filemap: do not use folio_contains for swap cache folios

filemap: do not use folio_contains for swap cache folios

Currently, none of the folio_contains callers should encounter swap cache
folios.

For fs/ callers, swap cache folios are never part of their workflow.

For filemap and truncate, folio_contains is only used for sanity checks to
verify the folio index matches the expected lookup / invalidation target.

The swap cache does not utilize filemap or truncate helpers in ways that
would trigger these checks, as it mostly implements its own cache
management.

Shmem won't trigger these sanity checks either unless thing went wrong, as
it would directly trigger a BUG because swap cache index are unrelated and
almost never matches shmem index.  Shmem have to handle mixed values of
folios, shadows, and swap entries, so it has its own way of handling the
mapping.

While some filemap helpers works for swap cache space, the swap cache is
different from the page cache in many ways.  So this particular helper
will unlikely to work in a helpful way for swap cache folios.

So make it explicit here that folio_contains should not be used for swap
cache folios.  This helps to avoid misuse, make swap cache less exposed
and remove the folio_index usage here.

[akpm@linux-foundation.org: s/VM_WARN_ON_FOLIO/VM_WARN_ON_ONCE_FOLIO/, per Kairui]
Link: https://lkml.kernel.org/r/20250430181052.55698-5-ryncsn@gmail.com
Signed-off-by: Kairui Song <kasong@tencent.com>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Chao Yu <chao@kernel.org>
Cc: Chris Li <chrisl@kernel.org>
Cc: Chris Mason <clm@fb.com>
Cc: Christian Brauner <brauner@kernel.org>
Cc: David Sterba <dsterba@suse.com>
Cc: "Huang, Ying" <ying.huang@linux.alibaba.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Jaegeuk Kim <jaegeuk@kernel.org>
Cc: Joanne Koong <joannelkoong@gmail.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Josef Bacik <josef@toxicpanda.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Miklos Szeredi <miklos@szeredi.hu>
Cc: Nhat Pham <nphamcs@gmail.com>
Cc: Qu Wenruo <wqu@suse.com>
Cc: Yosry Ahmed <yosryahmed@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>