Re: [PATCH v1] x86/mm/pat: fix VM_PAT handling in COW mappings

[David Hildenbrand]

> > > try the trivial restriction approach first, and only go with your original
> > > patch if that fails?
> >
> > Which version would you prefer, I had two alternatives (excluding comment
> > changes, white-space expected to be broken).
> >
> >
> > 1) Disallow when we would have set VM_PAT on is_cow_mapping()
> >
> > diff --git a/arch/x86/mm/pat/memtype.c b/arch/x86/mm/pat/memtype.c
> > index 0d72183b5dd0..6979912b1a5d 100644
> > --- a/arch/x86/mm/pat/memtype.c
> > +++ b/arch/x86/mm/pat/memtype.c
> > @@ -994,6 +994,9 @@ int track_pfn_remap(struct vm_area_struct *vma, pgprot_t *prot,
> >                                  && size == (vma->vm_end - vma->vm_start))) {
> >                  int ret;
> > +               if (is_cow_mapping(vma->vm_flags))
> > +                       return -EINVAL;
> > +
> >                  ret = reserve_pfn_range(paddr, size, prot, 0);
> >                  if (ret == 0 && vma)
> >                          vm_flags_set(vma, VM_PAT);
> >
> >
> > 2) Fallback to !VM_PAT
> >
> > diff --git a/arch/x86/mm/pat/memtype.c b/arch/x86/mm/pat/memtype.c
> > index 0d72183b5dd0..8e97156c9be8 100644
> > --- a/arch/x86/mm/pat/memtype.c
> > +++ b/arch/x86/mm/pat/memtype.c
> > @@ -990,8 +990,8 @@ int track_pfn_remap(struct vm_area_struct *vma, pgprot_t *prot,
> >          enum page_cache_mode pcm;
> >          /* reserve the whole chunk starting from paddr */
> > -       if (!vma || (addr == vma->vm_start
> > -                               && size == (vma->vm_end - vma->vm_start))) {
> > +       if (!vma || (!is_cow_mapping(vma->vm_flags) && addr == vma->vm_start &&
> > +                    size == (vma->vm_end - vma->vm_start))) {
> >                  int ret;
> >                  ret = reserve_pfn_range(paddr, size, prot, 0);
> >
> >
> >
> > Personally, I'd go for 2).
>
> So what's the advantage of #2? This is clearly something the user didn't
> really intend or think about much. Isn't explicitly failing that mapping a
> better option than silently downgrading it to !VM_PAT?
>
> (If I'm reading it right ...)

I think a simple mmap(MAP_PRIVATE) of /dev/mem will unconditionally fail 
with 1), while it keeps on working for 2).

Note that I think we currently set VM_PAT on each and every system if 
remap_pfn_range() will cover the whole VMA, even if pat is not actually 
enabled.

It's all a bit of a mess TBH, but I got my hands dirty enough on that.

So 1) can be rather destructive ... 2) at least somehow keeps it working.

For that reason I went with the current patch, because it's hard to tell 
which use case you will end up breaking ... :/

--
Cheers,

David / dhildenb