Re: [patch 2/8] x86 PAT: set VM_PFNMAP flag in vm_insert_pfn

[Nick Piggin]

On Fri, Nov 14, 2008 at 01:35:38PM -0800, Pallipadi, Venkatesh wrote:
> 
> 
> >-----Original Message-----
> >From: Nick Piggin [mailto:npiggin@xxxxxxx]
> >Sent: Thursday, November 13, 2008 6:06 PM
> >To: Pallipadi, Venkatesh
> >Cc: Ingo Molnar; Thomas Gleixner; H Peter Anvin; Hugh Dickins;
> >Roland Dreier; Jesse Barnes; Jeremy Fitzhardinge; Arjan van de
> >Ven; linux-kernel@xxxxxxxxxxxxxxx; Siddha, Suresh B
> >Subject: Re: [patch 2/8] x86 PAT: set VM_PFNMAP flag in vm_insert_pfn
> >
> >On Thu, Nov 13, 2008 at 10:47:23AM -0800, Pallipadi, Venkatesh wrote:
> >>
> >> Yes. It does. But, it calls a lower level insert_pfn()
> >function. The lower
> >> level insert_pfn() does not have any bug checks. But the higher level
> >> vm_insert_pfn() checks for PFNMAP or MIXEDMAP.
> >
> >Yes, but is there anything extra you need to check for cache aliases in
> >MIXEDMAP mappings?
> >
> 
> Yes. We need additional things to track MIXEDMAP and we are looking at that.
> But, that is slightly more trickier than the general PFNMAP case. And
> only in-tree user of MIXEDMAP is xip and that too it only uses it for
> regular WB mapping. So, we thought we should fix the more common case
> first here.
> 
> With MIXEDMAP there is no way whether to distinguish whether insert_pfn
> Or insert_page was used while looking at VMA. We can probably use PFNMAP
> in addition to MIXEDMAP to indicate that, which will make things easier.

It's difficult because it can have either method for a single VMA, and
a given address in the vma may even change over time (not with current
code in kernel AFAIKS, but AXFS eventually might get to that point).


> But, we are still looking at that and trying to understand the change
> implication.

OK: now I understand correctly. Getting PFNMAP working is an important
first step. I agree.

Thanks,
Nick
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