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

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

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.

/Thomas