linux-kernel - Re: [PATCH] hugetlb: detect race if fail to COW

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Message-ID: <CAONaPpHybQL38PSq-hux5X44zuHDCQg=8L1fb+geWv00ktQq7g@mail.gmail.com>
Date:	Fri, 18 Nov 2011 15:21:26 +0100
From:	John Kacur <jkacur@...hat.com>
To:	Hillf Danton <dhillf@...il.com>
Cc:	Andrea Arcangeli <aarcange@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Michal Hocko <mhocko@...e.cz>,
	Johannes Weiner <jweiner@...hat.com>, linux-mm@...ck.org,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] hugetlb: detect race if fail to COW

On Fri, Nov 18, 2011 at 3:16 PM, John Kacur <jkacur@...hat.com> wrote:
> On Fri, Nov 18, 2011 at 3:04 PM, Hillf Danton <dhillf@...il.com> wrote:
>> In the error path that we fail to allocate new huge page, before try again, we
>> have to check race since page_table_lock is re-acquired.
>>
>> If racing, our job is done.
>>
>> Signed-off-by: Hillf Danton <dhillf@...il.com>
>> ---
>>
>> --- a/mm/hugetlb.c      Fri Nov 18 21:38:30 2011
>> +++ b/mm/hugetlb.c      Fri Nov 18 21:48:15 2011
>> @@ -2407,7 +2407,14 @@ retry_avoidcopy:
>>                                BUG_ON(page_count(old_page) != 1);
>>                                BUG_ON(huge_pte_none(pte));
>>                                spin_lock(&mm->page_table_lock);
>> -                               goto retry_avoidcopy;
>> +                               ptep = huge_pte_offset(mm, address & huge_page_mask(h));
>> +                               if (likely(pte_same(huge_ptep_get(ptep), pte)))
>> +                                       goto retry_avoidcopy;
>> +                               /*
>> +                                * race occurs while re-acquiring page_table_lock, and
>> +                                * our job is done.
>> +                                */
>> +                               return 0;
>>                        }
>>                        WARN_ON_ONCE(1);
>>                }
>
>
> 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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