linux-kernel - Re: [patch 4/6] MMU notifier: invalidate

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Message-ID: <20080129142452.GH7233@v2.random>
Date:	Tue, 29 Jan 2008 15:24:52 +0100
From:	Andrea Arcangeli <andrea@...ranet.com>
To:	Christoph Lameter <clameter@....com>
Cc:	Robin Holt <holt@....com>, Avi Kivity <avi@...ranet.com>,
	Izik Eidus <izike@...ranet.com>, Nick Piggin <npiggin@...e.de>,
	kvm-devel@...ts.sourceforge.net,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>, steiner@....com,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	daniel.blueman@...drics.com, Hugh Dickins <hugh@...itas.com>
Subject: Re: [patch 4/6] MMU notifier: invalidate_page callbacks using
	Linux rmaps

This should fix the aging bugs you introduced through the faulty cpp
expansion. This is hard to write for me, given any time somebody does
a ptep_clear_flush_young w/o manually cpp-expandin "|
mmu_notifier_age_page" after it, it's always a bug that needs fixing,
similar bugs can emerge with time for ptep_clear_flush too. What will
happen is that somebody will cleanup in 26+ and we'll remain with a
#ifdef KERNEL_VERSION() < 2.6.26 in ksm.c to call
mmu_notifier(invalidate_page) explicitly. Performance and
optimizations or unnecessary invalidate_page are a red-herring, it can
be fully optimized both ways. 99% of the time when somebody calls
ptep_clear_flush and ptep_clear_flush_young, the respective mmu
notifier can't be forgotten (and calling them once more even if a
later invalidate_range is invoked, is always safer and preferable than
not calling them at all) so I fail to see how this will not be cleaned
up eventually, the same way the tlb flushes have been cleaned up
already. Nevertheless I back your implementation and I'm not even
trying at changing it with the risk to slowdown merging.

Signed-off-by: Andrea Arcangeli <andrea@...ranet.com>

diff --git a/mm/rmap.c b/mm/rmap.c
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -285,10 +285,8 @@ static int page_referenced_one(struct pa
 	if (!pte)
 		goto out;

-	if (ptep_clear_flush_young(vma, address, pte))
-		referenced++;
-
-	if (mmu_notifier_age_page(mm, address))
+	if (ptep_clear_flush_young(vma, address, pte) |
+	    mmu_notifier_age_page(mm, address))
 		referenced++;

 	/* Pretend the page is referenced if the task has the
@@ -684,7 +682,7 @@ static int try_to_unmap_one(struct page 
 	 * skipped over this mm) then we should reactivate it.
 	 */
 	if (!migration && ((vma->vm_flags & VM_LOCKED) ||
-			(ptep_clear_flush_young(vma, address, pte) ||
+			(ptep_clear_flush_young(vma, address, pte) |
 				mmu_notifier_age_page(mm, address)))) {
 		ret = SWAP_FAIL;
 		goto out_unmap;
@@ -818,10 +816,8 @@ static void try_to_unmap_cluster(unsigne
 		page = vm_normal_page(vma, address, *pte);
 		BUG_ON(!page || PageAnon(page));

-		if (ptep_clear_flush_young(vma, address, pte))
-			continue;
-
-		if (mmu_notifier_age_page(mm, address))
+		if (ptep_clear_flush_young(vma, address, pte) | 
+		    mmu_notifier_age_page(mm, address))
 			continue;

 		/* Nuke the page table entry. */
--
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