Re: [syzbot ci] Re: mm, swap: swap table phase III: remove swap_map

[Kairui Song <ryncsn@gmail.com>]

On Sun, Jan 25, 2026 at 02:13:41PM +0800, syzbot ci wrote:
> syzbot ci has tested the following series
> 
> [v1] mm, swap: swap table phase III: remove swap_map
> https://lore.kernel.org/all/20260126-swap-table-p3-v1-0-a74155fab9b0@tencent.com
> * [PATCH 01/12] mm, swap: protect si->swap_file properly and use as a mount indicator
> * [PATCH 02/12] mm, swap: clean up swapon process and locking
> * [PATCH 03/12] mm, swap: remove redundant arguments and locking for enabling a device
> * [PATCH 04/12] mm, swap: consolidate bad slots setup and make it more robust
> * [PATCH 05/12] mm/workingset: leave highest bits empty for anon shadow
> * [PATCH 06/12] mm, swap: implement helpers for reserving data in the swap table
> * [PATCH 07/12] mm, swap: mark bad slots in swap table directly
> * [PATCH 08/12] mm, swap: simplify swap table sanity range check
> * [PATCH 09/12] mm, swap: use the swap table to track the swap count
> * [PATCH 10/12] mm, swap: no need to truncate the scan border
> * [PATCH 11/12] mm, swap: simplify checking if a folio is swapped
> * [PATCH 12/12] mm, swap: no need to clear the shadow explicitly
> 
> and found the following issue:
> WARNING in swap_cluster_lock
> 
> Full report is available here:
> https://ci.syzbot.org/series/3f6169fc-e24a-4a19-ba56-e5907b448edc
> 
> ***
> 
> WARNING in swap_cluster_lock
> 
> tree:      mm-new
> URL:       https://kernel.googlesource.com/pub/scm/linux/kernel/git/akpm/mm.git
> base:      5a3704ed2dce0b54a7f038b765bb752b87ee8cc2
> arch:      amd64
> compiler:  Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8
> config:    https://ci.syzbot.org/builds/0eabd97a-86d8-4606-9d94-dbe4e7fd7c07/config
> C repro:   https://ci.syzbot.org/findings/5b039fd0-70da-4954-817d-8bf86315c684/c_repro
> syz repro: https://ci.syzbot.org/findings/5b039fd0-70da-4954-817d-8bf86315c684/syz_repro
> 
> ------------[ cut here ]------------
> offset >= si->max
> WARNING: mm/swap.h:88 at __swap_offset_to_cluster mm/swap.h:88 [inline], CPU#1: syz.0.548/6508
> WARNING: mm/swap.h:88 at __swap_cluster_lock mm/swap.h:101 [inline], CPU#1: syz.0.548/6508
> WARNING: mm/swap.h:88 at swap_cluster_lock+0xef/0x130 mm/swap.h:132, CPU#1: syz.0.548/6508

This is a good catch from the bot. It's caused by the patch "[PATCH 10/12] mm, swap: no need to truncate the scan border", however that patch is not wrong, it just have to update the debug check too:

diff --git a/mm/swap.h b/mm/swap.h
index 087cef49cf69..386a289ef8e7 100644
--- a/mm/swap.h
+++ b/mm/swap.h
@@ -85,7 +85,7 @@ static inline struct swap_cluster_info *__swap_offset_to_cluster(
                struct swap_info_struct *si, pgoff_t offset)
 {
        VM_WARN_ON_ONCE(percpu_ref_is_zero(&si->users)); /* race with swapoff */
-       VM_WARN_ON_ONCE(offset >= si->max);
+       VM_WARN_ON_ONCE(offset >= roundup(si->max, SWAPFILE_CLUSTER));
        return &si->cluster_info[offset / SWAPFILE_CLUSTER];
 }

I'll update this in V2.