From: Boaz Harrosh <boaz@plexistor.com>
To: Vivek Goyal <vgoyal@redhat.com>, Boaz Harrosh <boaz@plexistor.com>
Cc: linux-fsdevel <linux-fsdevel@vger.kernel.org>,
virtio-fs@redhat.com, linux-nvdimm <linux-nvdimm@lists.01.org>
Subject: Re: [PATCH] dax: dax_layout_busy_page() should not unmap cow pages
Date: Mon, 5 Aug 2019 22:16:45 +0300 [thread overview]
Message-ID: <9c0ec951-01e7-7ae0-2d69-1b26f3450d65@plexistor.com> (raw)
In-Reply-To: <20190805184951.GC13994@redhat.com>
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
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next prev parent reply other threads:[~2019-08-05 19:19 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-08-02 19:29 [PATCH] dax: dax_layout_busy_page() should not unmap cow pages Vivek Goyal
2019-08-02 19:37 ` Dan Williams
2019-08-05 11:53 ` Boaz Harrosh
2019-08-05 18:49 ` Vivek Goyal
2019-08-05 19:16 ` Boaz Harrosh [this message]
2019-08-05 20:11 ` Dan Williams
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