With ongoing work to support large folio swapin, it is important to make sure we do not pass large folios to zswap_load() without implementing proper support. For example, if a swapin fault observes that contiguous PTEs are pointing to contiguous swap entries and tries to swap them in as a large folio, swap_read_folio() will pass in a large folio to zswap_load(), but zswap_load() will only effectively load the first page in the folio. If the first page is not in zswap, the folio will be read from disk, even though other pages may be in zswap. In both cases, this will lead to silent data corruption. Proper large folio swapin support needs to go into zswap before zswap can be enabled in a system that supports large folio swapin. Looking at callers of swap_read_folio(), it seems like they are either allocated from __read_swap_cache_async() or do_swap_page() in the SWP_SYNCHRONOUS_IO path. Both of which allocate order-0 folios, so we are fine for now. Add a VM_BUG_ON() in zswap_load() to make sure that we detect changes in the order of those allocations without proper handling of zswap. Alternatively, swap_read_folio() (or its callers) can be updated to have a fallback mechanism that splits large folios or reads subpages separately. Similar logic may be needed anyway in case part of a large folio is already in the swapcache and the rest of it is swapped out. Signed-off-by: Yosry Ahmed <yosryahmed@xxxxxxxxxx> --- Sorry for the long CC list, I just found myself repeatedly looking at new series that add swap support for mTHPs / large folios, making sure they do not break with zswap or make incorrect assumptions. This debug check should give us some peace of mind. Hopefully this patch will also raise awareness among people who are working on this. --- mm/zswap.c | 3 +++ 1 file changed, 3 insertions(+) diff --git a/mm/zswap.c b/mm/zswap.c index b9b35ef86d9be..6007252429bb2 100644 --- a/mm/zswap.c +++ b/mm/zswap.c @@ -1577,6 +1577,9 @@ bool zswap_load(struct folio *folio) if (!entry) return false; + /* Zswap loads do not handle large folio swapins correctly yet */ + VM_BUG_ON(folio_test_large(folio)); + if (entry->length) zswap_decompress(entry, folio); else -- 2.45.2.505.gda0bf45e8d-goog
