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@xxxxxxxxx>
>> ---
>> 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
