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

Dave Chinner
2021-01-12T23:30:20Z

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.

We are trying to avoid external sub-block IO outside the range of
the user data IO (COW, sub-block zeroing, etc) so that we don't
trash adjacent sub-block IO in flight. This means we can't do
sub-block zeroing and that then means we can't map unwritten extents
or allocate new extents for the sub-block IO. It also means the IO
range cannot span EOF because that triggers unconditional sub-block
zeroing in iomap_dio_rw_actor().

And because we may have to map multiple extents to fully span an IO
range, we have to guarantee that subsequent extents for the IO are
also written otherwise we have a partial write abort case. Hence we
have single extent limitations as well.

So "no allocation" really doesn't describe what we want this flag to
at all.

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.

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).

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.

Cheers,

Dave.
--
Dave Chinner
david@fromorbit.com