Re: Adding read_exact and write_all OPs?

[Pavel Begunkov <asml.silence@gmail.com>]

On 8/16/22 19:46, Stefan Metzmacher wrote:
> Hi Jens,
> 
>>> In application code it's quite common to write/read a whole buffer and
>>> only then continue task execution. The traditional approach is to wrap
>>> read/write sycall/OP in loop, which is often done as part of a
>>> language std. In synchronous context it makes sense because it allows
>>> to process things like EINTR. But in asynchronous (event-driven)
>>> context I think it makes a bit less sense.
>>>
>>> What do you think about potential addition of OPs like read_exact and
>>> write_all, i.e. OPs which on successful CQE guarantee that an input
>>> buffer was processed completely? They would allow to simplify user
>>> code and in some cases to significantly reduce ring traffic.
>>
>> That may make sense, and there's some precedence there for sockets with
>> the WAITALL flags.
> 
> I remember we got short reads/writes from some early kernel versions
> and had to add a retry in samba.
> 
> But don't we already have a retry in current kernels?

Right, both short reads and writes will be retried when possible. Not
all file types are covered but fs and block should be fine. We also try
to handle short send/recv.

> At least for the need_complete_io() case?
> io_rw_should_reissue() also seem to be related and also handles S_ISREG,
> but it's hidden behind CONFIG_BLOCK.
> 
> Are there really systems without CONFIG_BLOCK?

-- 
Pavel Begunkov