linux-kernel - Re: [PATCH v5] mm: softdirty: enable write notifications on VMAs after VM

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Message-ID: <20140826155351.GC8952@moon>
Date:	Tue, 26 Aug 2014 19:53:51 +0400
From:	Cyrill Gorcunov <gorcunov@...il.com>
To:	"Kirill A. Shutemov" <kirill@...temov.name>
Cc:	Hugh Dickins <hughd@...gle.com>, Peter Feiner <pfeiner@...gle.com>,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Pavel Emelyanov <xemul@...allels.com>,
	Jamie Liu <jamieliu@...gle.com>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Magnus Damm <magnus.damm@...il.com>
Subject: Re: [PATCH v5] mm: softdirty: enable write notifications on VMAs
 after VM_SOFTDIRTY cleared

On Tue, Aug 26, 2014 at 06:43:55PM +0300, Kirill A. Shutemov wrote:
> On Tue, Aug 26, 2014 at 07:18:13PM +0400, Cyrill Gorcunov wrote:
> > > Basically, it's safe if only soft-dirty is allowed to modify vm_flags
> > > without down_write(). But why is soft-dirty so special?
> > 
> > because how we use this bit, i mean in normal workload this bit won't
> > be used intensively i think so it's not widespread in kernel code
> 
> Weak argument to me.
> 
> What about walk through vmas twice: first with down_write() to modify
> vm_flags and vm_page_prot, then downgrade_write() and do
> walk_page_range() on every vma?

I still it's undeeded, but for sure using write-lock/downgrade won't hurt,
so no argues from my side.
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