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

["Pallipadi, Venkatesh" <venkatesh.pallipadi@intel.com>]

 

>-----Original Message-----
>From: linux-kernel-owner@vger.kernel.org 
>[mailto:linux-kernel-owner@vger.kernel.org] On Behalf Of Ingo Molnar
>Sent: Tuesday, November 18, 2008 1:38 PM
>To: Nick Piggin
>Cc: Pallipadi, Venkatesh; Thomas Gleixner; H Peter Anvin; Hugh 
>Dickins; Roland Dreier; Jesse Barnes; Jeremy Fitzhardinge; 
>Arjan van de Ven; linux-kernel@vger.kernel.org; Siddha, Suresh B
>Subject: Re: [patch 2/8] x86 PAT: set VM_PFNMAP flag in vm_insert_pfn
>
>
>* Nick Piggin <npiggin@suse.de> wrote:
>
>> On Fri, Nov 14, 2008 at 01:35:38PM -0800, Pallipadi, Venkatesh wrote:
>> > 
>> > 
>> > >-----Original Message-----
>> > >From: Nick Piggin [mailto:npiggin@suse.de]
>> > >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@vger.kernel.org; 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.
>
>Venki, a patch logistics sidenote: the final mm/* bits of this 
>patchset need acks from MM folks - Andrew, Nick or Hugh - we cannot 
>just queue them up in the x86/pat tree without agreement from MM 
>maintainers.
>

OK. Feedback based on the discussions on this patchset, this particular patch
set VM_PFNMAP flag in vm_insert_pfn
is not really needed and we should rather BUG_ON on that not being set. 
And there were few more comments on one other generic vm patch from Nick.

I will resend the entire patchset with changes soon and then we can work
out the logistics.

Thanks,
Venki