From: Kairui Song Date: Wed, 28 Jan 2026 16:19:23 +0000 (+0800) Subject: mm, shmem: prevent infinite loop on truncate race X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=2030dddf95451b4e7a389f052091e7c4b7b274c6;p=thirdparty%2Fkernel%2Flinux.git mm, shmem: prevent infinite loop on truncate race When truncating a large swap entry, shmem_free_swap() returns 0 when the entry's index doesn't match the given index due to lookup alignment. The failure fallback path checks if the entry crosses the end border and aborts when it happens, so truncate won't erase an unexpected entry or range. But one scenario was ignored. When `index` points to the middle of a large swap entry, and the large swap entry doesn't go across the end border, find_get_entries() will return that large swap entry as the first item in the batch with `indices[0]` equal to `index`. The entry's base index will be smaller than `indices[0]`, so shmem_free_swap() will fail and return 0 due to the "base < index" check. The code will then call shmem_confirm_swap(), get the order, check if it crosses the END boundary (which it doesn't), and retry with the same index. The next iteration will find the same entry again at the same index with same indices, leading to an infinite loop. Fix this by retrying with a round-down index, and abort if the index is smaller than the truncate range. Link: https://lkml.kernel.org/r/aXo6ltB5iqAKJzY8@KASONG-MC4 Fixes: 809bc86517cc ("mm: shmem: support large folio swap out") Fixes: 8a1968bd997f ("mm/shmem, swap: fix race of truncate and swap entry split") Signed-off-by: Kairui Song Reported-by: Chris Mason Closes: https://lore.kernel.org/linux-mm/20260128130336.727049-1-clm@meta.com/ Reviewed-by: Baolin Wang Cc: Baoquan He Cc: Barry Song Cc: Chris Li Cc: Hugh Dickins Cc: Kemeng Shi Cc: Nhat Pham Cc: Signed-off-by: Andrew Morton --- diff --git a/mm/shmem.c b/mm/shmem.c index 6c3485d24d66..79af5f9f8b90 100644 --- a/mm/shmem.c +++ b/mm/shmem.c @@ -1211,17 +1211,22 @@ whole_folios: swaps_freed = shmem_free_swap(mapping, indices[i], end - 1, folio); if (!swaps_freed) { - /* - * If found a large swap entry cross the end border, - * skip it as the truncate_inode_partial_folio above - * should have at least zerod its content once. - */ + pgoff_t base = indices[i]; + order = shmem_confirm_swap(mapping, indices[i], radix_to_swp_entry(folio)); - if (order > 0 && indices[i] + (1 << order) > end) - continue; - /* Swap was replaced by page: retry */ - index = indices[i]; + /* + * If found a large swap entry cross the end or start + * border, skip it as the truncate_inode_partial_folio + * above should have at least zerod its content once. + */ + if (order > 0) { + base = round_down(base, 1 << order); + if (base < start || base + (1 << order) > end) + continue; + } + /* Swap was replaced by page or extended, retry */ + index = base; break; } nr_swaps_freed += swaps_freed;