Messages in this thread | | | Subject | Re: [PATCH v2 2/2] drm/ttm: Fix vm page protection handling | From | Thomas Hellström (VMware) <> | Date | Wed, 4 Dec 2019 16:19:27 +0100 |
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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
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