mm/shmem: fix race in shmem_undo_range w/THP

commit 55ac8bbe358bdd2f3c044c12f249fd22d48fe015
Author: David Stevens <stevensd@chromium.org>
Date:   Tue Apr 18 17:40:31 2023 +0900

    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>

JIRA: https://issues.redhat.com/browse/RHEL-5619
Signed-off-by: Nico Pache <npache@redhat.com>

mm/shmem.c

@@ -1030,7 +1030,24 @@ 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);
 		}