linux-kernel - RE: [RFC V2] mm:change smaps/pagemap

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Date:	Fri, 30 Jan 2015 17:15:08 +0800
From:	"Wang, Yalin" <Yalin.Wang@...ymobile.com>
To:	'Naoya Horiguchi' <n-horiguchi@...jp.nec.com>
CC:	"'akpm@...ux-foundation.org'" <akpm@...ux-foundation.org>,
	"'kirill.shutemov@...ux.intel.com'" <kirill.shutemov@...ux.intel.com>,
	"'oleg@...hat.com'" <oleg@...hat.com>,
	"'gorcunov@...nvz.org'" <gorcunov@...nvz.org>,
	"'pfeiner@...gle.com'" <pfeiner@...gle.com>,
	"'aquini@...hat.com'" <aquini@...hat.com>,
	"'linux-kernel@...r.kernel.org'" <linux-kernel@...r.kernel.org>
Subject: RE: [RFC V2] mm:change smaps/pagemap_read calculation behavior

> -----Original Message-----
> From: Naoya Horiguchi [mailto:n-horiguchi@...jp.nec.com]
> Sent: Friday, January 30, 2015 4:24 PM
> To: Wang, Yalin
> Cc: 'akpm@...ux-foundation.org'; 'kirill.shutemov@...ux.intel.com';
> 'oleg@...hat.com'; 'gorcunov@...nvz.org'; 'pfeiner@...gle.com';
> 'aquini@...hat.com'; 'linux-kernel@...r.kernel.org'
> Subject: Re: [RFC V2] mm:change smaps/pagemap_read calculation behavior
> 
> On Fri, Jan 30, 2015 at 03:47:54PM +0800, Wang, Yalin wrote:
> > This patch change smaps/pagemap_read pagetable walk behavior, to make
> > sure not skip VM_PFNMAP pagetables,
> > so that we can calculate COW pages of VM_PFNMAP as normal pages.
> >
> > Signed-off-by: Yalin Wang <yalin.wang@...ymobile.com>
> 
> Hi Yalin,
> 
> The original motivation of the VM_PFNMAP code in pagewalk.c comes from the
> following patch:
> 
>   commit a9ff785e4437c83d2179161e012f5bdfbd6381f0
>   Author: Cliff Wickman <cpw@....com>
>   Date:   Fri May 24 15:55:36 2013 -0700
> 
>       mm/pagewalk.c: walk_page_range should avoid VM_PFNMAP areas
> 
> , where Cliff stated that some kind of vma(VM_PFNMAP) caused kernel panic
> when walk_page_range() was called over it. So I don't think that re-
> enabling
> to walk over every vma(VM_PFNMAP) unexceptionally is a good idea.
> 
> If you really want to get some information from a vma(VM_PFNMAP) via these
> interfaces, I recommend you to implement proper judging code which returns
> 0 for your vma(VM_PFNMAP) and returns 1 for Cliff's vma(VM_PFNMAP).
> 
I see, but I am curious that why kernel panic when I just
access process pagetables in page_table_walk()?
Is it caused by hardware problem?

The reason that I want to enable it is to see some drivers map some COW pages 
With VM_PFNMAP, so that user space can get correct page allocation info for 
These COW special pages with VM_PFNMAP flag.

Thanks 

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