On 06.07.20 18:30, Catalin Marinas wrote: > On Mon, Jul 06, 2020 at 04:16:13PM +0200, David Hildenbrand wrote: >> On 03.07.20 17:36, Catalin Marinas wrote: >>> When a huge page is split into normal pages, part of the head page flags >>> are transferred to the tail pages. However, the PG_arch_* flags are not >>> part of the preserved set. >>> >>> PG_arch_1 is currently used by the arch code to handle cache maintenance >>> for user space (either for I-D cache coherency or for D-cache aliases >>> consistent with the kernel mapping). Since splitting a huge page does >>> not change the physical or virtual address of a mapping, additional >>> cache maintenance for the tail pages is unnecessary. Preserving the >>> PG_arch_1 flag from the head page in the tail pages would not break the >>> current use-cases. >> >> ^ is fairly arm64 specific, no? (I remember that the semantics are >> different e.g., on s390x). > > Not entirely arm64 specific. Apart from s390 and x86, I think all the > other architectures use this flag for cache maintenance (I guess they > followed the cachetlb.rst suggestion). My understanding of the s390 and > x86 is that transferring this flag from the head of a compound page to > the tail pages should not cause any issue. We don't even document > anywhere that this flag is meant to disappear on huge page splitting. I > guess no-one noticed because clearing it is relatively benign. On s390x, PG_arch_1 indicates (s390/kernel/uv.c:arch_make_page_accessible()) - kernel page tables - for hugetlbfs pages, that storage keys are initialized for that page (IIRC KVM only) - a user space page might be encrypted/secure (KVM only) The latter does not support hugetlbfs/THP. KVM does not support THP. So on s390x the bit should never be set in that context and, therefore, also won't be affected by this change. > > But if there are concerns, I'm happy to guard it with something like > __ARCH_WANT_PG_ARCH_HEAD_TAIL (I need to think of a more suggestive > name). I guess we can avoid that if we properly check+document all users. (ignoring x86 and s390x behavior here might be dangerous, although my gut feeling is that it's ok for both) -- Thanks, David / dhildenb
