661f7188edc9456c1f1d1727d76535ae3d 100644 (file)
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2055,9 +2055,9 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
                young = pmd_young(old_pmd);
                soft_dirty = pmd_soft_dirty(old_pmd);
                uffd_wp = pmd_uffd_wp(old_pmd);
+               VM_BUG_ON_PAGE(!page_count(page), page);
+               page_ref_add(page, HPAGE_PMD_NR - 1);
        }
-       VM_BUG_ON_PAGE(!page_count(page), page);
-       page_ref_add(page, HPAGE_PMD_NR - 1);
 
        /*
         * Withdraw the table only after we mark the pmd entry invalid.