mm/slab/kvfree_rcu: Switch to WQ_MEM_RECLAIM wq

In the Linux kernel, the following vulnerability has been resolved:

mm/slab/kvfree_rcu: Switch to WQ_MEM_RECLAIM wq

Currently kvfree_rcu() APIs use a system workqueue which is

"system_unbound_wq" to driver RCU machinery to reclaim a memory.

Recently, it has been noted that the following kernel warning can

be observed:

<snip>

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) ...

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 ...

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 ]---

<snip>

To address this switch to use of independent WQ_MEM_RECLAIM

workqueue, so the rules are not violated from workqueue framework

point of view.

Apart of that, since kvfree_rcu() does reclaim memory it is worth

to go with WQ_MEM_RECLAIM type of wq because it is designed for

this purpose.