[PATCH mmotm] mm/thp: refix __split_huge_pmd_locked() for migration PMD

[Hugh Dickins <hughd@google.com>]

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
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).

Fixes: ec0abae6dcdf ("mm/thp: fix __split_huge_pmd_locked() for migration PMD")
Signed-off-by: Hugh Dickins <hughd@google.com>
---
That's an unfair "Fixes": it did not introduce the problem, but it
missed this aspect of the problem; and will be a good guide to where this
refix should go if stable backports are asked for.

 mm/huge_memory.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2039,9 +2039,9 @@ 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.