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

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Message-ID: <20140821214601.GD16042@google.com>
Date:	Thu, 21 Aug 2014 17:46:01 -0400
From:	Peter Feiner <pfeiner@...gle.com>
To:	"Kirill A. Shutemov" <kirill@...temov.name>
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 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.
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