[Virtio-fs] [PATCH] dax: dax_layout_busy_page() should not unmap cow pages

[Boaz Harrosh]

On 05/08/2019 21:49, Vivek Goyal wrote:
> On Mon, Aug 05, 2019 at 02:53:06PM +0300, Boaz Harrosh wrote:
<>
>> So as I understand the man page:
>> fallocate(FL_PUNCH_HOLE); means user is asking to get rid also of COW pages.
>> On the other way fallocate(FL_ZERO_RANGE) only the pmem portion is zeroed and COW (private pages) stays
> 
> I tested fallocate(FL_PUNCH_HOLE) on xfs (non-dax) and it does not seem to
> get rid of COW pages and my test case still can read the data it wrote
> in private pages.
> 

It seems you are right and I am wrong. This is what the Kernel code has to say about it:

	/*
	 * Unlike in truncate_pagecache, unmap_mapping_range is called only
	 * once (before truncating pagecache), and without "even_cows" flag:
	 * hole-punching should not remove private COWed pages from the hole.
	 */

For me this is confusing but that is what it is. So remove private COWed pages
is only done when we do an setattr(ATTR_SIZE).

>>
>> Just saying I have not followed the above code path
>> (We should have an xfstest for this?)
> 
> I don't know either. It indeed is interesting to figure out what's the
> expected behavior with fallocate() and truncate() for COW pages and cover
> that using xfstest (if not already done).
> 

I could not find any test for the COW positive FL_PUNCH_HOLE (I have that bug)
could be nice to make one, and let FSs like mine fail.
Any way very nice catch.

> 
> Thanks
> Vivek
> 

Thanks
Boaz