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

[Thomas HellstrÃm (VMware)]

On 12/4/19 3:42 PM, Michal Hocko wrote:
> On Wed 04-12-19 15:36:58, Thomas HellstrÃm (VMware) wrote:
> > On 12/4/19 3:35 PM, Michal Hocko wrote:
> > > On Wed 04-12-19 15:16:09, Thomas HellstrÃm (VMware) wrote:
> > > > On 12/4/19 2:52 PM, Michal Hocko wrote:
> > > > > On Tue 03-12-19 11:48:53, Thomas HellstrÃm (VMware) wrote:
> > > > > > From: Thomas Hellstrom <thellstrom@xxxxxxxxxx>
> > > > > >
> > > > > > 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@xxxxxxxxxxxx/
> > > >
> > > > > 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