mm: vmscan: remove folio_test_private() check in pageout()

mm: vmscan: remove folio_test_private() check in pageout()

Patch series "some cleanups for pageout()", v2.

Since we no longer attempt to write back filesystem folios in pageout(),
and only tmpfs/shmem folios and anonymous swapcache folios can be written
back, we can remove the redundant folio_test_private() related logic to
simplify the logic of pageout(), as tmpfs/shmem and swapcache folios do
not use the PG_private flag.

This patch (of 2):

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.

Link: https://lkml.kernel.org/r/cover.1758166683.git.baolin.wang@linux.alibaba.com
Link: https://lkml.kernel.org/r/9ef0e560dc83650bc538eb5dcd1594e112c1369f.1758166683.git.baolin.wang@linux.alibaba.com
Link: https://git.kernel.org/pub/scm/linux/kernel/git/tglx/history.git
Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Acked-by: David Hildenbrand <david@redhat.com>
Acked-by: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Hugh Dickins <hughd@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Qi Zheng <zhengqi.arch@bytedance.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>