[PATCH] mm: memcontrol: convert page cache to a new mem_cgroup_charge() API fix

[Johannes Weiner]

Incorporate Hugh's feedback:

- shmem_getpage_gfp() needs to handle the new -ENOENT that was
  previously implied in the -EEXIST when a swap entry changed under us
  in any way. Otherwise hole punching could cause a racing fault to
  SIGBUS instead of allocating a new page.

- It is indeed page reclaim that picks up any swapcache we leave
  stranded when free_swap_and_cache() runs on a page locked by
  somebody else. Document that our delete_from_swap_cache() is an
  optimization, not something we rely on for correctness.

- Style cleanup: testing `expected' to decide on -EEXIST vs -ENOENT
  differentiates the callsites, but is a bit awkward to read. Test
  `entry' instead.

Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>
---
 mm/shmem.c | 15 +++++++++------
 1 file changed, 9 insertions(+), 6 deletions(-)

diff --git a/mm/shmem.c b/mm/shmem.c
index afd5a057ebb7..00fb001e8f3e 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -638,7 +638,7 @@ static int shmem_add_to_page_cache(struct page *page,
 		xas_lock_irq(&xas);
 		entry = xas_find_conflict(&xas);
 		if (entry != expected)
-			xas_set_err(&xas, expected ? -ENOENT : -EEXIST);
+			xas_set_err(&xas, entry ? -EEXIST : -ENOENT);
 		xas_create_range(&xas);
 		if (xas_error(&xas))
 			goto unlock;
@@ -1686,10 +1686,13 @@ static int shmem_swapin_page(struct inode *inode, pgoff_t index,
 		 * We already confirmed swap under page lock, but
 		 * free_swap_and_cache() only trylocks a page, so it
 		 * is just possible that the entry has been truncated
-		 * or holepunched since swap was confirmed.
-		 * shmem_undo_range() will have done some of the
-		 * unaccounting, now delete_from_swap_cache() will do
-		 * the rest.
+		 * or holepunched since swap was confirmed. This could
+		 * occur at any time while the page is locked, and
+		 * usually page reclaim will take care of the stranded
+		 * swapcache page. But when we catch it, we may as
+		 * well clean up after ourselves: shmem_undo_range()
+		 * will have done some of the unaccounting, now
+		 * delete_from_swap_cache() will do the rest.
 		 */
 		if (error == -ENOENT)
 			delete_from_swap_cache(page);
@@ -1765,7 +1768,7 @@ static int shmem_getpage_gfp(struct inode *inode, pgoff_t index,
 	if (xa_is_value(page)) {
 		error = shmem_swapin_page(inode, index, &page,
 					  sgp, gfp, vma, fault_type);
-		if (error == -EEXIST)
+		if (error == -EEXIST || error == -ENOENT)
 			goto repeat;
 
 		*pagep = page;
-- 
2.26.2