linux-kernel - Re: [PATCH] mm: Don't forget to set softdirty on file mapped fault

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Date:	Tue, 8 Jul 2014 13:45:11 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Cyrill Gorcunov <gorcunov@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>, Linux MM <linux-mm@...ck.org>,
	Pavel Emelyanov <xemul@...allels.com>
Subject: Re: [PATCH] mm: Don't forget to set softdirty on file mapped fault

On Wed, 9 Jul 2014 00:40:17 +0400 Cyrill Gorcunov <gorcunov@...il.com> wrote:

> On Tue, Jul 08, 2014 at 01:19:20PM -0700, Andrew Morton wrote:
> > On Tue, 8 Jul 2014 23:21:51 +0400 Cyrill Gorcunov <gorcunov@...il.com> wrote:
> > 
> > > Otherwise we may not notice that pte was softdirty because pte_mksoft_dirty
> > > helper _returns_ new pte but not modifies argument.
> > 
> > When fixing a bug, please describe the end-user visible effects of that
> > bug.
> > 
> > [for the 12,000th time :(]
> 
> "we may not notice that pte was softdirty" I thought it's enough, because
> that's the effect user sees -- pte is not dirtified where it should.
> 
> Really sorry Andrew if I were not clear enough. What about: In case if page
> fault happend on dirty filemapping the newly created pte may not
> notice if old one were already softdirtified because pte_mksoft_dirty
> doesn't modify its argument but rather returns new pte value.

The user doesn't know or care about pte bits.

What actually *happens*?  Does criu migration hang?  Does it lose data?
Does it take longer?

IOW, what would an end-user's bug report look like?

It's important to think this way because a year from now some person
we've never heard of may be looking at a user's bug report and
wondering whether backporting this patch will fix it.  Amongst other
reasons.
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