linux-kernel - Re: [PATCH 1/1] mm: pagemap: limit scan to virtual region being asked

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Date:	Wed, 11 Feb 2015 23:16:18 +0000
From:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	Shiraz Hashim <shashim@...eaurora.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"oleg@...hat.com" <oleg@...hat.com>,
	"gorcunov@...nvz.org" <gorcunov@...nvz.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/1] mm: pagemap: limit scan to virtual region being
 asked

On Wed, Feb 11, 2015 at 02:09:15PM -0800, Andrew Morton wrote:
> On Wed, 14 Jan 2015 01:08:40 +0000 Naoya Horiguchi <n-horiguchi@...jp.nec.com> wrote:
> 
> > On Tue, Jan 13, 2015 at 05:57:04PM +0530, Shiraz Hashim wrote:
> > > pagemap_read scans through the virtual address space of a
> > > task till it prepares 'count' pagemaps or it reaches end
> > > of task.
> > > 
> > > This presents a problem when the page walk doesn't happen
> > > for vma with VM_PFNMAP set. In which case walk is silently
> > > skipped and no pagemap is prepare, in turn making
> > > pagemap_read to scan through task end, even crossing beyond
> > > 'count', landing into a different vma region. This leads to
> > > wrong presentation of mappings for that vma.
> > > 
> > > Fix this by limiting end_vaddr to the end of the virtual
> > > address region being scanned.
> > > 
> > > Signed-off-by: Shiraz Hashim <shashim@...eaurora.org>
> > 
> > This patch works in some case, but there still seems a problem in another case.
> > 
> > Consider that we have two vmas within some narrow (PAGEMAP_WALK_SIZE) region.
> > One vma in lower address is VM_PFNMAP, and the other vma in higher address is not.
> > Then a single call of walk_page_range() skips the first vma and scans the
> > second vma, but the pagemap record of the second vma will be stored on the
> > wrong offset in the buffer, because we just skip vma(VM_PFNMAP) without calling
> > any callbacks (within which add_to_pagemap() increments pm.pos).
> > 
> > So calling pte_hole() for vma(VM_PFNMAP) looks a better fix to me.
> > 
> 
> Can we get this finished off?  ASAP, please.

Yes, I think so.
The patch "mm: pagewalk: call pte_hole() for VM_PFNMAP during walk_page_range"
(replacing this patch) is already in mainline.

Thanks,
Naoya Horiguchi--
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