[PATCH] xfs: don't invalidate whole file on DAX read/write

[Christoph Hellwig]

On Wed, Aug 03, 2016 at 09:40:26AM +1000, Dave Chinner wrote:
> From: Dave Chinner <dchinner at redhat.com>
> 
> When we do DAX IO, we try to invalidate the entire page cache held
> on the file. This is incorrect as it will trash the entire mapping
> tree that now tracks dirty state in exceptional entries in the radix
> tree slots.
> 
> What we are trying to do is remove cached pages (e.g from reads
> into holes) that sit in the radix tree over the range we are about
> to write to. Hence we should just limit the invalidation to the
> range we are about to overwrite.

Looks fine (for a broad defintion of "fine"):


Reviewed-by: Christoph Hellwig <hch at lst.de>

> +	 * XXX: This is racy against mmap, and there's nothing we can do about
> +	 * it. dax_do_io() should really do this invalidation internally as
> +	 * it will know if we've allocated over a holei for this specific IO and
> +	 * if so it needs to update the mapping tree and invalidate existing
> +	 * PTEs over the newly allocated range. Remove this invalidation when
> +	 * dax_do_io() is fixed up.

FYI, I've got a basically working version of an iomap based DAX I/O path
(still fails a few corner cases), and I'll see if I can add that to it.