mm: filemap: retain mapped dropbehind folios

Fault-around can map ready dropbehind folios without going through the
normal page-cache lookup that clears dropbehind.  A mapping represents a
competing cached user, so retain the folio instead of forcibly unmapping
it when writeback completes.

For a mapped folio, folio_unmap_invalidate() can call
unmap_mapping_folio(), which takes i_mmap_rwsem and may sleep.  Retaining
mapped folios avoids this path when folio_end_dropbehind() runs in
non-preemptible task context.

Tal was able to trigger a sleeping-in-atomic warning due to this [1].

Unmapped dropbehind folios continue through the existing invalidation path.

Link: https://lore.kernel.org/4aba05e1a2c3b61cb337d373eb9b7a8db4ddd822.1788024049.git.qiwenjie@xiaomi.com
Link: https://lore.kernel.org/076bb01b-6fcf-4691-be8c-0e8507c9fe64@columbia.edu [1]
Fixes: fb7d3bc414 ("mm/filemap: drop streaming/uncached pages when writeback completes")
Signed-off-by: Wenjie Qi <qiwenjie@xiaomi.com>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Tal Zussman <tz2294@columbia.edu>
Tested-by: Tal Zussman <tz2294@columbia.edu>
Cc: Barry Song <baohua@kernel.org>
Cc: Jan Kara <jack@suse.cz>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Trond Myklebust <trond.myklebust@hammerspace.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/filemap.c

@@ -1616,7 +1616,7 @@ static void filemap_end_dropbehind(struct folio *folio)
 		return;
 	if (!folio_test_clear_dropbehind(folio))
 		return;
-	if (mapping)
+	if (mapping && !folio_mapped(folio))
 		folio_unmap_invalidate(mapping, folio, 0);
 }