Re: [PATCH v3 14/20] mm: swap: decouple virtual swap slot from backing store
From: Dan Carpenter
Date: Tue Feb 10 2026 - 01:32:08 EST
Hi Nhat,
kernel test robot noticed the following build warnings:
url: https://github.com/intel-lab-lkp/linux/commits/Nhat-Pham/mm-swap-decouple-swap-cache-from-physical-swap-infrastructure/20260209-120606
base: 05f7e89ab9731565d8a62e3b5d1ec206485eeb0b
patch link: https://lore.kernel.org/r/20260208215839.87595-15-nphamcs%40gmail.com
patch subject: [PATCH v3 14/20] mm: swap: decouple virtual swap slot from backing store
config: powerpc-randconfig-r073-20260209 (https://download.01.org/0day-ci/archive/20260209/202602092300.lZO4Ee4N-lkp@xxxxxxxxx/config)
compiler: powerpc-linux-gcc (GCC) 15.2.0
smatch version: v0.5.0-8994-gd50c5a4c
If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <lkp@xxxxxxxxx>
| Reported-by: Dan Carpenter <dan.carpenter@xxxxxxxxxx>
| Closes: https://lore.kernel.org/r/202602092300.lZO4Ee4N-lkp@xxxxxxxxx/
smatch warnings:
mm/vswap.c:733 vswap_alloc_swap_slot() warn: variable dereferenced before check 'folio' (see line 701)
vim +/folio +733 mm/vswap.c
19a5fe94e9aae4 Nhat Pham 2026-02-08 694 bool vswap_alloc_swap_slot(struct folio *folio)
19a5fe94e9aae4 Nhat Pham 2026-02-08 695 {
19a5fe94e9aae4 Nhat Pham 2026-02-08 696 int i, nr = folio_nr_pages(folio);
19a5fe94e9aae4 Nhat Pham 2026-02-08 697 struct vswap_cluster *cluster = NULL;
19a5fe94e9aae4 Nhat Pham 2026-02-08 698 struct swap_info_struct *si;
19a5fe94e9aae4 Nhat Pham 2026-02-08 699 struct swap_cluster_info *ci;
19a5fe94e9aae4 Nhat Pham 2026-02-08 700 swp_slot_t slot = { .val = 0 };
19a5fe94e9aae4 Nhat Pham 2026-02-08 @701 swp_entry_t entry = folio->swap;
folio dereference here
19a5fe94e9aae4 Nhat Pham 2026-02-08 702 struct swp_desc *desc;
19a5fe94e9aae4 Nhat Pham 2026-02-08 703 bool fallback = false;
19a5fe94e9aae4 Nhat Pham 2026-02-08 704
19a5fe94e9aae4 Nhat Pham 2026-02-08 705 /*
19a5fe94e9aae4 Nhat Pham 2026-02-08 706 * We might have already allocated a backing physical swap slot in past
19a5fe94e9aae4 Nhat Pham 2026-02-08 707 * attempts (for instance, when we disable zswap). If the entire range is
19a5fe94e9aae4 Nhat Pham 2026-02-08 708 * already swapfile-backed we can skip swapfile case.
