linux-kernel - Re: [PATCH] mm: fix anon_vma->degree underflow in anon

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Date:	Tue, 3 Mar 2015 14:00:28 -0800 (PST)
From:	David Rientjes <rientjes@...gle.com>
To:	Leon Yu <chianglungyu@...il.com>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Konstantin Khlebnikov <koct9i@...il.com>,
	Rik van Riel <riel@...hat.com>,
	Michal Hocko <mhocko@...e.cz>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH] mm: fix anon_vma->degree underflow in anon_vma endless
 growing prevention

On Tue, 3 Mar 2015, Leon Yu wrote:

> I have constantly stumbled upon "kernel BUG at mm/rmap.c:399!" after upgrading
> to 3.19 and had no luck with 4.0-rc1 neither.
> 
> So, after looking into new logic introduced by commit 7a3ef208e662, ("mm:
> prevent endless growth of anon_vma hierarchy"), I found chances are that
> unlink_anon_vmas() is called without incrementing dst->anon_vma->degree in
> anon_vma_clone() due to allocation failure. If dst->anon_vma is not NULL in
> error path, its degree will be incorrectly decremented in unlink_anon_vmas()
> and eventually underflow when exiting as a result of another call to
> unlink_anon_vmas(). That's how "kernel BUG at mm/rmap.c:399!" is triggered
> for me.
> 
> This patch fixes the underflow by dropping dst->anon_vma when allocation
> fails. It's safe to do so regardless of original value of dst->anon_vma
> because dst->anon_vma doesn't have valid meaning if anon_vma_clone() fails.
> Besides, callers don't care dst->anon_vma in such case neither.
> 
> Signed-off-by: Leon Yu <chianglungyu@...il.com>
> Fixes: 7a3ef208e662 ("mm: prevent endless growth of anon_vma hierarchy")
> Cc: stable@...r.kernel.org # v3.19

Acked-by: David Rientjes <rientjes@...gle.com>
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