[PATCH v2 1/2] mm: vmscan: remove folio_test_private() check in pageout()

[Baolin Wang <baolin.wang@linux.alibaba.com>]

The folio_test_private() check in pageout() was introduced by commit
ce91b575332b ("orphaned pagecache memleak fix") in 2005 (checked from
a history tree[1]). As the commit message mentioned, it was to address
the issue where reiserfs pagecache may be truncated while still pinned.
To further explain, the truncation removes the page->mapping, but the
page is still listed in the VM queues because it still has buffers.

In 2008, commit a2b345642f530 ("Fix dirty page accounting leak with ext3
data=journal") seems to be dealing with a similar issue, where the page
becomes dirty after truncation, and it provides a very useful call stack:
truncate_complete_page()
      cancel_dirty_page() // PG_dirty cleared, decr. dirty pages
      do_invalidatepage()
        ext3_invalidatepage()
          journal_invalidatepage()
            journal_unmap_buffer()
              __dispose_buffer()
                __journal_unfile_buffer()
                  __journal_temp_unlink_buffer()
                    mark_buffer_dirty(); // PG_dirty set, incr. dirty pages

In this commit a2b345642f530, we forcefully clear the page's dirty flag
during truncation (in truncate_complete_page()).

Now it seems this was just a peculiar usage specific to reiserfs. Maybe
reiserfs had some extra refcount on these pages, which caused them to pass
the is_page_cache_freeable() check. With the fix provided by commit a2b345642f530
and reiserfs being removed in 2024 by commit fb6f20ecb121 ("reiserfs: The
last commit"), such a case is unlikely to occur again. So let's remove the
redundant folio_test_private() checks and related buffer_head release logic,
and just leave a warning here to catch such a bug.

[1] https://git.kernel.org/pub/scm/linux/kernel/git/tglx/history.git
Acked-by: David Hildenbrand <david@redhat.com>
Acked-by: Shakeel Butt <shakeel.butt@linux.dev>
Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
---
 mm/vmscan.c | 12 +++---------
 1 file changed, 3 insertions(+), 9 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index f1fc36729ddd..930add6d90ab 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -701,16 +701,10 @@ static pageout_t pageout(struct folio *folio, struct address_space *mapping,
 		return PAGE_KEEP;
 	if (!mapping) {
 		/*
-		 * Some data journaling orphaned folios can have
-		 * folio->mapping == NULL while being dirty with clean buffers.
+		 * Is it still possible to have a dirty folio with
+		 * a NULL mapping? I think not.
 		 */
-		if (folio_test_private(folio)) {
-			if (try_to_free_buffers(folio)) {
-				folio_clear_dirty(folio);
-				pr_info("%s: orphaned folio\n", __func__);
-				return PAGE_CLEAN;
-			}
-		}
+		VM_WARN_ON_FOLIO(true, folio);
 		return PAGE_KEEP;
 	}
 
-- 
2.43.7