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

[Vlastimil Babka <vbabka@suse.cz>]

On 2/21/25 17:30, Keith Busch wrote:
> 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?

I think it's reasonable to expect a memory reclaim related action would
destroy a kmem cache. Mateusz's suggestion would work around the issue, but
then we could get another surprising warning elsewhere. Also making the
kmem_cache destroys async can be tricky when a recreation happens
immediately under the same name (implications with sysfs/debugfs etc). We
managed to make the destroying synchronous as part of this series and it
would be great to keep it that way.

>   ------------[ cut here ]------------
>   workqueue: WQ_MEM_RECLAIM nvme-wq:nvme_scan_work is flushing !WQ_MEM_RECLAIM events_unbound:kfree_rcu_work

Maybe instead kfree_rcu_work should be using a WQ_MEM_RECLAIM workqueue? It
is after all freeing memory. Ulad, what do you think?

>   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 ]---