Re: [RFC] fixed files

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

On 2/8/2020 11:15 PM, Jens Axboe wrote:
> On 2/8/20 6:28 AM, Pavel Begunkov wrote:
>> Hi,
>>
>> As you remember, splice(2) needs two fds, and it's a bit of a pain
>> finding a place for the second REQ_F_FIXED_FILE flag. So, I was
>> thinking, can we use the last (i.e. sign) bit to mark an fd as fixed? A
>> lot of userspace programs consider any negative result of open() as an
>> error, so it's more or less safe to reuse it.
>>
>> e.g.
>> fill_sqe(fd) // is not fixed
>> fill_sqe(buf_idx | LAST_BIT) // fixed file
> 
> Right now we only support 1024 fixed buffers anyway, so we do have some
> space there. If we steal a bit, it'll still allow us to expand to 32K of
> fixed buffers in the future.
> 
> It's a bit iffy, but like you, I don't immediately see a better way to
> do this that doesn't include stealing an IOSQE bit or adding a special
> splice flag for it. Might still prefer the latter, to be honest...

"fixed" is clearly a per-{fd,buffer} attribute. If I'd now design it
from the scratch, I would store fixed-resource index in the same field
as fds and addr (but not separate @buf_index), and have per-resource
switch-flag somewhere. And then I see 2 convenient ways:

1. encode the fixed bit into addr and fd, as supposed above.

2. Add N generic IOSQE_FIXED bits (i.e. IOSQE_FIXED_RESOURSE{1,2,...}),
which correspond to resources (fd, buffer, etc) in order of occurrence
in an sqe. I wouldn't expect having more than 3-4 flags.

And then IORING_OP_{READ,WRITE}_FIXED would have been the same opcode as
the corresponding non-fixed version. But backward-compatibility is a pain.

-- 
Pavel Begunkov