Re: [PATCH] mm/page-writeback: drop usage of folio_index

[Kairui Song <ryncsn@gmail.com>]

On Fri, Aug 15, 2025 at 9:48 PM Matthew Wilcox <willy@infradead.org> wrote:
>
> On Fri, Aug 15, 2025 at 08:12:52PM +0800, Kairui Song wrote:
> > +++ b/mm/page-writeback.c
> > @@ -2739,8 +2739,8 @@ void __folio_mark_dirty(struct folio *folio, struct address_space *mapping,
> >       if (folio->mapping) {   /* Race with truncate? */
> >               WARN_ON_ONCE(warn && !folio_test_uptodate(folio));
> >               folio_account_dirtied(folio, mapping);
> > -             __xa_set_mark(&mapping->i_pages, folio_index(folio),
> > -                             PAGECACHE_TAG_DIRTY);
> > +             __xa_set_mark(&mapping->i_pages, folio->index,
> > +                           PAGECACHE_TAG_DIRTY);
> >       }
> >       xa_unlock_irqrestore(&mapping->i_pages, flags);
> >  }
>
> What about a shmem folio that's been moved to the swap cache?  I used
> folio_index() here because I couldn't prove to my satisfaction that this
> couldn't happen.

I just checked all callers of __folio_mark_dirty:

- block_dirty_folio
    __folio_mark_dirty

- filemap_dirty_folio
    __folio_mark_dirty

For these two, all their caller are from other fs not related to
shmem/swap cache)

- mark_buffer_dirty
    __folio_mark_dirty (mapping is folio->mapping)

- folio_redirty_for_writepage
    filemap_dirty_folio
      __folio_mark_dirty (mapping is folio->mapping)

For these two, __folio_mark_dirty is called with folio->mapping, and
swap cache space is never set to folio->mapping. If the folio is a
swap cache here, folio_index returns its swap cache index, which is
not equal to its index in shmem or any other map, things will go very
wrong.

And, currently both shmem / swap cache uses noop_dirty_folio, so they
should never call into the helper here.

I think I can add below sanity check here, just to clarify things and
for debugging:

/*
 * Shmem writeback relies on swap, and swap writeback
 * is LRU based, not using the dirty mark.
 */
VM_WARN_ON(shmem_mapping(mapping) || folio_test_swapcache(folio))

And maybe we can also have a VM_WARN_ON for `folio->mapping != mapping` here?