linux-kernel - Re: 2.6.29 pat issue

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Message-ID: <49AE5039.5050105@vmware.com>
Date:	Wed, 04 Mar 2009 10:56:09 +0100
From:	Thomas Hellstrom <thellstrom@...are.com>
To:	"Pallipadi, Venkatesh" <venkatesh.pallipadi@...el.com>
CC:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Linux kernel mailing list <linux-kernel@...r.kernel.org>,
	"Siddha, Suresh B" <suresh.b.siddha@...el.com>,
	Nick Piggin <npiggin@...e.de>
Subject: Re: 2.6.29 pat issue

Hi!

Pallipadi, Venkatesh wrote:
> On Fri, Feb 06, 2009 at 01:43:08AM -0800, Thomas Hellström wrote:
>   
>> Eric W. Biederman wrote:
>>     
>>> Thomas Hellstrom <thellstrom@...are.com> writes:
>>>
>>>
>>>
>>>       
>>>> Indeed, it's crucial to keep the mappings consistent, but failure to do so is a
>>>> kernel driver bug, it should never be the result of invalid user data.
>>>>
>>>>         
>>> It easily can be.  Think of an X server mmaping frame buffers. Or other
>>> device bars.
>>>
>>>
>>>       
>> Hmm, Yes  you're right, although I'm still a bit doubtful about RAM pages.
>>
>> Wait. Now I see what's causing the problems. The code is  assuming that
>> VM_PFNMAP vmas never map RAM pages. That's also an invalid assumption.
>> See comments in mm/memory.c
>>
>> So probably the attribute check should be done for the insert_pfn path
>> of VM_MIXEDMAP as well. That's not done today.
>>
>> So there are three distinct bugs at this point:
>>
>> 1) VMAs with VM_PFNMAP are incorrectly assumed to be linear if
>> vma->vm_pgoff non-null.
>> 2) VM_PFNMAP VMA PTEs are incorrectly assumed to never point to physical
>> RAM.
>> 3) There is no check for the insert_pfn  path of vm_insert_mixed().
>>
>>     
>
> Patch below will solve (1) above.
>   

Yes, hmm, but how about remap_pfn_range() having an optimized function 
to call into directly, instead of
track_pfn_vma_new?

> About (2), Yes. we can optimize the PAT code if we use struct page to track
> PFNMAP as long at memory is backed by a struct page. It has some complications
> with refcounting the number of mappings and related things. We are actively
> looking at it. 

Cool. Still, there needs to be a check for non-io pages in vm_insert_pfn 
to avoid hitting the WARN_ON_ONCE:
+ if (!pfn_valid(pfn)) //Pfn pointing to a non-ram page.
    track_pfn_vma_new()

> About (3), vm_insert_mixed was not used by any in kernel driver,
> so, we did not add checks there, with the intention of fixing most commonly
> used remap_pfn_range and vm_insert_pfn first.
>
> Below patch should fix the regression upstream. 

Not completely, I think the above check is needed for (2) as well.

> I don't like the way we
> overloaded a bit here. But, we don't seem to see any other option.
> Nick: Do you see any cleaner way to do this?
>
> Thanks,
> Venki
>
>   

Thanks,
Thomas


> Subject: [PATCH] VM, x86 PAT: Change implementation of is_linear_pfn_mapping
>
> Use of vma->vm_pgoff to identify the pfnmaps that are fully mapped at
> mmap time is broken, as vm_pgoff can also be set when full mapping is
> not setup at mmap time.
> http://marc.info/?l=linux-kernel&m=123383810628583&w=2
>
> Change the logic to overload VM_NONLINEAR flag along with VM_PFNMAP to
> mean full mapping setup at mmap time. This distinction is needed by x86 PAT
> code.
>
> Signed-off-by: Venkatesh Pallipadi <venkatesh.pallipadi@...el.com>
> Signed-off-by: Suresh Siddha <suresh.b.siddha@...el.com>
> ---
>  include/linux/mm.h |    8 +++++++-
>  mm/memory.c        |    2 ++
>  2 files changed, 9 insertions(+), 1 deletions(-)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 065cdf8..6c3fc3a 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -127,6 +127,12 @@ extern unsigned int kobjsize(const void *objp);
>  #define VM_SPECIAL (VM_IO | VM_DONTEXPAND | VM_RESERVED | VM_PFNMAP)
>  
>  /*
> + * pfnmap vmas that are fully mapped at mmap time (not mapped on fault).
> + * Used by x86 PAT to identify such PFNMAP mappings and optimize their handling.
> + */
> +#define VM_PFNMAP_AT_MMAP (VM_NONLINEAR | VM_PFNMAP)
> +
> +/*
>   * mapping from the currently active vm_flags protection bits (the
>   * low four bits) to a page protection mask..
>   */
> @@ -145,7 +151,7 @@ extern pgprot_t protection_map[16];
>   */
>  static inline int is_linear_pfn_mapping(struct vm_area_struct *vma)
>  {
> -	return ((vma->vm_flags & VM_PFNMAP) && vma->vm_pgoff);
> +	return ((vma->vm_flags & VM_PFNMAP_AT_MMAP) == VM_PFNMAP_AT_MMAP);
>  }
>  
>  static inline int is_pfn_mapping(struct vm_area_struct *vma)
> diff --git a/mm/memory.c b/mm/memory.c
> index baa999e..457e97e 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -1671,6 +1671,7 @@ int remap_pfn_range(struct vm_area_struct *vma, unsigned long addr,
>  		return -EINVAL;
>  
>  	vma->vm_flags |= VM_IO | VM_RESERVED | VM_PFNMAP;
> +	vma->vm_flags |= VM_PFNMAP_AT_MMAP;
>  
>  	err = track_pfn_vma_new(vma, &prot, pfn, PAGE_ALIGN(size));
>  	if (err) {
> @@ -1679,6 +1680,7 @@ int remap_pfn_range(struct vm_area_struct *vma, unsigned long addr,
>  		 * needed from higher level routine calling unmap_vmas
>  		 */
>  		vma->vm_flags &= ~(VM_IO | VM_RESERVED | VM_PFNMAP);
> +		vma->vm_flags &= ~VM_PFNMAP_AT_MMAP;
>  		return -EINVAL;
>  	}
>  
>   

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