On Tue, Nov 21, 2017 at 3:17 PM, Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx> wrote: > On 11/21/2017 02:46 PM, Andy Lutomirski wrote: >>> GDT: R/O TSS: R/W at least because of trampoline stack entry code: >>> EXEC+R/O exception stacks: R/W >> Can you avoid code duplication by adding some logic right after the >> kernel cpu_entry_area is set up to iterate page by page over the PTEs >> in the cpu_entry_area for that CPU and just install exactly the same >> PTEs into the kaiser table? E.g. just call kaiser_add_mapping once >> per page but with the parameters read out from the fixmap PTEs >> instead of hard coded? > > Yes, we could do that. But, what's the gain? We end up removing > effectively three (long) lines of code from three kaiser_add_mapping() > calls. I'm hoping we can remove kaiser_add_mapping() entirely. Maybe that's silly optimism. > > To do this, we need to special-case the kernel page table walker to deal > with PTEs only since we can't just grab PMD or PUD flags and stick them > in a PTE. We would only be able to use this path when populating things > that we know are 4k-mapped in the kernel. I'm not sure I'm understanding the issue. We'd promise to map the cpu_entry_area without using large pages, but I'm not sure I know what you're referring to. The only issue I see is that we'd have to be quite careful when tearing down the user tables to avoid freeing the shared part. > > I guess the upside is that we don't open-code the permissions in the > KAISER code that *have* to match the permissions that the kernel itself > established. > > It also means that theoretically you could not touch the KAISER code the > next time we expand the cpu entry area. I definitely like that part. --Andy -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
