linux-kernel - Re: [PATCH v3] mm/pagewalk.c: walk_page_range should avoid VM

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Message-ID: <1368625360-cebir27q-mutt-n-horiguchi@ah.jp.nec.com>
Date:	Wed, 15 May 2013 09:42:40 -0400
From:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
To:	Cliff Wickman <cpw@....com>
Cc:	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	mgorman@...e.de, aarcange@...hat.com, dave.hansen@...el.com,
	dsterba@...e.cz, hannes@...xchg.org, kosaki.motohiro@...il.com,
	kirill.shutemov@...ux.intel.com, mpm@...enic.com,
	rdunlap@...radead.org
Subject: Re: [PATCH v3] mm/pagewalk.c: walk_page_range should avoid VM_PFNMAP
 areas

On Wed, May 15, 2013 at 07:46:36AM -0500, Cliff Wickman wrote:
> 
> /proc/<pid>/smaps and similar walks through a user page table should not
> be looking at VM_PFNMAP areas.
> 
> v2: 
> - moves the VM_BUG_ON out of the loop
> - adds the needed test for  vma->vm_start <= addr
> 
> v3 adds comments to make this clearer, as N. Horiguchi recommends:
>   > I recommend that you check VM_PFNMAP in the possible callers' side.
>   > But this patch seems to solve your problem, so with properly commenting
>   > this somewhere, I do not oppose it.
> 
> Certain tests in walk_page_range() (specifically split_huge_page_pmd())
> assume that all the mapped PFN's are backed with page structures. And this is
> not usually true for VM_PFNMAP areas. This can result in panics on kernel
> page faults when attempting to address those page structures.
> 
> There are a half dozen callers of walk_page_range() that walk through
> a task's entire page table (as N. Horiguchi pointed out). So rather than
> change all of them, this patch changes just walk_page_range() to ignore 
> VM_PFNMAP areas.
> 
> The logic of hugetlb_vma() is moved back into walk_page_range(), as we
> want to test any vma in the range.
> 
> VM_PFNMAP areas are used by:
> - graphics memory manager   gpu/drm/drm_gem.c
> - global reference unit     sgi-gru/grufile.c
> - sgi special memory        char/mspec.c
> - and probably several out-of-tree modules
> 
> I'm copying everyone who has changed this file recently, in case
> there is some reason that I am not aware of to provide
> /proc/<pid>/smaps|clear_refs|maps|numa_maps for these VM_PFNMAP areas.
> 
> Signed-off-by: Cliff Wickman <cpw@....com>

Thank you.

Reviewed-by: Naoya Horiguchi <n-horiguchi@...jp.nec.com>
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