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@xxxxxxxxxxxxx
