FROMLIST: 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.
Bug: 315281107
Change-Id: I455f964a9f21c13089890073777388236b6669d7
[willy@infradead.org: modifications to the changelog & tweaked the fix]
Fixes: 3417013e0d18 ("mm/migrate: Add folio_migrate_mapping()")
Link: https://lkml.kernel.org/r/20231214045841.961776-1-willy@infradead.org
Link: https://lore.kernel.org/linux-mm/20231214045841.961776-1-willy@infradead.org/
Signed-off-by: Charan Teja Kalla <quic_charante@quicinc.com>
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reported-by: Charan 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: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Charan Teja Kalla <quic_charante@quicinc.com>
diff --git a/mm/migrate.c b/mm/migrate.c
index ef49097..9f5f52d 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -393,6 +393,7 @@
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 */
@@ -430,8 +431,10 @@
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 */
@@ -441,7 +444,11 @@
folio_set_dirty(newfolio);
}
- xas_store(&xas, 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
