Re: [x86 PAT PATCH 0/2] x86 PAT vm_flag code refactoring

[Konstantin Khlebnikov]

Suresh Siddha wrote:
> On Tue, 2012-04-03 at 10:03 +0400, Konstantin Khlebnikov wrote:
> > Suresh Siddha wrote:
> > > Konstantin,
> > >
> > > On Sat, 2012-03-31 at 21:09 +0400, Konstantin Khlebnikov wrote:
> > > > v2: Do not use batched pfn reserving for single-page VMA. This is not optimal
> > > > and breaks something, because I see glitches on the screen with i915/drm driver.
> > > > With this version glitches are gone, and I see the same regions in
> > > > /sys/kernel/debug/x86/pat_memtype_list as before patch. So, please review this
> > > > carefully, probably I'm wrong somewhere, or I have triggered some hidden bug.
> > >
> > > Actually it is not a hidden bug. In the original code, we were setting
> > > VM_PFN_AT_MMAP only for remap_pfn_range() but not for the vm_insert_pfn().
> > > Also the value of 'vm_pgoff' depends on the driver/mmap_region() in the case of
> > > vm_insert_pfn(). But with your proposed code, you were setting
> > > the VM_PAT for the single-page VMA also and end-up using wrong vm_pgoff in
> > > untrack_pfn_vma().
> >
> > But I set correct vma->vm_pgoff together with VM_PAT. But, it shouldn't work if vma is expandable...
>
> Also, I am not sure if we can override vm_pgoff in the fault handling
> path. For example, looking at unmap_mapping_range_tree() it does depend
> on the vm_pgoff value and it might break if we change the vm_pgoff in
> track_pfn_vma_new() (which gets called from vm_insert_pfn() as part of
> the i915_gem_fault()).

Yes, and we shouldn't change vma under mm->mmap_sem read-lock.

> thanks,
> suresh

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