From: David Stevens <stevensd@chromium.org>
Date: Tue, 18 Apr 2023 08:40:31 +0000 (+0900)
Subject: mm/shmem: Fix race in shmem_undo_range w/THP
X-Git-Url: https://www.infradead.org/git/?a=commitdiff_plain;h=baac0f9f362db62d80ce0017b530436c34a53e8c;p=users%2Fjedix%2Flinux-maple.git

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.

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 448f393d8ab2..347c84a9f2ff 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1022,7 +1022,22 @@ 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);
 		}