linux-kernel - Re: [FYI] tux3: Core changes

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Date:	Tue, 26 May 2015 14:49:55 -0700
From:	Daniel Phillips <daniel@...nq.net>
To:	Rik van Riel <riel@...hat.com>, Jan Kara <jack@...e.cz>
CC:	David Lang <david@...g.hm>, tux3@...3.org,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
Subject: Re: [FYI] tux3: Core changes

On 05/26/2015 02:36 PM, Rik van Riel wrote:
> On 05/26/2015 04:22 PM, Daniel Phillips wrote:
>> On 05/26/2015 02:00 AM, Jan Kara wrote:
>>> So my opinion is: Don't fork the page if page_count is elevated. You can
>>> just wait for the IO if you need stable pages in that case. It's slow but
>>> it's safe and it should be pretty rare. Is there any problem with that?
>>
>> That would be our fallback if anybody discovers a specific case where page
>> fork breaks something, which so far has not been demonstrated.
>>
>> With a known fallback, it is hard to see why we should delay merging over
>> that. Perfection has never been a requirement for merging filesystems. On
> 
> However, avoiding data corruption by erring on the side of safety is
> a pretty basic requirement.

Erring on the side of safety is still an error. As a community we have
never been fond of adding code or overhead to fix theoretical bugs. I
do not see why we should relax that principle now.

We can fix actual bugs, but theoretical bugs are only shapeless specters
passing in the night. We should not become frozen in fear of them.

>> the contrary, imperfection is a reason for merging, so that the many
>> eyeballs effect may prove its value.
> 
> If you skip the page fork when there is an elevated page count, tux3
> should be safe (at least from that aspect). Only do the COW when there
> is no "strange" use of the page going on.

Then you break the I in ACID. There must be a compelling reason to do
that.

Regards,

Daniel


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