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

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Message-ID: <20140708205448.GH17860@moon.sw.swsoft.com>
Date:	Wed, 9 Jul 2014 00:54:48 +0400
From:	Cyrill Gorcunov <gorcunov@...il.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
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 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).
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