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@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? -- Michal Hocko SUSE Labs
