mm: fix swapops.h:131 bug if remap_file_pages raced migration

Add remove_linear_migration_ptes_from_nonlinear(), to fix an interesting
little include/linux/swapops.h:131 BUG_ON(!PageLocked) found by trinity:
indicating that remove_migration_ptes() failed to find one of the
migration entries that was temporarily inserted.

The problem comes from remap_file_pages()'s switch from vma_interval_tree
(good for inserting the migration entry) to i_mmap_nonlinear list (no good
for locating it again); but can only be a problem if the remap_file_pages()
range does not cover the whole of the vma (zap_pte() clears the range).

remove_migration_ptes() needs a file_nonlinear method to go down the
i_mmap_nonlinear list, applying linear location to look for migration
entries in those vmas too, just in case there was this race.

The file_nonlinear method does need rmap_walk_control.arg to do this;
but it never needed vma passed in - vma comes from its own iteration.

Reported-and-tested-by: Dave Jones <davej@redhat.com>
Reported-and-tested-by: Sasha Levin <sasha.levin@oracle.com>
Signed-off-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 3 insertions, 2 deletions

diff --git a/mm/rmap.c b/mm/rmap.c
index d9d4231..8fc049f 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1360,8 +1360,9 @@ static int try_to_unmap_cluster(unsigned long cursor, unsigned int *mapcount,
 }
 
 static int try_to_unmap_nonlinear(struct page *page,
-		struct address_space *mapping, struct vm_area_struct *vma)
+		struct address_space *mapping, void *arg)
 {
+	struct vm_area_struct *vma;
 	int ret = SWAP_AGAIN;
 	unsigned long cursor;
 	unsigned long max_nl_cursor = 0;
@@ -1663,7 +1664,7 @@ static int rmap_walk_file(struct page *page, struct rmap_walk_control *rwc)
 	if (list_empty(&mapping->i_mmap_nonlinear))
 		goto done;
 
-	ret = rwc->file_nonlinear(page, mapping, vma);
+	ret = rwc->file_nonlinear(page, mapping, rwc->arg);
 
 done:
 	mutex_unlock(&mapping->i_mmap_mutex);