Commit fc346d0a authored by Charan Teja Kalla's avatar Charan Teja Kalla Committed by Andrew Morton
Browse files

mm: migrate high-order folios in swap cache correctly 

Large folios occupy N consecutive entries in the swap cache instead of
using multi-index entries like the page cache.  However, if a large folio
is re-added to the LRU list, it can be migrated.  The migration code was
not aware of the difference between the swap cache and the page cache and
assumed that a single xas_store() would be sufficient.

This leaves potentially many stale pointers to the now-migrated folio in
the swap cache, which can lead to almost arbitrary data corruption in the
future.  This can also manifest as infinite loops with the RCU read lock
held.

[willy@infradead.org: modifications to the changelog & tweaked the fix]
Fixes: 3417013e ("mm/migrate: Add folio_migrate_mapping()")
Link: https://lkml.kernel.org/r/20231214045841.961776-1-willy@infradead.org


Signed-off-by: default avatarCharan Teja Kalla <quic_charante@quicinc.com>
Signed-off-by: default avatarMatthew Wilcox (Oracle) <willy@infradead.org>
Reported-by: default avatarCharan Teja Kalla <quic_charante@quicinc.com>
Closes: https://lkml.kernel.org/r/1700569840-17327-1-git-send-email-quic_charante@quicinc.com


Cc: David Hildenbrand <david@redhat.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Cc: Shakeel Butt <shakeelb@google.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
parent 4249f13c

mm/migrate.c

+8 −1
Original line number Diff line number Diff line
@@ -405,6 +405,7 @@ int folio_migrate_mapping(struct address_space *mapping, 
	int dirty;

	int expected_count = folio_expected_refs(mapping, folio) + extra_count;

	long nr = folio_nr_pages(folio);

	long entries, i;



	if (!mapping) {

		/* Anonymous page without mapping */
@@ -442,8 +443,10 @@ int folio_migrate_mapping(struct address_space *mapping, 
			folio_set_swapcache(newfolio);

			newfolio->private = folio_get_private(folio);

		}

		entries = nr;

	} else {

		VM_BUG_ON_FOLIO(folio_test_swapcache(folio), folio);

		entries = 1;

	}



	/* Move dirty while page refs frozen and newpage not yet exposed */
@@ -453,7 +456,11 @@ int folio_migrate_mapping(struct address_space *mapping, 
		folio_set_dirty(newfolio);

	}



	/* Swap cache still stores N entries instead of a high-order entry */

	for (i = 0; i < entries; i++) {

		xas_store(&xas, newfolio);

		xas_next(&xas);

	}



	/*

	 * Drop cache reference from old page by unfreezing