mm/huge_memory.c: fix data loss when splitting a file pmd

From: Hugh Dickins <[email protected]>
Subject: mm/huge_memory.c: fix data loss when splitting a file pmd

__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().

---

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

diff -puN mm/huge_memory.c~thp-fix-data-loss-when-splitting-a-file-pmd mm/huge_memory.c
@@ -2084,6 +2084,8 @@ 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);

_