We have a check in do_swap_page that page from lookup_swap_cache should
have private field equal to the swapcache index we searched it at
(page_private(page) != entry.val). So folio_migrate_mapping should
preserve page private for each page of a huge folio to satisfy this
check else we get infinite loop in:
+-> mmap_read_lock
+-> __get_user_pages_locked
+-> for-loop # taken once
+-> __get_user_pages
+-> retry-loop # constantly spinning
+-> faultin_page # return 0 to trigger retry
+-> handle_mm_fault
+-> __handle_mm_fault
+-> handle_pte_fault
+-> do_swap_page
+-> lookup_swap_cache # returns non-NULL
+-> if (swapcache)
+-> if (!folio_test_swapcache || page_private(page) !=
entry.val)
+-> goto out_page
+-> return 0
And this loop is under mmap_lock so we have secondary problem of
multiple processes hanging on attempt to take mmap_lock.
Note: We don't need this after we stop using private in this ms patch:
cfeed8ffe55b ("mm/swap: stop using page->private on tail pages for THP_SWAP")
https://virtuozzo.atlassian.net/browse/PSBM-153264
Signed-off-by: Pavel Tikhomirov <ptikhomi...@virtuozzo.com>
---
mm/migrate.c | 4 ++++
1 file changed, 4 insertions(+)
diff --git a/mm/migrate.c b/mm/migrate.c
index d950f42c0708..357a0a4c59e6 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -424,6 +424,10 @@ int folio_migrate_mapping(struct address_space *mapping,
if (folio_test_swapcache(folio)) {
folio_set_swapcache(newfolio);
newfolio->private = folio_get_private(folio);
+
+ /* Swap cache needs private for each page of huge page
*/
+ for (i = 1; i < nr; i++)
+ set_page_private(folio_page(newfolio, i),
folio_page(folio, i)->private);
}
entries = nr;
} else {
--
2.43.0
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