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

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Date:	Wed, 24 Jul 2013 10:42:24 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	James Bottomley <James.Bottomley@...senpartnership.com>
Cc:	Cyrill Gorcunov <gorcunov@...il.com>,
	Linux MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Pavel Emelyanov <xemul@...allels.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Matt Mackall <mpm@...enic.com>,
	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...il.com>,
	Stephen Rothwell <sfr@...b.auug.org.au>
Subject: Re: [PATCH] mm: Save soft-dirty bits on swapped pages

On Wed, Jul 24, 2013 at 10:36 AM, James Bottomley
<James.Bottomley@...senpartnership.com> wrote:
> On Wed, 2013-07-24 at 21:17 +0400, Cyrill Gorcunov wrote:
>> On Wed, Jul 24, 2013 at 10:06:53AM -0700, Andy Lutomirski wrote:
>> > > Hi Andy, if I understand you correctly "file-backed pages" are carried
>> > > in pte with _PAGE_FILE bit set and the swap soft-dirty bit won't be
>> > > used on them but _PAGE_SOFT_DIRTY will be set on write if only I've
>> > > not missed something obvious (Pavel?).
>> >
>> > If I understand this stuff correctly, the vmscan code calls
>> > try_to_unmap when it reclaims memory, which makes its way into
>> > try_to_unmap_one, which clears the pte (and loses the soft-dirty bit).
>>
>> Indeed, I was so stareing into swap that forgot about files. I'll do
>> a separate patch for that, thanks!
>
> Lets just be clear about the problem first: the vmscan pass referred to
> above happens only on clean pages, so the soft dirty bit could only be
> set if the page was previously dirty and got written back.  Now it's an
> exercise for the reader whether we want to reinstantiate a cleaned
> evicted page for the purpose of doing an iterative migration or whether
> we want to flip the page in the migrated entity to be evicted (so if it
> gets referred to, it pulls in an up to date copy) ... assuming the
> backing file also gets transferred, of course.

I think I understand your distinction.  Nonetheless, given the loss of
the soft-dirty bit, the migration tool could fail to notice that the
pages was dirtied and subsequently cleaned and evicted.  I'm
unconvinced that doing this on a per-PTE basis is the right way,
though.

I've long wanted a feature to efficiently see what changed on a
filesystem by comparing, say, a hash tree.  NTFS can do this (sort
of), but I don't think that anything else can.  I think that btrfs
should be able to, but there's no API that I've ever seen.

--Andy
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