[151/227] mm/thp: refix __split_huge_pmd_locked() for migration PMD

Commit Message

Andrew Morton March 22, 2022, 9:46 p.m. UTC

From: Hugh Dickins <[e-mail]>
Subject: 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/[e-mail]
Fixes: ec0abae6dcdf ("mm/thp: fix __split_huge_pmd_locked() for migration PMD")
Signed-off-by: Hugh Dickins <[e-mail]>
Reviewed-by: Yang Shi <[e-mail]>
Cc: Ralph Campbell <[e-mail]>
Cc: Zi Yan <[e-mail]>
Cc: "Kirill A. Shutemov" <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
---

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

Patch

--- a/mm/huge_memory.c~mm-thp-refix-__split_huge_pmd_locked-for-migration-pmd
+++ a/mm/huge_memory.c
@@ -2055,9 +2055,9 @@  static void __split_huge_pmd_locked(stru
 		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.