From 12e9ac7bc5b254048f886bf421e3a15491106c1f Mon Sep 17 00:00:00 2001 From: Nhat Pham Date: Fri, 28 Aug 2026 12:14:33 -0700 Subject: [PATCH] mm, swap: fix SWAP_USAGE_OFFLIST_BIT collision with real usage count SWAP_USAGE_OFFLIST_BIT is embedded in the si->inuse_pages usage counter, and is meant to sit above any value that counter can reach. However, it is defined from BITS_PER_TYPE(atomic_t), so it is bit 30. On a system with 4 KiB pages the flag collides with the usage count once that count reaches 4 TiB. swap_usage_in_pages() masks bit 30 out, so whenever the real count has that bit set, every caller of it reads 4 TiB low: * /proc/swaps understates Used by 4 TiB. * A raw count of exactly 2^30 masks to zero, so try_to_unuse() takes its "if (!swap_usage_in_pages(si)) goto success;" early exit and swapoff tears the device down while pages are still swapped out. Nothing in the rest of swapoff aborts the teardown, so those pages are lost. Independently of swapoff, the collision also corrupts the counter and the plist. On a device in normal use, a free that leaves bit 30 set in the count makes swap_usage_sub() see the flag where there is only count, and call add_to_avail_list(). It clears the bit with fetch_and(~SWAP_USAGE_OFFLIST_BIT), leaving the stored count 4 TiB below the real one, and calls plist_add() on a device that is already listed, tripping the WARN_ON(!plist_node_empty(node)) in plist_add() and linking the node a second time. Change the definition of SWAP_USAGE_OFFLIST_BIT to be based on atomic_long_t instead. Note that the usage counter field itself is of this same type, so it is still a valid bit. Link: https://lore.kernel.org/20260828191433.3304458-1-nphamcs@gmail.com Fixes: b228386cf237 ("mm, swap: clean up plist removal and adding") Signed-off-by: Nhat Pham Reported-by: Sashiko Closes: https://sashiko.dev/#/patchset/20260825153238.2695446-1-nphamcs%40gmail.com Suggested-by: Andrew Morton Reviewed-by: Andrew Morton Acked-by: Kairui Song Cc: Baoquan He Cc: Barry Song Cc: Chris Li Cc: Gregory Price Cc: Johannes Weiner Cc: Joshua Hahn Cc: Kemeng Shi Cc: Shakeel Butt Cc: Youngjun Park Cc: Signed-off-by: Andrew Morton --- mm/swapfile.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/mm/swapfile.c b/mm/swapfile.c index 53bf01d5f7f1..601979b97f95 100644 --- a/mm/swapfile.c +++ b/mm/swapfile.c @@ -156,7 +156,7 @@ static struct swap_info_struct *swap_entry_to_info(swp_entry_t entry) * This bit will be set if the device is not on the plist and not * usable, will be cleared if the device is on the plist. */ -#define SWAP_USAGE_OFFLIST_BIT (1UL << (BITS_PER_TYPE(atomic_t) - 2)) +#define SWAP_USAGE_OFFLIST_BIT (1UL << (BITS_PER_TYPE(atomic_long_t) - 2)) #define SWAP_USAGE_COUNTER_MASK (~SWAP_USAGE_OFFLIST_BIT) static long swap_usage_in_pages(struct swap_info_struct *si) {