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

[Ingo Molnar]

* David Hildenbrand <david@xxxxxxxxxx> wrote:

> > > > 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 ... :/

Yeah, so I think you make valid observations, i.e. your first patch is 
probably the best one.

But since it changes mm/memory.c, I'd like to pass that over to Andrew 
and the MM folks.

The x86 bits:

  Acked-by: Ingo Molnar <mingo@xxxxxxxxxx>

Thanks,

	Ingo