Re: copy on write for splice() from file to pipe?

Jens Axboe
2023-02-11T14:13:55Z

On 2/10/23 8:18?PM, Ming Lei wrote:
> On Fri, Feb 10, 2023 at 02:08:35PM -0800, Linus Torvalds wrote:
>> On Fri, Feb 10, 2023 at 1:51 PM Jens Axboe <axboe@kernel.dk> wrote:
>>>
>>> Speaking of splice/io_uring, Ming posted this today:
>>>
>>> https://lore.kernel.org/io-uring/20230210153212.733006-1-ming.lei@redhat.com/
>>
>> Ugh. Some of that is really ugly. Both 'ignore_sig' and
>> 'ack_page_consuming' just look wrong. Pure random special cases.
>>
>> And that 'ignore_sig' is particularly ugly, since the only thing that
>> sets it also sets SPLICE_F_NONBLOCK.
>>
>> And the *only* thing that actually then checks that field is
>> 'splice_from_pipe_next()', where there are exactly two
>> signal_pending() checks that it adds to, and
>>
>> (a) the first one is to protect from endless loops
>>
>> (b) the second one is irrelevant when SPLICE_F_NONBLOCK is set
>>
>> So honestly, just NAK on that series.
>>
>> I think that instead of 'ignore_sig' (which shouldn't exist), that
>> first 'signal_pending()' check in splice_from_pipe_next() should just
>> be changed into a 'fatal_signal_pending()'.
>
> Good point, here the signal is often from task_work_add() called by
> io_uring.

Usually you'd use task_sigpending() to distinguis the two, but
fatal_signal_pending() as Linus suggests would also work. The only
concern here is that since you'll be potentially blocking on waiting for
the pipe to be readable - if task does indeed have task_work pending and
that very task_work is the one that will ensure that the pipe is now
readable, then you're waiting condition will never be satisfied.

--
Jens Axboe