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From: Hugh Dickins <hughd@google.com>
To: Andrea Arcangeli <aarcange@redhat.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>,
	Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>,
	"linux-mm@kvack.org" <linux-mm@kvack.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"akpm@linux-foundation.org" <akpm@linux-foundation.org>,
	"hannes@cmpxchg.org" <hannes@cmpxchg.org>
Subject: Re: [mmotm] BUG: Bad page state in process khugepaged ?
Date: Thu, 10 Feb 2011 23:02:50 -0800 (PST)	[thread overview]
Message-ID: <alpine.LSU.2.00.1102102243160.2331@sister.anvils> (raw)
In-Reply-To: <20110209200728.GQ3347@random.random>

On Wed, 9 Feb 2011, Andrea Arcangeli wrote:
> On Wed, Feb 09, 2011 at 03:52:46PM +0900, KAMEZAWA Hiroyuki wrote:
> > On Wed, 9 Feb 2011 15:50:01 +0900
> > Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp> wrote:
> > 
> > > > 
> > > > In hex, pc->flags was 7A00000000004 and this means PCG_USED bit is set.
> > > > This implies page_remove_rmap() may not be called but ->mapping is NULL. Hmm?
> > > > (7A is encoding of section number.)
> > > > 
> > > Sigh.. it seems another freed-but-not-uncharged problem..
> > > 
> > 
> > Ah, ok, this is maybe caused by this. I'm sorry that I missed this.
> > ==
> > static inline int free_pages_check(struct page *page)
> > {
> >         if (unlikely(page_mapcount(page) |
> >                 (page->mapping != NULL)  |
> >                 (atomic_read(&page->_count) != 0) |
> >                 (page->flags & PAGE_FLAGS_CHECK_AT_FREE) |
> >                 (mem_cgroup_bad_page_check(page)))) {    <==========(*)
> >                 bad_page(page);
> >                 return 1;
> > ==
> > 
> > Then, ok, this is a memcgroup and hugepage issue.
> > 
> > I'll look into.
> 
> Yes, the rest of the info on the page looked ok and shouldn't have
> triggered a bad_page call. Thanks so much for looking into it.

There is a separate little issue here, Andrea.

Although we went to some trouble for bad_page() to take the page out
of circulation yet let the system continue, your VM_BUG_ON(!PageBuddy)
inside __ClearPageBuddy(page), from two callsites in bad_page(), is
turning it into a fatal error when CONFIG_DEBUG_VM.

You could that only MM developers switch CONFIG_DEBUG_VM=y, and they
would like bad_page() to be fatal; maybe, but if so we should do that
as an intentional patch, rather than as an unexpected side-effect ;)

I noticed this a few days ago, but hadn't quite decided whether just to
remove the VM_BUG_ON, or move it to __ClearPageBuddy's third callsite,
or... doesn't matter much.

I do also wonder if PageBuddy would better be _mapcount -something else:
if we've got a miscounted page (itself unlikely of course), there's a
chance that its _mapcount will be further decremented after it has been
freed: whereupon it will go from -1 to -2, PageBuddy at present.  The
special avoidance of PageBuddy being that it can pull a whole block of
pages into misuse if its mistaken.

Hugh

  reply	other threads:[~2011-02-11  7:03 UTC|newest]

Thread overview: 14+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2011-02-09  6:10 KAMEZAWA Hiroyuki
2011-02-09  6:40 ` KAMEZAWA Hiroyuki
2011-02-09  6:50 ` Daisuke Nishimura
2011-02-09  6:52   ` KAMEZAWA Hiroyuki
2011-02-09 20:07     ` Andrea Arcangeli
2011-02-11  7:02       ` Hugh Dickins [this message]
2011-02-11 10:49         ` Andrea Arcangeli
2011-02-11 19:58           ` Hugh Dickins
2011-02-11 20:24             ` Andrea Arcangeli
2011-02-14 22:24           ` Johannes Weiner
2011-02-09  7:23 ` [PATCH][BUGFIX] memcg: fix leak of accounting at failure path of hugepage collapsing KAMEZAWA Hiroyuki
2011-02-09  7:51   ` Daisuke Nishimura
2011-02-09  9:51   ` Johannes Weiner
2011-02-10  2:49   ` Minchan Kim

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