Date: Tue, 14 Oct 2014 16:35:06 -0700
From: David Cohen <david.a.cohen@linux.intel.com>
To: Sasha Levin <sasha.levin@oracle.com>
Cc: Yu Zhao <yuzhao@google.com>, Andrew Morton <akpm@linux-foundation.org>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Mel Gorman <mgorman@suse.de>, Rik van Riel <riel@redhat.com>,
        Ingo Molnar <mingo@kernel.org>, Hugh Dickins <hughd@google.com>,
        Bob Liu <lliubbo@gmail.com>, Johannes Weiner <hannes@cmpxchg.org>,
        David Rientjes <rientjes@google.com>, Vlastimil Babka <vbabka@suse.cz>,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 2/2] mm: verify compound order when freeing a page
Message-ID: <20141014233506.GB2889@psi-dev26.jf.intel.com>
References: <1413317800-25450-1-git-send-email-yuzhao@google.com>
 <1413317800-25450-2-git-send-email-yuzhao@google.com>
 <20141014202955.GA2889@psi-dev26.jf.intel.com>
 <543DACFB.2060405@oracle.com>
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On Tue, Oct 14, 2014 at 07:08:43PM -0400, Sasha Levin wrote:
> On 10/14/2014 04:29 PM, David Cohen wrote:
> >> +	VM_BUG_ON(PageTail(page));
> >> > +	VM_BUG_ON(PageHead(page) && compound_order(page) != order);
> > It may be too severe. AFAIU we're not talking about a fatal error.
> > How about VM_WARN_ON()?
> 
> VM_BUG_ON() should catch anything which is not "supposed" to happen,
> and not just the severe stuff. Unlike BUG_ON, VM_BUG_ON only gets
> hit with mm debugging enabled.

Thanks for pointing that out :)
VM_WARN_ON*() is recent, so there isn't much examples when to use it.
I considered the below case similar to this patch. But your point does
make sense anyway.

commit 82f71ae4a2b829a25971bdf54b4d0d3d69d3c8b7
Author: Konstantin Khlebnikov <koct9i@gmail.com>
Date:   Wed Aug 6 16:06:36 2014 -0700

    mm: catch memory commitment underflow
    
    Print a warning (if CONFIG_DEBUG_VM=y) when memory commitment becomes
    too negative.
    
    This shouldn't happen any more - the previous two patches fixed the
    committed_as underflow issues.

    [akpm@linux-foundation.org: use VM_WARN_ONCE, per Dave]


Br, David

> 
> 
> Thanks,
> Sasha
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