Re: [PATCH UPDATE] x86: ignore spurious faults

Nick Piggin
2008-01-24T23:41:39Z

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?