Re: unnecessary tlb flush in mprotect

[Andi Kleen <ak@linux.intel.com>]

On Tue, May 22, 2012 at 09:08:47AM +0800, Alex Shi wrote:
> On 05/21/2012 04:30 PM, Alex Shi wrote:
> 
> > when mprotect just change prots of non-present pages, current mprotect
> > still do the tlb flush in check_protection().
> > but according to 4.10.2.3 Intel SDM V3A (
> > www.intel.com/Assets/ja_JP/PDF/manual/253668.pdf ) at that time, TLB has
> > no this lines for this page. So, tlb flush is just waste time. (for cr3
> > rewrite, flush all tlb, or invlpg, like a 'nop' in intel cpu)
> > 
> > Do we need to add the pte_present similar check here to prevent the
> > unnecessary tlb flushing? I mean, are there real case in word, User like
> > to change page prots before assign a physical page to it?
> > 
> 
> 
> Any comments from linux-mm emailing list?

I would be careful with TLB optimizations if it's not a clear performance
wins. A lot of these interactions are tricky and it's very easy to break
things in subtle and hard to debug ways.

-Andi
-- 
ak@linux.intel.com -- Speaking for myself only