mm: fix filemap_get_folios_contig returning batches of identical folios

filemap_get_folios_contig() is supposed to return distinct folios found
within [start, end].  Large folios in the Xarray become multi-index
entries.  xas_next() can iterate through the sub-indexes before finding a
sibling entry and breaking out of the loop.

This can result in a returned folio_batch containing an indeterminate
number of duplicate folios, which forces the callers to skeptically handle
the returned batch.  This is inefficient and incurs a large maintenance
overhead.

We can fix this by calling xas_advance() after we have successfully adding
a folio to the batch to ensure our Xarray is positioned such that it will
correctly find the next folio - similar to filemap_get_read_batch().

Link: https://lkml.kernel.org/r/Z-8s1-kiIDkzgRbc@fedora
Fixes: 35b471467f88 ("filemap: add filemap_get_folios_contig()")
Signed-off-by: Vishal Moola (Oracle) <vishal.moola@gmail.com>
Reported-by: Qu Wenruo <quwenruo.btrfs@gmx.com>
Closes: https://lkml.kernel.org/r/b714e4de-2583-4035-b829-72cfb5eb6fc6@gmx.com
Tested-by: Qu Wenruo <quwenruo.btrfs@gmx.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Vivek Kasireddy <vivek.kasireddy@intel.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/filemap.c b/mm/filemap.c
index b5e784f34d980b64242d6e45a8c98a18e7be00ca..7b90cbeb4a1adfe44da53ea76a60ccc2f0e9609d 100644 (file)
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2244,6 +2244,7 @@ unsigned filemap_get_folios_contig(struct address_space *mapping,
                        *start = folio->index + nr;
                        goto out;
                }
+               xas_advance(&xas, folio_next_index(folio) - 1);
                continue;
 put_folio:
                folio_put(folio);