Re: [PATCH 1/2] loop: respect REQ_NOWAIT for memory allocation

[Jens Axboe <axboe@kernel.dk>]

On 11/17/25 10:21 PM, hch@lst.de wrote:
> On Sun, Nov 16, 2025 at 05:43:53AM +0000, Chaitanya Kulkarni wrote:
>> On 11/15/25 19:50, Damien Le Moal wrote:
>>> On 11/16/25 11:52, Chaitanya Kulkarni wrote:
>>>>    6. Loop driver:
>>>>     loop_queue_rq()
>>>>      lo_rw_aio()
>>>>       kmalloc_array(..., GFP_NOIO) <-- BLOCKS (REQ_NOWAIT violation)
>>>>        -> Should use GFP_NOWAIT when rq->cmd_flags & REQ_NOWAIT
>>> Same comment as for zloop. Re-read the code and see that loop_queue_rq() calls
>>> loop_queue_work(). That function has a memory allocation that is already marked
>>> with GFP_NOWAIT, and that this function does not directly execute lo_rw_aio() as
>>> that is done from loop_workfn(), in the work item context.
>>> So again, no blocking violation that I can see here.
>>> As far as I can tell, this patch is not needed.
>>>
>> Thanks for pointing that out. Since REQ_NOWAIT is not valid in the
>> workqueue, then REQ_NOWAIT flag needs to be cleared before
>> handing it over to workqueue ? is that the right interpretation?
> 
> Having it cleared does seem useful as there is no need to skip blocking
> operations.  I don't think it actually is required, just a lot more
> efficient.

Agree, it doesn't make any sense to carry the REQ_NOWAIT into a blocking
out-of-line submit.

-- 
Jens Axboe