linux-kernel - Re: [PATCH] flush icache before set_pte() on ia64 take9 [2/2] flush icache at set

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Date:	Sat, 11 Aug 2007 07:23:52 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	"Luck, Tony" <tony.luck@...el.com>
Cc:	linux-ia64@...r.kernel.org, linux-kernel@...r.kernel.org,
	davem@...emloft.net, Zoltan.Menyhart@...l.net,
	nickpiggin@...oo.com.au
Subject: Re: [PATCH] flush icache before set_pte() on ia64  take9 [2/2]
 flush icache at set_pte

On Fri, 10 Aug 2007 11:17:30 -0700
"Luck, Tony" <tony.luck@...el.com> wrote:

> 1) In arch/ia64/mm/init.c: __ia64_sync_icache_dcache()
> 
> -	if (!pte_exec(pte))
> -		return;				/* not an executable page... */
> +	BUG_ON(!pte_exec(pte));
> 
> In this latest version the only route to this routine is from set_pte()
> inside the test :
> 
> 	if (pte_exec(pteval) && ....) {
> 	}
> 
> So this BUG_ON is now redundant.
> 
I see.

> 2) In include/asm-ia64/pgtable.h
> 
> +	if (pte_exec(pteval) &&    // flush only new executable page.
> +	    pte_present(pteval) && // swap out ?
> +	    pte_user(pteval) &&    // ignore kernel page
> +	    (!pte_present(*ptep) ||// do_no_page or swap in, migration,
> +		pte_pfn(*ptep) != pte_pfn(pteval))) // do_wp_page(), page copy
> +		/* load_module() calles flush_icache_range() explicitly*/
> +		__ia64_sync_icache_dcache(pteval);
> 
> Just above this there is a comment saying that pte_exec() only works
> when pte_present() is true.  So we must re-order the conditions so that
> we check that the pteval satisfies pte_present() before using either of
> pte_exec() or pte_user() on it like this:
> 
> 	if (pte_present(pteval) &&
>           pte_exec(pteval) &&
>           pte_user(pteval) &&
> 
> I put in some crude counters to see whether we should check pte_exec() or
> pte_user() next ... and it was very clear that the pte_exec() check gets
> us out of the if() faster (at least during a kernel build).
> 
ok.

I'm sorry that I'll be offlined until next Wednesday. So, I'll post above
fix in a week or so.


> I also compared how often the old code called lazy_mmu_prot_update()
> with how often the new code calls __ia64_sync_icache_dcache() (again
> using kernel build as my workload) ... and the answer is about the
> same (less than 0.2% change ... probably less than run-to-run variation).
> 
> 
> So now the only remaining task is to convince myself that this
> new version covers all the cases.
> 
yes. I want more eyes for review. 

Thanks,
-Kame

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