Re: [PATCH] mm/shmem, swap: fix softlockup with mTHP swapin

[Kairui Song <ryncsn@gmail.com>]

On Mon, Jun 9, 2025 at 4:55 PM Barry Song <21cnbao@gmail.com> wrote:
>
> On Mon, Jun 9, 2025 at 8:49 PM Baolin Wang
> <baolin.wang@linux.alibaba.com> wrote:
> >
> >
> >
> > On 2025/6/9 16:36, Kairui Song wrote:
> > > On Mon, Jun 9, 2025 at 4:27 PM Baolin Wang
> > > <baolin.wang@linux.alibaba.com> wrote:
> > >> On 2025/6/9 03:27, Kairui Song wrote:
> > >>> From: Kairui Song <kasong@tencent.com>
> > >>>
> > >>> Following softlockup can be easily reproduced on my test machine with:
> > >>>
> > >>> echo always > /sys/kernel/mm/transparent_hugepage/hugepages-64kB/enabled
> > >>> swapon /dev/zram0 # zram0 is a 48G swap device
> > >>> mkdir -p /sys/fs/cgroup/memory/test
> > >>> echo 1G > /sys/fs/cgroup/test/memory.max
> > >>> echo $BASHPID > /sys/fs/cgroup/test/cgroup.procs
> > >>> while true; do
> > >>>       dd if=/dev/zero of=/tmp/test.img bs=1M count=5120
> > >>>       cat /tmp/test.img > /dev/null
> > >>>       rm /tmp/test.img
> > >>> done
> > >>>
> > >>> Then after a while:
> > >>> watchdog: BUG: soft lockup - CPU#0 stuck for 763s! [cat:5787]
> > >>> Modules linked in: zram virtiofs
> > >>> CPU: 0 UID: 0 PID: 5787 Comm: cat Kdump: loaded Tainted: G             L      6.15.0.orig-gf3021d9246bc-dirty #118 PREEMPT(voluntary)·
> > >>> Tainted: [L]=SOFTLOCKUP
> > >>> Hardware name: Red Hat KVM/RHEL-AV, BIOS 0.0.0 02/06/2015
> > >>> RIP: 0010:mpol_shared_policy_lookup+0xd/0x70
> > >>> Code: e9 b8 b4 ff ff 31 c0 c3 cc cc cc cc 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 66 0f 1f 00 0f 1f 44 00 00 41 54 55 53 <48> 8b 1f 48 85 db 74 41 4c 8d 67 08 48 89 fb 48 89 f5 4c 89 e7 e8
> > >>> RSP: 0018:ffffc90002b1fc28 EFLAGS: 00000202
> > >>> RAX: 00000000001c20ca RBX: 0000000000724e1e RCX: 0000000000000001
> > >>> RDX: ffff888118e214c8 RSI: 0000000000057d42 RDI: ffff888118e21518
> > >>> RBP: 000000000002bec8 R08: 0000000000000001 R09: 0000000000000000
> > >>> R10: 0000000000000bf4 R11: 0000000000000000 R12: 0000000000000001
> > >>> R13: 00000000001c20ca R14: 00000000001c20ca R15: 0000000000000000
> > >>> FS:  00007f03f995c740(0000) GS:ffff88a07ad9a000(0000) knlGS:0000000000000000
> > >>> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > >>> CR2: 00007f03f98f1000 CR3: 0000000144626004 CR4: 0000000000770eb0
> > >>> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> > >>> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> > >>> PKRU: 55555554
> > >>> Call Trace:
> > >>>    <TASK>
> > >>>    shmem_alloc_folio+0x31/0xc0
> > >>>    shmem_swapin_folio+0x309/0xcf0
> > >>>    ? filemap_get_entry+0x117/0x1e0
> > >>>    ? xas_load+0xd/0xb0
> > >>>    ? filemap_get_entry+0x101/0x1e0
> > >>>    shmem_get_folio_gfp+0x2ed/0x5b0
> > >>>    shmem_file_read_iter+0x7f/0x2e0
> > >>>    vfs_read+0x252/0x330
> > >>>    ksys_read+0x68/0xf0
> > >>>    do_syscall_64+0x4c/0x1c0
> > >>>    entry_SYSCALL_64_after_hwframe+0x76/0x7e
> > >>> RIP: 0033:0x7f03f9a46991
> > >>> Code: 00 48 8b 15 81 14 10 00 f7 d8 64 89 02 b8 ff ff ff ff eb bd e8 20 ad 01 00 f3 0f 1e fa 80 3d 35 97 10 00 00 74 13 31 c0 0f 05 <48> 3d 00 f0 ff ff 77 4f c3 66 0f 1f 44 00 00 55 48 89 e5 48 83 ec
> > >>> RSP: 002b:00007fff3c52bd28 EFLAGS: 00000246 ORIG_RAX: 0000000000000000
> > >>> RAX: ffffffffffffffda RBX: 0000000000040000 RCX: 00007f03f9a46991
> > >>> RDX: 0000000000040000 RSI: 00007f03f98ba000 RDI: 0000000000000003
> > >>> RBP: 00007fff3c52bd50 R08: 0000000000000000 R09: 00007f03f9b9a380
> > >>> R10: 0000000000000022 R11: 0000000000000246 R12: 0000000000040000
> > >>> R13: 00007f03f98ba000 R14: 0000000000000003 R15: 0000000000000000
> > >>>    </TASK>
> > >>>
> > >>> The reason is simple, readahead brought some order 0 folio in swap
> > >>> cache, and the swapin mTHP folio being allocated is in confict with it,
> > >>> so swapcache_prepare fails and causes shmem_swap_alloc_folio to return
> > >>> -EEXIST, and shmem simply retries again and again causing this loop.
> > >>
> > >> If swapcache_prepare() fails and retries, the folio's order (order 0)
