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#L11...)
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!