mm: migrate high-order folios in swap cache correctly

commit fc346d0a70a13d52fe1c4bc49516d83a42cd7c4c upstream.

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: 3417013e0d18 ("mm/migrate: Add folio_migrate_mapping()")
Link: https://lkml.kernel.org/r/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: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/mm/migrate.c b/mm/migrate.c
index 06086dc..03bc206 100644 (file)
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -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);
        }
 
-       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