linux-kernel - Re: [PATCH UPDATE] x86: ignore spurious faults

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Date:	Fri, 25 Jan 2008 10:41:16 +1100
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	Matt Mackall <mpm@...enic.com>,
	Harvey Harrison <harvey.harrison@...il.com>,
	Ingo Molnar <mingo@...e.hu>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andi Kleen <ak@...e.de>
Subject: Re: [PATCH UPDATE] x86: ignore spurious faults

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?
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