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Date: Fri, 18 Nov 2011 15:16:02 +0100
Message-ID: <CAONaPpGQdpNDT9EuTq_xian+bRFDUsLn7AgjtG-=y0C6-9fDTQ@mail.gmail.com>
Subject: Re: [PATCH] hugetlb: detect race if fail to COW
From: John Kacur <jkacur@redhat.com>
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Sender: owner-linux-mm@kvack.org
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To: Hillf Danton <dhillf@gmail.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>, Andrew Morton <akpm@linux-foundation.org>, Michal Hocko <mhocko@suse.cz>, Johannes Weiner <jweiner@redhat.com>, linux-mm@kvack.org, LKML <linux-kernel@vger.kernel.org>

On Fri, Nov 18, 2011 at 3:04 PM, Hillf Danton <dhillf@gmail.com> wrote:
> In the error path that we fail to allocate new huge page, before try agai=
n, we
> have to check race since page_table_lock is re-acquired.
>
> If racing, our job is done.
>
> Signed-off-by: Hillf Danton <dhillf@gmail.com>
> ---
>
> --- a/mm/hugetlb.c =A0 =A0 =A0Fri Nov 18 21:38:30 2011
> +++ b/mm/hugetlb.c =A0 =A0 =A0Fri Nov 18 21:48:15 2011
> @@ -2407,7 +2407,14 @@ retry_avoidcopy:
> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0BUG_ON(pag=
e_count(old_page) !=3D 1);
> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0BUG_ON(hug=
e_pte_none(pte));
> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0spin_lock(=
&mm->page_table_lock);
> - =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 goto retry_=
avoidcopy;
> + =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 ptep =3D hu=
ge_pte_offset(mm, address & huge_page_mask(h));
> + =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 if (likely(=
pte_same(huge_ptep_get(ptep), pte)))
> + =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0=
 =A0 goto retry_avoidcopy;
> + =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 /*
> + =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0* race o=
ccurs while re-acquiring page_table_lock, and
> + =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0* our jo=
b is done.
> + =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0*/
> + =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 return 0;
> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0}
> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0WARN_ON_ONCE(1);
> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0}


I'm not sure about the veracity of the race condition, but you better
do spin_unlock before you return.

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