linux-kernel - Re: [PATCH] mm: softdirty: write protect PTEs created for read faults after VM

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Message-ID: <20140821215147.GA15482@node.dhcp.inet.fi>
Date:	Fri, 22 Aug 2014 00:51:47 +0300
From:	"Kirill A. Shutemov" <kirill@...temov.name>
To:	Peter Feiner <pfeiner@...gle.com>
Cc:	Cyrill Gorcunov <gorcunov@...il.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Pavel Emelyanov <xemul@...allels.com>,
	Jamie Liu <jamieliu@...gle.com>,
	Hugh Dickins <hughd@...gle.com>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Magnus Damm <damm@...nsource.se>
Subject: Re: [PATCH] mm: softdirty: write protect PTEs created for read
 faults after VM_SOFTDIRTY cleared

On Thu, Aug 21, 2014 at 05:46:01PM -0400, Peter Feiner wrote:
> On Fri, Aug 22, 2014 at 12:39:42AM +0300, Kirill A. Shutemov wrote:
> > On Fri, Aug 22, 2014 at 12:51:15AM +0400, Cyrill Gorcunov wrote:
> > 
> > Looks good to me.
> > 
> > Would you mind to apply the same pgprot_modify() approach on the
> > clear_refs_write(), test and post the patch?
> > 
> > Feel free to use my singed-off-by (or suggested-by if you prefer) once
> > it's tested (see merge case below).
> 
> Sure thing :-)
> 
> > One thing: there could be (I haven't checked) complications on
> > vma_merge(): since vm_flags are identical it assumes that it can reuse
> > vma->vm_page_prot of expanded vma. But VM_SOFTDIRTY is excluded from
> > vm_flags compatibility check. What should we do with vm_page_prot there?
> 
> Since the merged VMA will have VM_SOFTDIRTY set, it's OK that it's vm_page_prot
> won't be setup for write notifications. For the purpose of process migration,
> you'll just get some false positives, which is tolerable.

Right. But should we disable writenotify back to avoid exessive wp-faults
if it was enabled due to soft-dirty (the case when expanded vma is
soft-dirty)?

-- 
 Kirill A. Shutemov
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