From: Hugh Dickins hughd@google.com
commit 9d84604b845c3888d1bede43d16ab3ebedb13e24 upstream.
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 Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- mm/huge_memory.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/mm/huge_memory.c b/mm/huge_memory.c index 406a3c28c026..468fca576bc2 100644 --- 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.