mm/shmem: fix race in shmem_undo_range w/THP

Split folios during the second loop of shmem_undo_range.  It's not
sufficient to only split folios when dealing with partial pages, since
it's possible for a THP to be faulted in after that point.  Calling
truncate_inode_folio in that situation can result in throwing away data
outside of the range being targeted.

[akpm@linux-foundation.org: tidy up comment layout]
Link: https://lkml.kernel.org/r/20230418084031.3439795-1-stevensd@google.com
Fixes: b9a8a4195c7d ("truncate,shmem: Handle truncates that split large folios")
Signed-off-by: David Stevens <stevensd@chromium.org>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Suleiman Souhlal <suleiman@google.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/shmem.c b/mm/shmem.c
index 91e2620148b2f6d789420e6736daef7a53e2cc5c..0d1ce70bce38028970802375b2409e9e6400364f 100644 (file)
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1080,7 +1080,24 @@ whole_folios:
                                }
                                VM_BUG_ON_FOLIO(folio_test_writeback(folio),
                                                folio);
-                               truncate_inode_folio(mapping, folio);
+
+                               if (!folio_test_large(folio)) {
+                                       truncate_inode_folio(mapping, folio);
+                               } else if (truncate_inode_partial_folio(folio, lstart, lend)) {
+                                       /*
+                                        * If we split a page, reset the loop so
+                                        * that we pick up the new sub pages.
+                                        * Otherwise the THP was entirely
+                                        * dropped or the target range was
+                                        * zeroed, so just continue the loop as
+                                        * is.
+                                        */
+                                       if (!folio_test_large(folio)) {
+                                               folio_unlock(folio);
+                                               index = start;
+                                               break;
+                                       }
+                               }
                        }
                        folio_unlock(folio);
                }