Re: [PATCH v5 0/2] io_uring: add support for IORING_OP_GETDENTS

[Tavian Barnes <tavianator@tavianator.com>]

On Sun, 4 Apr 2021 at 00:42, Clay Harris <bugs@claycon.org> wrote:
> On Tue, Mar 30 2021 at 14:17:21 +0300, Lennert Buytenhek quoth thus:
>
> > ...
> >
> > - Make IORING_OP_GETDENTS read from the directory's current position
> >   if the specified offset value is -1 (IORING_FEAT_RW_CUR_POS).
> >   (Requested / pointed out by Tavian Barnes.)
>
> This seems like a good feature.  As I understand it, this would
> allow submitting pairs of IORING_OP_GETDENTS with IOSQE_IO_HARDLINK
> wherein the first specifies the current offset and the second specifies
> offset -1, thereby halfing the number of kernel round trips for N getdents64.

Yep, that was my main motivation for this suggestion.

> If the entire directory fits into the first buffer, the second would
> indicate EOF.  This would certainly seem like a win, but note there
> are diminishing returns as the directory size increases, versus just
> doubling the buffer size.

True, but most directories are small, so I expect it would be a
benefit most of the time.  Even for big directories you still get two
buffers filled with one syscall, same as if you did a conventional
getdents64() with twice as big a buffer.

> An alternate / additional idea you may wish to consider is changing
> getdents64 itself.
>
> Ordinary read functions must return 0 length to indicate EOF, because
> they can return arbitrary data.  This is not the case for getdents64.
>
> 1) Define a struct linux_dirent of minimum size containing an abnormal
> value as a sentinel.  d_off = 0 or -1 should work.
>
> 2) Implement a flag for getdents64.

Sadly getdents64() doesn't take a flags argument.  We'd probably need
a new syscall.

> IF
>         the flag is set AND
>         we are returning a non-zero length buffer AND
>         there is room in the buffer for the sentinel structure AND
>         a getdents64 call using the d_off of the last struct in the
>                 buffer would return EOF
> THEN
>         append the sentinel struct to the buffer.
>
>
> Using the arrangement, we would still handle a 0 length return as an
> EOF, but if we see the sentinel struct, we can skip the additional call
> altogether.  The saves all of the pairing of buffers and extra logic,
> and unless we're unlucky and the sentinel structure did not fit in
> the buffer at EOF, would always reduce the number of getdents64
> calls by one.
>
> Moreover, if the flag was available outside of io_uring, for smaller
> directories, this feature would cut the number of directory reads
> of readdir(3) by up to half.

If we need a new syscall anyway, the calling convention could be
adjusted to indicate EOF more easily than that, e.g.

int getdents2(int fd, void *buf, size_t *size, unsigned long flags);

With 0 being EOF, 1 being not-EOF, and -1 for error, or something.

-- 
Tavian Barnes