Re: [PATCH v3] mm/shmem, swap: fix softlockup with mTHP swapin

[Kairui Song <ryncsn@gmail.com>]

On Wed, Jun 11, 2025 at 11:33 AM Baoquan He <bhe@redhat.com> wrote:
>
> On 06/11/25 at 02:16am, 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
>                                                   ^^^typo, conflict
>

Nice catch, thanks!

Hi Andrew, it's only one typo in the commit message here, will you
help update the commit message while merging it or should I send a new
version?