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 55930cb..2c5fe4f 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1399,7 +1399,15 @@
*/
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;
