linux-kernel - Re: [PATCH 6/7] mm: add CONFIG_DEBUG_VM

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Date:	Sun, 16 Sep 2012 12:07:40 -0700 (PDT)
From:	Hugh Dickins <hughd@...gle.com>
To:	Jiri Slaby <jslaby@...e.cz>
cc:	Michel Lespinasse <walken@...gle.com>,
	Sasha Levin <levinsasha928@...il.com>, linux-mm@...ck.org,
	riel@...hat.com, peterz@...radead.org, aarcange@...hat.com,
	daniel.santos@...ox.com, linux-kernel@...r.kernel.org,
	akpm@...ux-foundation.org, Dave Jones <davej@...hat.com>
Subject: Re: [PATCH 6/7] mm: add CONFIG_DEBUG_VM_RB build option

On Sat, 15 Sep 2012, Jiri Slaby wrote:
> On 09/15/2012 02:00 AM, Michel Lespinasse wrote:
> > All right. Hugh managed to reproduce the issue on his suse laptop, and
> > I came up with a fix.
> > 
> > The problem was that in mremap, the new vma's vm_{start,end,pgoff}
> > fields need to be updated before calling anon_vma_clone() so that the
> > new vma will be properly indexed.
> > 
> > Patch attached. I expect this should also explain Jiri's reported
> > failure involving splitting THP pages during mremap(), even though we
> > did not manage to reproduce that one.
> 
> Oh, great. This is BTW also machine with suse.

We guessed that for you it might be :)
I've not yet moved up from 11.4 by the way, if that makes a difference.

In fact, even before these reports, when Michel was wondering about the
uses of mremap, I did mention an mremap/THP bug from a year ago, which
the SuSE update had been good for reproducing.

> What was the way that
> Hugh used to reproduce the other issue?

I've lost track of which issue is "other".

To reproduce Sasha's interval_tree.c warnings, all I had to do was switch
on CONFIG_DEBUG_VM_RB (I regret not having done so before) and boot up.

I didn't look to see what was doing the mremap which caused the warning
until now: surprisingly, it's microcode_ctl.  I've not made much effort
to get the right set of sources and work out why that would be using
mremap (a realloc inside a library?).

I failed to reproduce your BUG in huge_memory.c, but what I was trying
was SuSE update via yast2, on several machines; but perhaps because
they were all fairly close to up-to-date, I didn't hit a problem.
(That was before I turned on DEBUG_VM_RB for Sasha's.)

Hugh

> For me it happened twice in a
> row when using zypper to upgrade packages. But it did not happen any
> more after that.
> 
> thanks,
> -- 
> js
> suse labs
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