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

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

On 12/4/19 4:26 PM, Michal Hocko wrote:
> On Wed 04-12-19 16:19:27, Thomas Hellström (VMware) wrote:
>> 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.
> Let me repeat that this belongs to a code somewhere everybody can see it
> rather than a "random" discussion at mailing list.
>
> Thanks!

Yes, I agree. I'll of course follow up with the comments added to the code.

Thomas