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

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Message-ID: <20140821225033.GE16042@google.com>
Date:	Thu, 21 Aug 2014 18:50:33 -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 12:51:47AM +0300, Kirill A. Shutemov wrote:
> > > 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)?

Ah, I understand now. I've got a patch in the works that disables the write
faults when a VMA is merged. I'll send a series with all of the changes
tomorrow.
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