Re: [PATCH 09/10] iomap: add a IOMAP_DIO_NOALLOC flag

[Christoph Hellwig <hch@lst.de>]

On Wed, Jan 13, 2021 at 10:29:23AM +1100, Dave Chinner wrote:
> On Tue, Jan 12, 2021 at 05:26:15PM +0100, Christoph Hellwig wrote:
> > Add a flag to request that the iomap instances do not allocate blocks
> > by translating it to another new IOMAP_NOALLOC flag.
> 
> Except "no allocation" that is not what XFS needs for concurrent
> sub-block DIO.

Well, this is just a quick draft.  I could not come up with a better
name, so I picked on that explains most but not all of what is going
on.

> If we're going to use a flag for this specific functionality, let's
> call it what it is: IOMAP_DIO_UNALIGNED/IOMAP_UNALIGNED and do two
> things with it.

Sounds fine with me.

> 
> 	1. Make unaligned IO a formal part of the iomap_dio_rw()
> 	behaviour so it can do the common checks to for things that
> 	need exclusive serialisation for unaligned IO (i.e. avoid IO
> 	spanning EOF, abort if there are cached pages over the
> 	range, etc).

Note that these all writes already fall back to buffered I/O if
invalidate_inode_pages2_range fails, so there must never be cached
pages for direct I/O these days.  This is different from NOWAIT
I/O where we simply give up if there are any cached pages and don't
even try to invalidate them.

> 	2. require the filesystem mapping callback do only allow
> 	unaligned IO into ranges that are contiguous and don't
> 	require mapping state changes or sub-block zeroing to be
> 	performed during the sub-block IO.

Yeah.