19a5fe94e9aae4 Nhat Pham 2026-02-08 709 */
19a5fe94e9aae4 Nhat Pham 2026-02-08 710 if (vswap_swapfile_backed(entry, nr))
19a5fe94e9aae4 Nhat Pham 2026-02-08 711 return true;
19a5fe94e9aae4 Nhat Pham 2026-02-08 712
19a5fe94e9aae4 Nhat Pham 2026-02-08 713 if (swap_slot_alloc(&slot, folio_order(folio)))
and here
19a5fe94e9aae4 Nhat Pham 2026-02-08 714 return false;
19a5fe94e9aae4 Nhat Pham 2026-02-08 715
19a5fe94e9aae4 Nhat Pham 2026-02-08 716 if (!slot.val)
19a5fe94e9aae4 Nhat Pham 2026-02-08 717 return false;
19a5fe94e9aae4 Nhat Pham 2026-02-08 718
7f88e3ea20f231 Nhat Pham 2026-02-08 719 /* establish the vrtual <-> physical swap slots linkages. */
7f88e3ea20f231 Nhat Pham 2026-02-08 720 si = __swap_slot_to_info(slot);
7f88e3ea20f231 Nhat Pham 2026-02-08 721 ci = swap_cluster_lock(si, swp_slot_offset(slot));
7f88e3ea20f231 Nhat Pham 2026-02-08 722 vswap_rmap_set(ci, slot, entry.val, nr);
7f88e3ea20f231 Nhat Pham 2026-02-08 723 swap_cluster_unlock(ci);
7f88e3ea20f231 Nhat Pham 2026-02-08 724
7f88e3ea20f231 Nhat Pham 2026-02-08 725 rcu_read_lock();
7f88e3ea20f231 Nhat Pham 2026-02-08 726 for (i = 0; i < nr; i++) {
7f88e3ea20f231 Nhat Pham 2026-02-08 727 desc = vswap_iter(&cluster, entry.val + i);
7f88e3ea20f231 Nhat Pham 2026-02-08 728 VM_WARN_ON(!desc);
7f88e3ea20f231 Nhat Pham 2026-02-08 729
19a5fe94e9aae4 Nhat Pham 2026-02-08 730 if (desc->type == VSWAP_FOLIO) {
19a5fe94e9aae4 Nhat Pham 2026-02-08 731 /* case 1: fallback from zswap store failure */
19a5fe94e9aae4 Nhat Pham 2026-02-08 732 fallback = true;
19a5fe94e9aae4 Nhat Pham 2026-02-08 @733 if (!folio)
So it can't be NULL here.
19a5fe94e9aae4 Nhat Pham 2026-02-08 734 folio = desc->swap_cache;
So we'll never do this assignment and it will never become NULL.
19a5fe94e9aae4 Nhat Pham 2026-02-08 735 else
19a5fe94e9aae4 Nhat Pham 2026-02-08 736 VM_WARN_ON(folio != desc->swap_cache);
19a5fe94e9aae4 Nhat Pham 2026-02-08 737 } else {
19a5fe94e9aae4 Nhat Pham 2026-02-08 738 /*
19a5fe94e9aae4 Nhat Pham 2026-02-08 739 * Case 2: zswap writeback.
19a5fe94e9aae4 Nhat Pham 2026-02-08 740 *
19a5fe94e9aae4 Nhat Pham 2026-02-08 741 * No need to free zswap entry here - it will be freed once zswap
19a5fe94e9aae4 Nhat Pham 2026-02-08 742 * writeback suceeds.
19a5fe94e9aae4 Nhat Pham 2026-02-08 743 */
19a5fe94e9aae4 Nhat Pham 2026-02-08 744 VM_WARN_ON(desc->type != VSWAP_ZSWAP);
19a5fe94e9aae4 Nhat Pham 2026-02-08 745 VM_WARN_ON(fallback);
19a5fe94e9aae4 Nhat Pham 2026-02-08 746 }
19a5fe94e9aae4 Nhat Pham 2026-02-08 747 desc->type = VSWAP_SWAPFILE;
7f88e3ea20f231 Nhat Pham 2026-02-08 748 desc->slot.val = slot.val + i;
7f88e3ea20f231 Nhat Pham 2026-02-08 749 }
7f88e3ea20f231 Nhat Pham 2026-02-08 750 spin_unlock(&cluster->lock);
7f88e3ea20f231 Nhat Pham 2026-02-08 751 rcu_read_unlock();
19a5fe94e9aae4 Nhat Pham 2026-02-08 752 return true;
7f88e3ea20f231 Nhat Pham 2026-02-08 753 }
--
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