linux-kernel - Re: [PATCH 2/2] mm: verify compound order when freeing a page

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Date:	Tue, 14 Oct 2014 16:35:06 -0700
From:	David Cohen <david.a.cohen@...ux.intel.com>
To:	Sasha Levin <sasha.levin@...cle.com>
Cc:	Yu Zhao <yuzhao@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	Ingo Molnar <mingo@...nel.org>,
	Hugh Dickins <hughd@...gle.com>, Bob Liu <lliubbo@...il.com>,
	Johannes Weiner <hannes@...xchg.org>,
	David Rientjes <rientjes@...gle.com>,
	Vlastimil Babka <vbabka@...e.cz>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] mm: verify compound order when freeing a page

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@...il.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@...ux-foundation.org: use VM_WARN_ONCE, per Dave]


Br, David

> 
> 
> Thanks,
> Sasha
> --
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