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

[Chaitanya Kulkarni <chaitanyak@nvidia.com>]

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?

e.g.

loop_queue_rq()
  loop_queue_work()
    ...
    ...
    rq->cmd_flags &= ~REQ_NOWAIT; <---
    
    list_add_tail(&cmd->list_entry, cmd_list);
    queue_work(lo->workqueue, work);
    spin_unlock_irq(&lo->lo_work_lock);

I have read the code [1] and the commit that added the flag [2] as well.
I could not find any mention of how switching to a workqueue context
affects the interpretation of REQ_NOWAIT, or whether its scope is
strictly limited to XXX_queue_rq() in the request lifecycle.

-ck

[1]

fio context =============>>

loop_queue_rq()
  loop_queue_work()
    queue_work(lo->workqueue, work);

fio ===> workqueue context

Work queue context =====>>>

loop_workfn
  loop_process_work
   loop_handle_cmd
    do_req_filebacked()

     struct request *rq = blk_mq_rq_from_pdu(cmd);

[2]

 From 03a07c92a9ed9938d828ca7f1d11b8bc63a7bb89 Mon Sep 17 00:00:00 2001
From: Goldwyn Rodrigues <rgoldwyn@suse.com>
Date: Tue, 20 Jun 2017 07:05:46 -0500
Subject: [PATCH] block: return on congested block device

A new bio operation flag REQ_NOWAIT is introduced to identify bio's
orignating from iocb with IOCB_NOWAIT. This flag indicates
to return immediately if a request cannot be made instead
of retrying.

Stacked devices such as md (the ones with make_request_fn hooks)
currently are not supported because it may block for housekeeping.
For example, an md can have a part of the device suspended.
For this reason, only request based devices are supported.
In the future, this feature will be expanded to stacked devices
by teaching them how to handle the REQ_NOWAIT flags.