On Thu 13-07-17 08:53:52, Benjamin Herrenschmidt wrote: > On Wed, 2017-07-12 at 09:23 +0200, Michal Hocko wrote: > > > > > > > > Ideally the MMU looks at the PTE for keys, in order to enforce > > > protection. This is the case with x86 and is the case with power9 Radix > > > page table. Hence the keys have to be programmed into the PTE. > > > > But x86 doesn't update ptes for PKEYs, that would be just too expensive. > > You could use standard mprotect to do the same... > > What do you mean ? x86 ends up in mprotect_fixup -> change_protection() > which will update the PTEs just the same as we do. > > Changing the key for a page is a form mprotect. Changing the access > permissions for keys is different, for us it's a special register > (AMR). > > I don't understand why you think we are doing any differently than x86 > here. That was a misunderstanding on my side as explained in other reply. > > > However with HPT on power, these keys do not necessarily have to be > > > programmed into the PTE. We could bypass the Linux Page Table Entry(PTE) > > > and instead just program them into the Hash Page Table(HPTE), since > > > the MMU does not refer the PTE but refers the HPTE. The last version > > > of the page attempted to do that. It worked as follows: > > > > > > a) when a address range is requested to be associated with a key; by the > > > application through key_mprotect() system call, the kernel > > > stores that key in the vmas corresponding to that address > > > range. > > > > > > b) Whenever there is a hash page fault for that address, the fault > > > handler reads the key from the VMA and programs the key into the > > > HPTE. __hash_page() is the function that does that. > > > > What causes the fault here? > > The hardware. With the hash MMU, the HW walks a hash table which is > effectively a large in-memory TLB extension. When a page isn't found > there, a "hash fault" is generated allowing Linux to populate that > hash table with the content of the corresponding PTE. Thanks for the clarification -- Michal Hocko SUSE Labs
