Re: [PATCH 0/2] iomap: Waiting for IO in iomap_dio_rw()

[Dave Chinner <david@fromorbit.com>]

On Wed, Oct 09, 2019 at 10:41:24PM +0200, Jan Kara wrote:
> Hello,
> 
> when doing the ext4 conversion of direct IO code to iomap, we found it very
> difficult to handle inode extension with what iomap code currently provides.
> Ext4 wants to do inode extension as sync IO (so that the whole duration of
> IO is protected by inode->i_rwsem), also we need to truncate blocks beyond
> end of file in case of error or short write. Now in ->end_io handler we don't
> have the information how long originally the write was (to judge whether we
> may have allocated more blocks than we actually used) and in ->write_iter
> we don't know whether / how much of the IO actually succeeded in case of AIO.
> 
> Thinking about it for some time I think iomap code makes it unnecessarily
> complex for the filesystem in case it decides it doesn't want to perform AIO
> and wants to fall back to good old synchronous IO. In such case it is much
> easier for the filesystem if it just gets normal error return from
> iomap_dio_rw() and not just -EIOCBQUEUED.

Yeah, that'd be nice. :)

> The first patch in the series adds argument to iomap_dio_rw() to wait for IO
> completion (internally iomap_dio_rw() already supports this!) and the second
> patch converts XFS waiting for unaligned DIO write to this new API.
> 
> What do people think?

I've just caught up on the ext4 iomap dio thread where this came up,
so I have some idea of what is going on now :)

My main issue is that I don't like the idea of a "force_wait"
parameter to iomap_dio_rw() that overrides what the kiocb says to
do inside iomap_dio_rw(). It just seems ... clunky.

I'd much prefer that the entire sync/async IO decision is done in
one spot, and the result of that is passed into iomap_dio_rw(). i.e.
the caller always determines the behaviour.

That would mean the callers need to do something like this by
default:

	ret = iomap_dio_rw(iocb, iter, ops, dops, is_sync_kiocb(iocb));

And filesystems like XFS will need to do:

	ret = iomap_dio_rw(iocb, iter, ops, dops,
			is_sync_kiocb(iocb) || unaligned);

and ext4 will calculate the parameter in whatever way it needs to.

In fact, it may be that a wrapper function is better for existing
callers:

static inline ssize_t iomap_dio_rw()
{
	return iomap_dio_rw_wait(iocb, iter, ops, dops, is_sync_kiocb(iocb));
}

And XFS/ext4 writes call iomap_dio_rw_wait() directly. That way we
don't need to change the read code at all...

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com