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: 35b4714 ("filemap: add filemap_get_folios_contig()")
Signed-off-by: Vishal Moola (Oracle) <[email protected]>
Reported-by: Qu Wenruo <[email protected]>
Closes: https://lkml.kernel.org/r/[email protected]
Tested-by: Qu Wenruo <[email protected]>
Cc: Matthew Wilcox (Oracle) <[email protected]>
Cc: Vivek Kasireddy <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: VMoola

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);