Re: [PATCH] /dev/mem: nowait zero/null ops

[Jens Axboe <axboe@kernel.dk>]

On 9/8/21 4:06 AM, Pavel Begunkov wrote:
> Make read_iter_zero() to honor IOCB_NOWAIT, so /dev/zero can be
> advertised as FMODE_NOWAIT. This helps subsystems like io_uring to use
> it more effectively. Set FMODE_NOWAIT for /dev/null as well, it never
> waits and therefore trivially meets the criteria.
> 
> Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
> ---
>  drivers/char/mem.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/char/mem.c b/drivers/char/mem.c
> index 1c596b5cdb27..531f144d7132 100644
> --- a/drivers/char/mem.c
> +++ b/drivers/char/mem.c
> @@ -495,6 +495,8 @@ static ssize_t read_iter_zero(struct kiocb *iocb, struct iov_iter *iter)
>  		written += n;
>  		if (signal_pending(current))
>  			return written ? written : -ERESTARTSYS;
> +		if (iocb->ki_flags & IOCB_NOWAIT)
> +			return written ? written : -EAGAIN;
>  		cond_resched();
>  	}

I don't think this part is needed.

>  	return written;
> @@ -696,11 +698,11 @@ static const struct memdev {
>  #ifdef CONFIG_DEVMEM
>  	 [DEVMEM_MINOR] = { "mem", 0, &mem_fops, FMODE_UNSIGNED_OFFSET },
>  #endif
> -	 [3] = { "null", 0666, &null_fops, 0 },
> +	 [3] = { "null", 0666, &null_fops, FMODE_NOWAIT },
>  #ifdef CONFIG_DEVPORT
>  	 [4] = { "port", 0, &port_fops, 0 },
>  #endif
> -	 [5] = { "zero", 0666, &zero_fops, 0 },
> +	 [5] = { "zero", 0666, &zero_fops, FMODE_NOWAIT },
>  	 [7] = { "full", 0666, &full_fops, 0 },
>  	 [8] = { "random", 0666, &random_fops, 0 },
>  	 [9] = { "urandom", 0666, &urandom_fops, 0 },
> 

This looks fine.

-- 
Jens Axboe