+ mm-shmem-fix-race-in-shmem_undo_range-w-thp.patch added to mm-hotfixes-unstable branch

+ mm-shmem-fix-race-in-shmem_undo_range-w-thp.patch added to mm-hotfixes-unstable branch

[Andrew Morton]

The patch titled
     Subject: mm/shmem: Fix race in shmem_undo_range w/THP
has been added to the -mm mm-hotfixes-unstable branch.  Its filename is
     mm-shmem-fix-race-in-shmem_undo_range-w-thp.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-shmem-fix-race-in-shmem_undo_range-w-thp.patch

This patch will later appear in the mm-hotfixes-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: David Stevens <stevensd@chromium.org>
Subject: mm/shmem: Fix race in shmem_undo_range w/THP
Date: Tue, 18 Apr 2023 17:40:31 +0900

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>
---

 mm/shmem.c |   17 ++++++++++++++++-
 1 file changed, 16 insertions(+), 1 deletion(-)

--- a/mm/shmem.c~mm-shmem-fix-race-in-shmem_undo_range-w-thp
+++ a/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);
 		}
_

Patches currently in -mm which might be from stevensd@chromium.org are

mm-shmem-fix-race-in-shmem_undo_range-w-thp.patch
mm-khugepaged-drain-lru-after-swapping-in-shmem.patch
mm-khugepaged-refactor-collapse_file-control-flow.patch
mm-khugepaged-skip-shmem-with-userfaultfd.patch
mm-khugepaged-maintain-page-cache-uptodate-flag.patch