linux-kernel - Re: [RFC][PATCH 06/26] mm: Migrate misplaced page

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Date:	Tue, 03 Apr 2012 10:32:13 -0700
From:	Dan Smith <danms@...ibm.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>, Paul Turner <pjt@...gle.com>,
	Suresh Siddha <suresh.b.siddha@...el.com>,
	Mike Galbraith <efault@....de>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Bharata B Rao <bharata.rao@...il.com>,
	Lee Schermerhorn <Lee.Schermerhorn@...com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Rik van Riel <riel@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [RFC][PATCH 06/26] mm: Migrate misplaced page

PZ> XXX: hnaz, dansmith saw some bad_page() reports when using memcg, I
PZ> could not reproduce -- is there something funny with the mem_cgroup
PZ> calls in the below patch?

I think the problem stems from the final put_page() on the old page
being called before the charge commit. I think something like the
following should do the trick (and appears to work for me):

diff --git a/mm/migrate.c b/mm/migrate.c
index b7fa472..fd88f4b 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -1590,7 +1590,6 @@ migrate_misplaced_page(struct page *page, struct mm_struct
                put_page(page);         /* drop       "          "  */
 
                unlock_page(page);
-               put_page(page);         /* drop fault path ref & free */
 
                page = newpage;
        }
@@ -1599,6 +1598,9 @@ out:
        if (!charge)
                mem_cgroup_end_migration(mcg, oldpage, newpage, !rc);
 
+       if (oldpage != page)
+               put_page(oldpage);
+
        if (rc) {
                unlock_page(newpage);
                __free_page(newpage);


-- 
Dan Smith
IBM Linux Technology Center
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