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SubjectRe: [PATCH v5] blkcg: allocate struct blkcg_gq outside request queue spinlock
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On 03/09/2017 01:05 AM, Tahsin Erdogan wrote:
> blkg_conf_prep() currently calls blkg_lookup_create() while holding
> request queue spinlock. This means allocating memory for struct
> blkcg_gq has to be made non-blocking. This causes occasional -ENOMEM
> failures in call paths like below:
>
> pcpu_alloc+0x68f/0x710
> __alloc_percpu_gfp+0xd/0x10
> __percpu_counter_init+0x55/0xc0
> cfq_pd_alloc+0x3b2/0x4e0
> blkg_alloc+0x187/0x230
> blkg_create+0x489/0x670
> blkg_lookup_create+0x9a/0x230
> blkg_conf_prep+0x1fb/0x240
> __cfqg_set_weight_device.isra.105+0x5c/0x180
> cfq_set_weight_on_dfl+0x69/0xc0
> cgroup_file_write+0x39/0x1c0
> kernfs_fop_write+0x13f/0x1d0
> __vfs_write+0x23/0x120
> vfs_write+0xc2/0x1f0
> SyS_write+0x44/0xb0
> entry_SYSCALL_64_fastpath+0x18/0xad
>
> In the code path above, percpu allocator cannot call vmalloc() due to
> queue spinlock.
>
> A failure in this call path gives grief to tools which are trying to
> configure io weights. We see occasional failures happen shortly after
> reboots even when system is not under any memory pressure. Machines
> with a lot of cpus are more vulnerable to this condition.
>
> Update blkg_create() function to temporarily drop the rcu and queue
> locks when it is allowed by gfp mask.

Added for 4.12, thanks for your persistence in pulling this through.

--
Jens Axboe

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