Re: [PATCH v3 1/3] mm/vmalloc: honor GFP constraints in pcpu_get_vm_areas()

[Dennis Zhou]

Hello,

On Fri, Jun 12, 2026 at 10:26:46AM +0800, Kaitao Cheng wrote:
> From: Kaitao Cheng <chengkaitao@xxxxxxxxxx>
> 
> pcpu_alloc_noprof() derives pcpu_gfp from the caller supplied GFP mask
> and passes it down to the backing percpu allocator. However, when the
> percpu vmalloc allocator has to create a new chunk, pcpu_create_chunk()
> calls pcpu_get_vm_areas() to allocate the corresponding vmalloc areas.
> 
> pcpu_get_vm_areas() currently performs its internal allocations with
> GFP_KERNEL, including vmap area metadata, vm_struct metadata and KASAN
> vmalloc shadow population. This means that a caller which deliberately
> uses GFP_NOFS or GFP_NOIO can still enter FS or IO reclaim while creating
> the vmalloc areas for a new percpu chunk.
> 
> One possible case is blk-cgroup after commit 5d726c4dbeed
> ("blk-cgroup: fix possible deadlock while configuring policy").
> blkg_conf_prep() now serializes against blkcg_deactivate_policy() with
> q->blkcg_mutex, and blkg_alloc() was changed to GFP_NOIO for that reason:
> 
>   CPU0: blkg_conf_prep()
>     mutex_lock(q->blkcg_mutex)
>     blkg_alloc(..., GFP_NOIO)
>       alloc_percpu_gfp(..., GFP_NOIO)
>         pcpu_alloc_noprof(..., GFP_NOIO)
> 	  pcpu_create_chunk(GFP_NOIO)
> 	    pcpu_get_vm_areas()
>               -> if percpu chunks are exhausted, chunk create may do
>                  internal GFP_KERNEL allocations
>               -> direct reclaim / writeback can issue IO to this queue
>               -> IO waits because the queue is frozen
> 
>   CPU1: blkcg_deactivate_policy()
>     blk_mq_freeze_queue(q)
>     mutex_lock(q->blkcg_mutex)
>       -> waits for CPU0
>     ... unfreeze only happens after q->blkcg_mutex is acquired/released
> 
> So the concern is that the caller deliberately uses GFP_NOIO because it
> may hold a lock which can be acquired after queue freeze, but the percpu
> slow path can temporarily lose that allocation context.
> 
> Pass the caller supplied GFP mask from pcpu_create_chunk() to
> pcpu_get_vm_areas(), and use it for the internal vmalloc metadata and
> KASAN shadow allocations.
> 
> Fixes: 9a5b183941b5 ("mm, percpu: do not consider sleepable allocations atomic")
> Signed-off-by: Kaitao Cheng <chengkaitao@xxxxxxxxxx>
> Reviewed-by: Uladzislau Rezki (Sony) <urezki@xxxxxxxxx>
> ---
 
Acked-by: Dennis Zhou <dennis@xxxxxxxxxx>

Thanks,
Dennis