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

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Date:	Tue, 8 Jul 2014 14:05:01 -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 00:54:48 +0400 Cyrill Gorcunov <gorcunov@...il.com> wrote:

> On Tue, Jul 08, 2014 at 01:45:11PM -0700, Andrew Morton wrote:
> > 
> > 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?
> 
> Ah, I see. Yes, the softdirty bit might be lost that usespace program
> won't see that a page was modified. So data lose is possible.
> 
> > 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.
> 
> Here is updated changelog, sounds better?
> ---
> 
> In case if page fault happend on dirty filemapping the newly created pte
> may loose softdirty bit thus if a userspace program is tracking memory
> changes with help of a memory tracker (CONFIG_MEM_SOFT_DIRTY) it might
> miss modification of a memory page (which in worts case may lead to
> data inconsistency).

Much better, thanks.

It's a rather gross-looking bug and data inconsistency sounds serious. 
Do you think a -stable backport is needed?  

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