Re: [PATCH 3/5] io_uring: add support for getdents

[Christian Brauner <brauner@kernel.org>]

On Mon, Jul 31, 2023 at 11:58:50AM +1000, Dave Chinner wrote:
> On Thu, Jul 27, 2023 at 06:28:52PM +0200, Christian Brauner wrote:
> > On Thu, Jul 27, 2023 at 05:17:30PM +0100, Pavel Begunkov wrote:
> > > On 7/27/23 16:52, Christian Brauner wrote:
> > > > On Thu, Jul 27, 2023 at 04:12:12PM +0100, Pavel Begunkov wrote:
> > > > It would also solve it for writes which is what my kiocb_modified()
> > > > comment was about. So right now you have:
> > > 
> > > Great, I assumed there are stricter requirements for mtime not
> > > transiently failing.
> > 
> > But I mean then wouldn't this already be a problem today?
> > kiocb_modified() can error out with EAGAIN today:
> > 
> >           ret = inode_needs_update_time(inode, &now);
> >           if (ret <= 0)
> >                   return ret;
> >           if (flags & IOCB_NOWAIT)
> >                   return -EAGAIN;
> > 
> >           return __file_update_time(file, &now, ret);
> > 
> > the thing is that it doesn't matter for ->write_iter() - for xfs at
> > least - because xfs does it as part of preparatory checks before
> > actually doing any real work. The problem happens when you do actual
> > work and afterwards call kiocb_modified(). That's why I think (2) is
> > preferable.
> 
> This has nothing to do with what "XFS does". It's actually an
> IOCB_NOWAIT API design constraint.
> 
> That is, IOCB_NOWAIT means "complete the whole operation without
> blocking or return -EAGAIN having done nothing".  If we have to do
> something that might block (like a timestamp update) then we need to
> punt the entire operation before anything has been modified.  This
> requires all the "do we need to modify this" checks to be done up
> front before we start modifying anything.
> 
> So while it looks like this might be "an XFS thing", that's because
> XFS tends to be the first filesystem that most io_uring NOWAIT
> functionality is implemented on. IOWs, what you see is XFS is doing
> things the way IOCB_NOWAIT requires to be done. i.e. it's a
> demonstration of how nonblocking filesystem modification operations
> need to be run, not an "XFS thing"...

Yes, I'm aware. I was trying to pay xfs a compliment for that but
somehow that didn't come through.

> 
> > > > I would prefer 2) which seems cleaner to me. But I might miss why this
> > > > won't work. So input needed/wanted.
> > > 
> > > Maybe I didn't fully grasp the (2) idea
> > > 
> > > 2.1: all read_iter, write_iter, etc. callbacks should do file_accessed()
> > > before doing IO, which sounds like a good option if everyone agrees with
> > > that. Taking a look at direct block io, it's already like this.
> > 
> > Yes, that's what I'm talking about. I'm asking whether that's ok for xfs
> > maintainers basically. i_op->write_iter() already works like that since
> > the dawn of time but i_op->read_iter doesn't and I'm proposing to make
> > it work like that and wondering if there's any issues I'm unaware of.
> 
> XFS already calls file_accessed() in the DIO read path before the
> read gets issued. I don't see any problem with lifting it to before
> the copy-out loop in filemap_read() because it is run regardless of
> whether any data is read or any error occurred.  Hence it just
> doesn't look like it matters if it is run before or after the
> copy-out loop to me....

Great.