mm/khugepaged: collapse_shmem() do not crash on Compound

collapse_shmem()'s VM_BUG_ON_PAGE(PageTransCompound) was unsafe: before
it holds page lock of the first page, racing truncation then extension
might conceivably have inserted a hugepage there already.  Fail with the
SCAN_PAGE_COMPOUND result, instead of crashing (CONFIG_DEBUG_VM=y) or
otherwise mishandling the unexpected hugepage - though later we might
code up a more constructive way of handling it, with SCAN_SUCCESS.

Link: http://lkml.kernel.org/r/alpine.LSU.2.11.1811261529310.2275@eggly.anvils
Fixes: f3f0e1d2150b2 ("khugepaged: add support of collapse for tmpfs/shmem pages")
Signed-off-by: Hugh Dickins <hughd@google.com>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Jerome Glisse <jglisse@redhat.com>
Cc: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: <stable@vger.kernel.org>	[4.8+]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 55930cbed3fd5e6f5ddc885b362ef4b0ac0a925f..2c5fe4f7a0c6eb5bec5b0f42a1c13f6bef40cb07 100644 (file)
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1399,7 +1399,15 @@ static void collapse_shmem(struct mm_struct *mm,
                 */
                VM_BUG_ON_PAGE(!PageLocked(page), page);
                VM_BUG_ON_PAGE(!PageUptodate(page), page);
-               VM_BUG_ON_PAGE(PageTransCompound(page), page);
+
+               /*
+                * If file was truncated then extended, or hole-punched, before
+                * we locked the first page, then a THP might be there already.
+                */
+               if (PageTransCompound(page)) {
+                       result = SCAN_PAGE_COMPOUND;
+                       goto out_unlock;
+               }
 
                if (page_mapping(page) != mapping) {
                        result = SCAN_TRUNCATED;