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

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Message-ID: <654d5c75-72fa-bfab-dc14-fa923a2a815a@gmail.com>
Date:   Wed, 8 Sep 2021 14:07:57 +0100
From:   Pavel Begunkov <asml.silence@...il.com>
To:     Jens Axboe <axboe@...nel.dk>, Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org
Cc:     io-uring@...r.kernel.org
Subject: Re: [PATCH] /dev/mem: nowait zero/null ops

On 9/8/21 1:57 PM, Jens Axboe wrote:
> 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@...il.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.

It can be clearing gigabytes in one go. Won't it be too much of a
delay when nowait is expected?
 
>>  	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.
> 

-- 
Pavel Begunkov