> > >> getting from swapcache will be different from the order stored in the
> > >> shmem mapping, so we will split the large swap entry by the following
> > >> logic in shmem_swapin_folio(). So I am not sure why causing a softlockup?
> > >>
> > >>          } else if (order != folio_order(folio)) {
> > >>                  /*
> > >>                   * Swap readahead may swap in order 0 folios into swapcache
> > >>                   * asynchronously, while the shmem mapping can still stores
> > >>                   * large swap entries. In such cases, we should split the
> > >>                   * large swap entry to prevent possible data corruption.
> > >>                   */
> > >>                  split_order = shmem_split_large_entry(inode, index, swap, gfp);
> > >>                  if (split_order < 0) {
> > >>                          error = split_order;
> > >>                          goto failed;
> > >>                  }
> > >>
> > >>                  /*
> > >>                   * If the large swap entry has already been split, it is
> > >>                   * necessary to recalculate the new swap entry based on
> > >>                   * the old order alignment.
> > >>                   */
> > >>                  if (split_order > 0) {
> > >>                          pgoff_t offset = index - round_down(index, 1 << split_order);
> > >>
> > >>                          swap = swp_entry(swp_type(swap), swp_offset(swap) + offset);
> > >>                  }
> > >>          }
> > >
> > > For example if the swap entry is 0x0 in shmem with order 4 (so it
> > > corresponds to swap entries 0x0 - 0x10), and a order 0 folio is
> > > currently cached with swap entry 0xa, then shmem swapin will try to
> > > use a folio with order 4, that will always fails swapcache_prepare,
> > > but filemap/swapcache lookup use entry 0x0 will return NULL, causing a
> > > loop.
> >
> > OK. Thanks for the explanation.
> >
> > >>> Fix it by applying a similar fix for anon mTHP swapin.
> > >>>
> > >>> The performance change is very slight, time of swapin 10g zero folios
> > >>> (test for 12 times):
> > >>> Before:  2.49s
> > >>> After:   2.52s
> > >>>
> > >>> Fixes: 1dd44c0af4fa1 ("mm: shmem: skip swapcache for swapin of synchronous swap device")
> > >>> Signed-off-by: Kairui Song <kasong@tencent.com>
> > >>>
> > >>> ---
> > >>>
> > >>> I found this issue while doing a performance comparing of mm-new with
> > >>> swap table series [1] on top of mm-new. This issue no longer exists
> > >>> if the swap table series is applied, because it elimated both
> > >>> SWAP_HAS_CACHE and SWP_SYNCHRONOUS_IO swapin completely while improving
> > >>> the performance and simplify the code, and the race swapin is solved
> > >>> differently by then.
> > >>>
> > >>> (The zero map fix might still need to stay for a while, but could be
> > >>> optimized too later with swap table).
> > >>
> > >> I don't understand why adding zeromap changes, and should explain this
> > >> explicitly.
> > >
> > > To stay in consistency with anon mTHP swapin, swap_zeromap_batch have
> > > it's own comments that a hybird folio with zero and non-zero pages
> > > can't be brought back as a whole. I can mention that in the commit
> > > message.
>
> For mTHP swapin, we need the zeromap check because we have no way to record
> whether there was a prior mTHP swap-out. So we rely on checking the
> continuity of swap offsets.
>
> It’s entirely possible that, in the past, several small folios were
> swapped out to consecutive locations, and one of them happened to be a
> zero folio, while the others were not.
>
> But for shmem, we have a place to record that information -   we swapped-out
> a mTHP, right?
>
> Regarding zeromap: for an mTHP swap-out, we currently can't mark subpages
> individually as zeromap—it’s either all-zero for every subpage or none are.

Thanks for the declaration! Yes, that's correct, I wasn't sure if zero
map will mark subpages so just left the check there. Will remove the
check in V2.

> So maybe we don't need swap_zeromap_batch()  for shmem?

Right, it's not needed here, the fix will be simpler.