Re: [PATCH RFC 2/2] io_uring: add support for getdents

[Clay Harris]

On Mon, Apr 24 2023 at 17:41:18 +0900, Dominique Martinet quoth thus:

> Thanks!
> 
> Clay Harris wrote on Mon, Apr 24, 2023 at 02:29:46AM -0500:
> > This also seems like a good place to bring up a point I made with
> > the last attempt at this code.  You're missing an optimization here.
> > getdents knows whether it is returning a buffer because the next entry
> > won't fit versus because there are no more entries.  As it doesn't
> > return that information, callers must always keep calling it back
> > until EOF.  This means a completely unnecessary call is made for
> > every open directory.  In other words, for a directory scan where
> > the buffers are large enough to not overflow, that literally twice
> > as many calls are made to getdents as necessary.  As io_uring is
> > in-kernel, it could use an internal interface to getdents which would
> > return an EOF indicator along with the (probably non-empty) buffer.
> > io_uring would then return that flag with the CQE.
> 
> Sorry I didn't spot that comment in the last iteration of the patch,
> that sounds interesting.
> 
> This isn't straightforward even in-kernel though: the ctx.actor callback
> (filldir64) isn't called when we're done, so we only know we couldn't
> fill in the buffer.
> We could have the callback record 'buffer full' and consider we're done
> if the buffer is full, or just single-handedly declare we are if we have
> more than `MAXNAMLEN + sizeof(struct linux_dirent64)` left over, but I
> assume a filesystem is allowed to return what it has readily available
> and expect the user to come back later?
> In which case we cannot use this as an heuristic...
> 
> So if we do this, it'll require a way for filesystems to say they're
> filling in as much as they can, or go the sledgehammer way of adding an
> extra dir_context dir_context callback, either way I'm not sure I want
> to deal with all that immediately unless I'm told all filesystems will
> fill as much as possible without ever failing for any temporary reason
> in the middle of iterate/iterate_shared().

I don't have a complete understanding of this area, but my thought was
not that we would look for any buffer full condition, but rather that
an iterator could be tested for next_entry == EOF.

> Call me greedy but I believe such a flag in the CQE could also be added
> later on without any bad side effects (as it's optional to check on it
> to stop calling early and there's no harm in not setting it)?

Certainly it could be added later, but I wanted to make sure some thought
was put into it now.  It would be nice to have it sooner rather than later
though...

> 
> > (* As an aside, the only place I've ever seen a non-zero lseek on a
> > directory, is in a very resource limited environment, e.g. too small
> > open files limit.  In the case of a depth-first directory scan, it
> > must close directories before completely reading them, and reopen /
> > lseek to their previous position in order to continue.  This scenario
> > is certainly not worth bothering with for io_uring.)
> 
> (I also thought of userspace NFS/9P servers are these two at least get
> requests from clients with an arbitrary offset, but I'll be glad to
> forget about them for now...)
> 
> -- 
> Dominique Martinet | Asmadeus