Re: [BUG] shmem: shmem_get_folio_gfp livelock

[Kairui Song]

On Wed, Jul 1, 2026 at 6:09 PM Baolin Wang
<baolin.wang@xxxxxxxxxxxxxxxxx> wrote:
>
> CC Hugh and Kairui.

Hello!

>
> On 6/30/26 9:15 PM, 马超 wrote:
> > Hello,
> > I encountered a bug in the shmem subsystem. Details below.
> >
> > [Summary]
> > shmem_get_folio_gfp() can livelock when multiple threads fault on the
> > same shmem page concurrently. The -EEXIST retry loop (goto repeat) has
> > no cond_resched(), causing busy-looping threads to starve the thread
> > that holds the swapcache slot, resulting in an indefinite RCU stall and
> > system hang.
> > [Environment]
> > 1.Kernel: 6.18.21 (ARM64, PREEMPT, CONFIG_LRU_GEN=y)
> > 2.Triggered by: multi-threaded app with threads constrained to 2 CPUs
> > via cpuset
> > [Root Cause]
> > When multiple threads in the same process fault on the same shmem
> > swap entry:
> > 1.Thread A enters shmem_swap_alloc_folio(), succeeds at
> > swapcache_prepare() (sets SWAP_HAS_CACHE), then enters
> > workingset_refault() → lru_gen_refault() → rcu_read_lock().
> > While inside the RCU read-side critical section, it is
> > preempted via preempt_schedule_irq (IRQ exit path detects
> > TIF_NEED_RESCHED).
> > 2.Threads B & C enter shmem_swap_alloc_folio(), fail at
> > swapcache_prepare() (slot already taken by A), return -EEXIST.
> > 3.In shmem_get_folio_gfp():
> >      error = shmem_swapin_folio(...);
> >      if (error == -EEXIST)
> >          goto repeat;// no cond_resched(), tight loop
> > 4.Threads B & C spin at 100% CPU on the retry loop. All three
> > threads share the same cpuset (CPU0-1,cpus_allowed=0x3).Thread A
> > is perpetually preempted and starved — it cannot complete the
> > few instructions needed to call rcu_read_unlock().
> > 5.The held RCU read lock blocks the grace period indefinitely,
> > causing all synchronize_rcu() callers (cgroup operations,
> > fd allocation, etc.) to hang, eventually blocking init.
> > [Scheduling Details]
> > Key observations:
> > 1.Thread A was RCU-boosted to prio 98 but accumulated only
> > 99ms of execution over the entire stall period (~1200s).
> > It was effectively starved despite the priority boost.
> > 2.Threads B & C have vruntime=0 and prio 91, indicating
> > they run in an RT-equivalent scheduling class (SCHED_FIFO/RT
> > policy). Each accumulated ~1134 seconds of execution with
> > only ~1600 context switches, meaning they ran uninterrupted
> > for ~700ms per scheduling quantum on average.
> > 3.Thread A cannot preempt Threads B & C: Although RCU boost
> > raised Thread A to prio 98, Threads B & C at prio 91 (lower
> > numeric value = higher priority in RT class) have equal or
> > higher effective priority. The busy-looping threads never
> > voluntarily yield (no cond_resched(), no blocking calls in
> > the loop), so Thread A never gets scheduled.
> > 4.CPU contention: CPU0 had nr_running=28 and CPU1 had
> > nr_running=24, with 3-4 RT tasks per CPU. Thread A competed
> > with Thread B on CPU0 but could not win scheduling.
> > [Observed Impact]
> > 1.RCU stall lasting 910+ seconds (19 consecutive stall
> > warnings, grace period g=4398761 never advanced)
> > 2.synchronize_rcu_expedited() callers blocked 742+ seconds
> > 3.init process hung > 720 seconds → system unresponsive
> > [Call Traces]
> > Thread A (RCU stall source, sampled 19 times identically):
> > __switch_to+0x1a4/0x360 (T)
> > __schedule+0x96c/0xf3c
> > preempt_schedule_irq+0xec/0x198
> > raw_irqentry_exit_cond_resched+0x2c/0x44
> > irqentry_exit+0x38/0x64
> > exit_to_kernel_mode+0x28/0x38
> > el1_interrupt+0x5c/0xa8
> > el1h_64_irq_handler+0x18/0x24
> > el1h_64_irq+0x84/0x88
> > workingset_refault+0x16c/0x79c (P)
> >    shmem_swapin_folio+0x8e4/0xd44
> >      shmem_get_folio_gfp+0xb8/0x710
> >        shmem_fault+0xa0/0x174
> >          __do_fault
> > do_pte_missing
> > handle_mm_fault
> > do_page_fault
> > el0_ia
> >
> > Thread B (busy-loop on CPU0, sum_exec_runtime=1134s):
> > xas_load+0x78/0xe4 (P)
> >    shmem_swapin_folio+0x950/0xd44
> >      shmem_get_folio_gfp+0xb8/0x710
> >        shmem_fault → ... → el0_ia
> >
> > Thread C (busy-loop on CPU1, sum_exec_runtime=1134s):
> > xas_load+0x50/0xe4 (P)
> >    shmem_swapin_folio+0xd8/0xd44
> >      shmem_get_folio_gfp+0xb8/0x710
> >        shmem_fault → ... → el0_ia
> > [Question]
> > What is the recommended approach to fix this livelock?
> > We are considering adding a cond_resched() before the
> > goto repeat in shmem_get_folio_gfp() to break the tight
> > loop and allow the swapcache-holding thread to make
> > progress. Would this be an acceptable fix, or is there
> > a better strategy (e.g., bounded retry with fallback,
> > or yielding to the specific waiter)?

So this is a 6.18 issue, the SWAP_HAS_CACHE design was a long time
issue and we had a workaround for anon in commit 13ddaf26be32, the
`schedule_timeout_uninterruptible(1)`. That's fine as a workaroud and
we remove the workaround with proper redesign later in commit
f1879e8a0c60. Shmem never used a workaround like
schedule_timeout_uninterruptible and we jump to the right design
directly.

For 6.18 LTS, do we need a fix though? I think we can have a similar
timeout for shmem as well, just do a
schedule_timeout_uninterruptible(1), yes it's really ugly, and we have
another long time workaround like that in mm/swap_state.c as well in
6.18. Without the later proper redesign there seems no better way.
Barry provided an extra improvement on top of that workaround in
commit 01626a182302, maybe can also be carried into shmem. So maybe
having both for shmem is a good choice for a 6.18 fix.

>
> IIRC, the scheduler maintainers are not a fan of continuing to sprinkle
> random cond_resched() calls throughout the kernel. The scheduling
> decisions should be left to the scheduler itself.
>
> Regarding your issue, could you try the latest kernel? IIUC, this
> problem has already been fixed there (likely from Kairui's swap
> refactoring work [1]).

Yes, exactly, the recent swap rework removed the root SWAP_HAS_CACHE
problem, so we should no longer see any similiar problems like this in
mainline.