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

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Date:	Fri, 10 Aug 2007 11:17:30 -0700
From:	"Luck, Tony" <tony.luck@...el.com>
To:	"KAMEZAWA Hiroyuki" <kamezawa.hiroyu@...fujitsu.com>
Cc:	<linux-ia64@...r.kernel.org>,
	"LKML" <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

This version looks really clean.  Thank for keeping working on
this through 9 versions!

A couple of small issues.

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.

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).

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.

-Tony
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