Re: Hang when swapping huge=within_size tmpfs from zram

[Lance Yang <ioworker0@gmail.com>]

On Wed, Feb 5, 2025 at 2:38 PM Baolin Wang
<baolin.wang@linux.alibaba.com> wrote:
>
>
>
> On 2025/2/5 09:55, Baolin Wang wrote:
> > Hi Alex,
> >
> > On 2025/2/5 09:23, Alex Xu (Hello71) wrote:
> >> Hi all,
> >>
> >> On 6.14-rc1, I found that creating a lot of files in tmpfs then deleting
> >> them reliably hangs when tmpfs is mounted with huge=within_size, and it
> >> is swapped out to zram (zstd/zsmalloc/no backing dev). I bisected this
> >> to acd7ccb284b "mm: shmem: add large folio support for tmpfs".
> >>
> >> When the issue occurs, rm uses 100% CPU, cannot be killed, and has no
> >> output in /proc/pid/stack or wchan. Eventually, an RCU stall is
> >> detected:
> >
> > Thanks for your report. Let me try to reproduce the issue locally and
> > see what happens.
> >
> >> rcu: INFO: rcu_preempt detected stalls on CPUs/tasks:
> >> rcu:     Tasks blocked on level-0 rcu_node (CPUs 0-11): P25160
> >> rcu:     (detected by 10, t=2102 jiffies, g=532677, q=4997 ncpus=12)
> >> task:rm              state:R  running task     stack:0     pid:25160
> >> tgid:25160 ppid:24309  task_flags:0x400000 flags:0x00004004
> >> Call Trace:
> >>   <TASK>
> >>   ? __schedule+0x388/0x1000
> >>   ? kmem_cache_free.part.0+0x23d/0x280
> >>   ? sysvec_apic_timer_interrupt+0xa/0x80
> >>   ? asm_sysvec_apic_timer_interrupt+0x16/0x20
> >>   ? xas_load+0x12/0xc0
> >>   ? xas_load+0x8/0xc0
> >>   ? xas_find+0x144/0x190
> >>   ? find_lock_entries+0x75/0x260
> >>   ? shmem_undo_range+0xe6/0x5f0
> >>   ? shmem_evict_inode+0xe4/0x230
> >>   ? mtree_erase+0x7e/0xe0
> >>   ? inode_set_ctime_current+0x2e/0x1f0
> >>   ? evict+0xe9/0x260
> >>   ? _atomic_dec_and_lock+0x31/0x50
> >>   ? do_unlinkat+0x270/0x2b0
> >>   ? __x64_sys_unlinkat+0x30/0x50
> >>   ? do_syscall_64+0x37/0xe0
> >>   ? entry_SYSCALL_64_after_hwframe+0x50/0x58
> >>   </TASK>
> >>
> >> Let me know what information is needed to further troubleshoot this
> >> issue.
>
> Sorry, I can't reproduce this issue, and my testing process is as follows:
> 1. Mount tmpfs with huge=within_size
> 2. Create and write a tmpfs file
> 3. Swap out the large folios of the tmpfs file to zram
> 4. Execute 'rm' command to remove the tmpfs file

I’m unable to reproduce the issue as well, and am following steps similar
to Baolin's process:

1) Mount tmpfs with the huge=within_size option and enable swap (using
zstd/zsmalloc without a backing device).
2) Create and write over 10,000 files in the tmpfs.
3) Swap out the large folios of these tmpfs files to zram.
4) Use the rm command to delete all the files from the tmpfs.

Testing with both 2MiB and 64KiB large folio sizes, and with
shmem_enabled=within_size, but everything works as expected.

Thanks,
Lance

>
> Do you have any reproduction scripts or reproduction steps guide? Thanks.