Sorry, I missed out Dave's email, so now I'm taking my time to page (hah!) all of this. On 25/07/23 15:21, Peter Zijlstra wrote: > On Mon, Jul 24, 2023 at 10:40:04AM -0700, Dave Hansen wrote: > >> TLB flushes for freed page tables are another game entirely. The CPU is >> free to cache any part of the paging hierarchy it wants at any time. >> It's also free to set accessed and dirty bits at any time, even for >> instructions that may never execute architecturally. >> >> That basically means that if you have *ANY* freed page table page >> *ANYWHERE* in the page table hierarchy of any CPU at any time ... you're >> screwed. >> >> There's no reasoning about accesses or ordering. As soon as the CPU >> does *anything*, it's out to get you. >> OK, I feel like I need to go back do some more reading now, but I think I get the difference. Thanks for spelling it out. >> You're going to need to do something a lot more radical to deal with >> free page table pages. > > Ha! IIRC the only thing we can reasonably do there is to have strict > per-cpu page-tables such that NOHZ_FULL CPUs can be isolated. That is, > as long we the per-cpu tables do not contain -- and have never contained > -- a particular table page, we can avoid flushing it. Because if it > never was there, it also couldn't have speculatively loaded it. > > Now, x86 doesn't really do per-cpu page tables easily (otherwise we'd > have done them ages ago) and doing them is going to be *major* surgery > and pain. > > Other than that, we must take the TLBI-IPI when freeing > page-table-pages. > > > But yeah, I think Nadav is right, vmalloc.c never frees page-tables (or > at least, I couldn't find it in a hurry either), but if we're going to > be doing this, then that file must include a very prominent comment > explaining it must never actually do so either. > I also couldn't find any freeing of the page-table-pages, I'll do another pass and sharpen my quill for a big fat comment. > Not being able to free page-tables might be a 'problem' if we're going > to be doing more of HUGE_VMALLOC, because that means it becomes rather > hard to swizzle from small to large pages.
