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Date: Thu, 25 Jul 2013 11:07:19 +0400
From: Cyrill Gorcunov <gorcunov@gmail.com>
Subject: Re: [PATCH] mm: Save soft-dirty bits on swapped pages
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To: Andy Lutomirski <luto@amacapital.net>
Cc: Pavel Emelyanov <xemul@parallels.com>, James Bottomley <James.Bottomley@hansenpartnership.com>, Linux MM <linux-mm@kvack.org>, LKML <linux-kernel@vger.kernel.org>, Andrew Morton <akpm@linux-foundation.org>, Matt Mackall <mpm@selenic.com>, Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>, Marcelo Tosatti <mtosatti@redhat.com>, KOSAKI Motohiro <kosaki.motohiro@gmail.com>, Stephen Rothwell <sfr@canb.auug.org.au>

On Wed, Jul 24, 2013 at 12:40:22PM -0700, Andy Lutomirski wrote:
> 
> Hmm.  So there are at least three kinds of memory:
> 
> Anonymous pages: soft-dirty works
> Shared file-backed pages: soft-dirty does not work
> Private file-backed pages: soft-dirty works (but see below)
> 
> Perhaps another bit should be allocated to expose to userspace either
> "soft-dirty", "soft-clean", or "soft-dirty unsupported"?

> There's another possible issue with private file-backed pages, though:
> how do you distinguish clean-and-not-cowed from cowed-but-soft-clean?
> (The former will reflect changes in the underlying file, I think, but
> the latter won't.)

When fault happens with cow allocation (on write) the pte get soft dirty
bit set (the code uses pte_mkdirty(entry) in __do_fault) and until we
explicitly clean the bit it remains set. Or you mean something else?

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