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

[David Hildenbrand <david@redhat.com>]

>>> 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