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

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Message-ID: <20130724181516.GI8508@moon>
Date:	Wed, 24 Jul 2013 22:15:16 +0400
From:	Cyrill Gorcunov <gorcunov@...il.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	James Bottomley <James.Bottomley@...senpartnership.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 10:42:24AM -0700, Andy Lutomirski wrote:
> >
> > 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.

Good question! I rather forward it to Pavel as an author for soft dirty
bit feature. Pavel?

> 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 fear for tracking soft-dirty-bit for swapped entries we sinply have
no other place than pte (still i'm quite open for ideas, maybe there
are a better way which I've missed).

> 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.
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