[PATCH 4.9 10/28] mm/huge_memory.c: fix data loss when splitting a file pmd

[Greg Kroah-Hartman]

4.9-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Hugh Dickins <hughd@xxxxxxxxxx>

commit e1f1b1572e8db87a56609fd05bef76f98f0e456a upstream.

__split_huge_pmd_locked() must check if the cleared huge pmd was dirty,
and propagate that to PageDirty: otherwise, data may be lost when a huge
tmpfs page is modified then split then reclaimed.

How has this taken so long to be noticed?  Because there was no problem
when the huge page is written by a write system call (shmem_write_end()
calls set_page_dirty()), nor when the page is allocated for a write fault
(fault_dirty_shared_page() calls set_page_dirty()); but when allocated for
a read fault (which MAP_POPULATE simulates), no set_page_dirty().

Link: http://lkml.kernel.org/r/alpine.LSU.2.11.1807111741430.1106@xxxxxxxxxxxx
Fixes: d21b9e57c74c ("thp: handle file pages in split_huge_pmd()")
Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
Reported-by: Ashwin Chaugule <ashwinch@xxxxxxxxxx>
Reviewed-by: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>
Reviewed-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Cc: "Huang, Ying" <ying.huang@xxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>	[4.8+]
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

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

--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1642,6 +1642,8 @@ static void __split_huge_pmd_locked(stru
 		if (vma_is_dax(vma))
 			return;
 		page = pmd_page(_pmd);
+		if (!PageDirty(page) && pmd_dirty(_pmd))
+			set_page_dirty(page);
 		if (!PageReferenced(page) && pmd_young(_pmd))
 			SetPageReferenced(page);
 		page_remove_rmap(page, true);