Messages in this thread | | | Date | Thu, 24 Jan 2008 16:26:26 -0800 | From | Jeremy Fitzhardinge <> | Subject | Re: [PATCH UPDATE] x86: ignore spurious faults |
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Nick Piggin wrote: > On Friday 25 January 2008 06:21, Jeremy Fitzhardinge wrote: > >> Matt Mackall wrote: >> >>> There's perhaps an opportunity to do this lazy TLB trick in the mmap >>> path as well, where RW mappings are initially mapped as RO so we can >>> catch processes dirtying them and then switched to RW. If the mapping is >>> shared across threads on multiple cores, we can defer synchronizing the >>> TLBs on the others. >>> >> I think spurious usermode faults are already dealt with. >> handle_pte_fault() does essentially the same thing as this patch: >> >> if (ptep_set_access_flags(vma, address, pte, entry, write_access)) { >> update_mmu_cache(vma, address, entry); >> } else { >> /* >> * This is needed only for protection faults but the arch code >> * is not yet telling us if this is a protection fault or not. >> * This still avoids useless tlb flushes for .text page faults >> * with threads. >> */ >> if (write_access) >> flush_tlb_page(vma, address); >> } >> > > I (obviously) don't know exactly how the TLB works in x86, but I > thought that on a miss, the CPU walks the pagetables first before > faulting? Maybe that's not the case if there is an RO entry > actually in the TLB? >
My understanding is that it will fault immediately if there's a TLB entry, and rewalk the tables on return from the fault before restarting the instruction, so there's no need for an explicit TLB flush. The TLB doesn't have a notion of negative cache entries, so any entry represents a present page of some variety.
J
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