linux-kernel - Re: [patch 1/2] [PATCH] mm: Save soft-dirty bits on swapped pages

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Date:	Wed, 7 Aug 2013 13:21:56 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Cyrill Gorcunov <gorcunov@...il.com>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	luto@...capital.net, gorcunov@...nvz.org, xemul@...allels.com,
	mpm@...enic.com, xiaoguangrong@...ux.vnet.ibm.com,
	mtosatti@...hat.com, kosaki.motohiro@...il.com,
	sfr@...b.auug.org.au, peterz@...radead.org,
	aneesh.kumar@...ux.vnet.ibm.com
Subject: Re: [patch 1/2] [PATCH] mm: Save soft-dirty bits on swapped pages

On Wed, 31 Jul 2013 00:41:55 +0400 Cyrill Gorcunov <gorcunov@...il.com> wrote:

> Andy Lutomirski reported that in case if a page with _PAGE_SOFT_DIRTY
> bit set get swapped out, the bit is getting lost and no longer
> available when pte read back.
> 
> To resolve this we introduce _PTE_SWP_SOFT_DIRTY bit which is
> saved in pte entry for the page being swapped out. When such page
> is to be read back from a swap cache we check for bit presence
> and if it's there we clear it and restore the former _PAGE_SOFT_DIRTY
> bit back.
> 
> One of the problem was to find a place in pte entry where we can
> save the _PTE_SWP_SOFT_DIRTY bit while page is in swap. The
> _PAGE_PSE was chosen for that, it doesn't intersect with swap
> entry format stored in pte.

So the implication is that if another architecture wants to support
this (and, realistically, wants to support CRIU), that architecture
must find a spare pte bit to implement _PTE_SWP_SOFT_DIRTY.  Yes?

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