Re: [PATCH v2 2/2] drm/ttm: Fix vm page protection handling

["Thomas Hellström (VMware)" <thomas_os@shipmail.org>]

On 12/4/19 3:42 PM, Michal Hocko wrote:
> On Wed 04-12-19 15:36:58, Thomas Hellström (VMware) wrote:
>> On 12/4/19 3:35 PM, Michal Hocko wrote:
>>> On Wed 04-12-19 15:16:09, Thomas Hellström (VMware) wrote:
>>>> On 12/4/19 2:52 PM, Michal Hocko wrote:
>>>>> On Tue 03-12-19 11:48:53, Thomas Hellström (VMware) wrote:
>>>>>> From: Thomas Hellstrom <thellstrom@vmware.com>
>>>>>>
>>>>>> TTM graphics buffer objects may, transparently to user-space,  move
>>>>>> between IO and system memory. When that happens, all PTEs pointing to the
>>>>>> old location are zapped before the move and then faulted in again if
>>>>>> needed. When that happens, the page protection caching mode- and
>>>>>> encryption bits may change and be different from those of
>>>>>> struct vm_area_struct::vm_page_prot.
>>>>>>
>>>>>> We were using an ugly hack to set the page protection correctly.
>>>>>> Fix that and instead use vmf_insert_mixed_prot() and / or
>>>>>> vmf_insert_pfn_prot().
>>>>>> Also get the default page protection from
>>>>>> struct vm_area_struct::vm_page_prot rather than using vm_get_page_prot().
>>>>>> This way we catch modifications done by the vm system for drivers that
>>>>>> want write-notification.
>>>>> So essentially this should have any new side effect on functionality it
>>>>> is just making a hacky/ugly code less so?
>>>> Functionality is unchanged. The use of a on-stack vma copy was severely
>>>> frowned upon in an earlier thread, which also points to another similar
>>>> example using vmf_insert_pfn_prot().
>>>>
>>>> https://lore.kernel.org/lkml/20190905103541.4161-2-thomas_os@shipmail.org/
>>>>
>>>>> In other words what are the
>>>>> consequences of having page protection inconsistent from vma's?
>>>> During the years, it looks like the caching- and encryption flags of
>>>> vma::vm_page_prot have been largely removed from usage. From what I can
>>>> tell, there are no more places left that can affect TTM. We discussed
>>>> __split_huge_pmd_locked() towards the end of that thread, but that doesn't
>>>> affect TTM even with huge page-table entries.
>>> Please state all those details/assumptions you are operating on in the
>>> changelog.
>> Thanks. I'll update the patchset and add that.
> And thinking about that this also begs for a comment in the code to
> explain that some (which?) mappings might have a mismatch and the
> generic code have to be careful. Because as things stand now this seems
> to be really subtle and happen to work _now_ and might break in the future.
> Or what does prevent a generic code to stumble over this discrepancy?

Yes we had that discussion in the thread I pointed to. I initially 
suggested and argued for updating the vma::vm_page_prot using a 
WRITE_ONCE() (we only have the mmap_sem in read mode), there seems to be 
other places in generic code that does the same.

But I was convinced by Andy that this was the right way and also was 
used elsewhere.

(See also 
https://elixir.bootlin.com/linux/latest/source/arch/x86/entry/vdso/vma.c#L116)

I guess to have this properly formulated, what's required is that 
generic code doesn't build page-table entries using vma::vm_page_prot 
for VM_PFNMAP and VM_MIXEDMAP outside of driver control.

/Thomas