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Date: Fri, 18 Nov 2011 15:21:26 +0100
Message-ID: <CAONaPpHybQL38PSq-hux5X44zuHDCQg=8L1fb+geWv00ktQq7g@mail.gmail.com>
Subject: Re: [PATCH] hugetlb: detect race if fail to COW
From: John Kacur <jkacur@redhat.com>
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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 3:16 PM, John Kacur <jkacur@redhat.com> wrote:
> 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 aga=
in, 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(pa=
ge_count(old_page) !=3D 1);
>> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0BUG_ON(hu=
ge_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 h=
uge_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 =
occurs while re-acquiring page_table_lock, and
>> + =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0* our j=
ob 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.
>

Ugh, sorry for the noise, I see that's not how it works here.

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