On Thu, 15 Feb 2007, Linus Torvalds wrote:
>
> So I think that a good implementation just does everything up-front, and
> doesn't _need_ a user buffer that is live over longer periods, except for
> the actual results. Exactly because the whole alloc/teardown is nasty.
Btw, this doesn't necessarily mean "not supporting multiple atoms at all".
I think the batching of async things is potentially a great idea. I think
it's quite workable for "open+fstat" kind of things, and I agree that it
can solve other things too (the "socket+bind+connect+sendmsg+rcv" kind of
complex setup things).
But I suspect that if we just said:
- we limit these atom sequences to just linear sequences of max "n" ops
- we read them all in in a single go at startup
we actually avoid several nasty issues. Not just the memory allocation
issue in user space (now it's perfectly ok to build up a sequence of ops
in temporary memory and throw it away once it's been submitted), but also
issues like the 32-bit vs 64-bit compatibility stuff (the compat handlers
would just convert it when they do the initial copying, and then the
actual run-time wouldn't care about user-level pointers having different
sizes etc).
Would it make the interface less cool? Yeah. Would it limit it to just a
few linked system calls (to avoid memory allocation issues in the kernel)?
Yes again. But it would simplify a lot of the interface issues.
It would _also_ allow the "sys_aio_read()" function to build up its
*own* set of atoms in kernel space to actually do the read, and there
would be no impact of the actual run-time wanting to read stuff from user
space. Again - it's actually the same issue as with the compat system
call: by making the interfaces do things up-front rather than dynamically,
it becomes more static, but also easier to do interface translations. You
can translate into any arbitrary internal format _once_, and be done with
it.
I dunno.
Linus
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