From: Kairui Song Date: Fri, 19 Dec 2025 19:43:38 +0000 (+0800) Subject: mm, swap: swap entry of a bad slot should not be considered as swapped out X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=f7ad377a9222a3e873cd20fd6fccf1a6f7f1347e;p=thirdparty%2Fkernel%2Flinux.git mm, swap: swap entry of a bad slot should not be considered as swapped out When checking if a swap entry is swapped out, we simply check if the bitwise result of the count value is larger than 0. But SWAP_MAP_BAD will also be considered as a swao count value larger than 0. SWAP_MAP_BAD being considered as a count value larger than 0 is useful for the swap allocator: they will be seen as a used slot, so the allocator will skip them. But for the swapped out check, this isn't correct. There is currently no observable issue. The swapped out check is only useful for readahead and folio swapped-out status check. For readahead, the swap cache layer will abort upon checking and updating the swap map. For the folio swapped out status check, the swap allocator will never allocate an entry of bad slots to folio, so that part is fine too. The worst that could happen now is redundant allocation/freeing of folios and waste CPU time. This also makes it easier to get rid of swap map checking and update during folio insertion in the swap cache layer. Link: https://lkml.kernel.org/r/20251220-swap-table-p2-v5-9-8862a265a033@tencent.com Signed-off-by: Kairui Song Reviewed-by: Baoquan He Cc: Baolin Wang Cc: Barry Song Cc: Chris Li Cc: Nhat Pham Cc: Rafael J. Wysocki (Intel) Cc: Yosry Ahmed Cc: Deepanshu Kartikey Cc: Johannes Weiner Cc: Kairui Song Signed-off-by: Andrew Morton --- diff --git a/mm/swap_state.c b/mm/swap_state.c index a82d0615f6466..d58bce532d959 100644 --- a/mm/swap_state.c +++ b/mm/swap_state.c @@ -526,7 +526,7 @@ struct folio *swap_cache_alloc_folio(swp_entry_t entry, gfp_t gfp_mask, if (folio) return folio; - /* Skip allocation for unused swap slot for readahead path. */ + /* Skip allocation for unused and bad swap slot for readahead. */ if (!swap_entry_swapped(si, entry)) return NULL; diff --git a/mm/swapfile.c b/mm/swapfile.c index eb394f30181a5..e47807fdf51b5 100644 --- a/mm/swapfile.c +++ b/mm/swapfile.c @@ -1766,10 +1766,10 @@ int __swap_count(swp_entry_t entry) return swap_count(si->swap_map[offset]); } -/* - * How many references to @entry are currently swapped out? - * This does not give an exact answer when swap count is continued, - * but does include the high COUNT_CONTINUED flag to allow for that. +/** + * swap_entry_swapped - Check if the swap entry is swapped. + * @si: the swap device. + * @entry: the swap entry. */ bool swap_entry_swapped(struct swap_info_struct *si, swp_entry_t entry) { @@ -1780,7 +1780,8 @@ bool swap_entry_swapped(struct swap_info_struct *si, swp_entry_t entry) ci = swap_cluster_lock(si, offset); count = swap_count(si->swap_map[offset]); swap_cluster_unlock(ci); - return !!count; + + return count && count != SWAP_MAP_BAD; } /* @@ -3677,10 +3678,10 @@ static int __swap_duplicate(swp_entry_t entry, unsigned char usage, int nr) count = si->swap_map[offset + i]; /* - * swapin_readahead() doesn't check if a swap entry is valid, so the - * swap entry could be SWAP_MAP_BAD. Check here with lock held. + * For swapin out, allocator never allocates bad slots. for + * swapin, readahead is guarded by swap_entry_swapped. */ - if (unlikely(swap_count(count) == SWAP_MAP_BAD)) { + if (WARN_ON(swap_count(count) == SWAP_MAP_BAD)) { err = -ENOENT; goto unlock_out; }