Re: [RFC] fixed files

[Jens Axboe <axboe@kernel.dk>]

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...

-- 
Jens Axboe