Re: [PATCH v2 6/7] mm, slab: call kvfree_rcu_barrier() from kmem_cache_destroy()

Keith Busch
2025-02-21T16:30:35Z

On Wed, Aug 07, 2024 at 12:31:19PM +0200, Vlastimil Babka wrote:
> We would like to replace call_rcu() users with kfree_rcu() where the
> existing callback is just a kmem_cache_free(). However this causes
> issues when the cache can be destroyed (such as due to module unload).
>
> Currently such modules should be issuing rcu_barrier() before
> kmem_cache_destroy() to have their call_rcu() callbacks processed first.
> This barrier is however not sufficient for kfree_rcu() in flight due
> to the batching introduced by a35d16905efc ("rcu: Add basic support for
> kfree_rcu() batching").
>
> This is not a problem for kmalloc caches which are never destroyed, but
> since removing SLOB, kfree_rcu() is allowed also for any other cache,
> that might be destroyed.
>
> In order not to complicate the API, put the responsibility for handling
> outstanding kfree_rcu() in kmem_cache_destroy() itself. Use the newly
> introduced kvfree_rcu_barrier() to wait before destroying the cache.
> This is similar to how we issue rcu_barrier() for SLAB_TYPESAFE_BY_RCU
> caches, but has to be done earlier, as the latter only needs to wait for
> the empty slab pages to finish freeing, and not objects from the slab.
>
> Users of call_rcu() with arbitrary callbacks should still issue
> rcu_barrier() before destroying the cache and unloading the module, as
> kvfree_rcu_barrier() is not a superset of rcu_barrier() and the
> callbacks may be invoking module code or performing other actions that
> are necessary for a successful unload.
>
> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
> ---
> mm/slab_common.c | 3 +++
> 1 file changed, 3 insertions(+)
>
> diff --git a/mm/slab_common.c b/mm/slab_common.c
> index c40227d5fa07..1a2873293f5d 100644
> --- a/mm/slab_common.c
> +++ b/mm/slab_common.c
> @@ -508,6 +508,9 @@ void kmem_cache_destroy(struct kmem_cache *s)
> if (unlikely(!s) || !kasan_check_byte(s))
> return;
>
> + /* in-flight kfree_rcu()'s may include objects from our cache */
> + kvfree_rcu_barrier();
> +
> cpus_read_lock();
> mutex_lock(&slab_mutex);

This patch appears to be triggering a new warning in certain conditions
when tearing down an nvme namespace's block device. Stack trace is at
the end.

The warning indicates that this shouldn't be called from a
WQ_MEM_RECLAIM workqueue. This workqueue is responsible for bringing up
and tearing down block devices, so this is a memory reclaim use AIUI.
I'm a bit confused why we can't tear down a disk from within a memory
reclaim workqueue. Is the recommended solution to simply remove the WQ
flag when creating the workqueue?

------------[ cut here ]------------
workqueue: WQ_MEM_RECLAIM nvme-wq:nvme_scan_work is flushing !WQ_MEM_RECLAIM events_unbound:kfree_rcu_work
WARNING: CPU: 21 PID: 330 at kernel/workqueue.c:3719 check_flush_dependency+0x112/0x120
Modules linked in: intel_uncore_frequency(E) intel_uncore_frequency_common(E) skx_edac(E) skx_edac_common(E) nfit(E) libnvdimm(E) x86_pkg_temp_thermal(E) intel_powerclamp(E) coretemp(E) kvm_intel(E) iTCO_wdt(E) xhci_pci(E) mlx5_ib(E) ipmi_si(E) iTCO_vendor_support(E) i2c_i801(E) ipmi_devintf(E) evdev(E) kvm(E) xhci_hcd(E) ib_uverbs(E) acpi_cpufreq(E) wmi(E) i2c_smbus(E) ipmi_msghandler(E) button(E) efivarfs(E) autofs4(E)
CPU: 21 UID: 0 PID: 330 Comm: kworker/u144:6 Tainted: G E 6.13.2-0_g925d379822da #1
Hardware name: Wiwynn Twin Lakes MP/Twin Lakes Passive MP, BIOS YMM20 02/01/2023
Workqueue: nvme-wq nvme_scan_work
RIP: 0010:check_flush_dependency+0x112/0x120
Code: 05 9a 40 14 02 01 48 81 c6 c0 00 00 00 48 8b 50 18 48 81 c7 c0 00 00 00 48 89 f9 48 c7 c7 90 64 5a 82 49 89 d8 e8 7e 4f 88 ff <0f> 0b eb 8c cc cc cc cc cc cc cc cc cc cc 0f 1f 44 00 00 41 57 41
RSP: 0018:ffffc90000df7bd8 EFLAGS: 00010082
RAX: 000000000000006a RBX: ffffffff81622390 RCX: 0000000000000027
RDX: 00000000fffeffff RSI: 000000000057ffa8 RDI: ffff88907f960c88
RBP: 0000000000000000 R08: ffffffff83068e50 R09: 000000000002fffd
R10: 0000000000000004 R11: 0000000000000000 R12: ffff8881001a4400
R13: 0000000000000000 R14: ffff88907f420fb8 R15: 0000000000000000
FS: 0000000000000000(0000) GS:ffff88907f940000(0000) knlGS:0000000000000000
CR2: 00007f60c3001000 CR3: 000000107d010005 CR4: 00000000007726f0
PKRU: 55555554
Call Trace:
<TASK>
? __warn+0xa4/0x140
? check_flush_dependency+0x112/0x120
? report_bug+0xe1/0x140
? check_flush_dependency+0x112/0x120
? handle_bug+0x5e/0x90
? exc_invalid_op+0x16/0x40
? asm_exc_invalid_op+0x16/0x20
? timer_recalc_next_expiry+0x190/0x190
? check_flush_dependency+0x112/0x120
? check_flush_dependency+0x112/0x120
__flush_work.llvm.1643880146586177030+0x174/0x2c0
flush_rcu_work+0x28/0x30
kvfree_rcu_barrier+0x12f/0x160
kmem_cache_destroy+0x18/0x120
bioset_exit+0x10c/0x150
disk_release.llvm.6740012984264378178+0x61/0xd0
device_release+0x4f/0x90
kobject_put+0x95/0x180
nvme_put_ns+0x23/0xc0
nvme_remove_invalid_namespaces+0xb3/0xd0
nvme_scan_work+0x342/0x490
process_scheduled_works+0x1a2/0x370
worker_thread+0x2ff/0x390
? pwq_release_workfn+0x1e0/0x1e0
kthread+0xb1/0xe0
? __kthread_parkme+0x70/0x70
ret_from_fork+0x30/0x40
? __kthread_parkme+0x70/0x70
ret_from_fork_asm+0x11/0x20
</TASK>
---[ end trace 0000000000000000 ]---