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Date: Thu, 15 May 2014 13:43:28 -0400
From: Sasha Levin <sasha.levin@oracle.com>
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To: Hugh Dickins <hughd@google.com>
CC: "Kirill A. Shutemov" <kirill@shutemov.name>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        David Rientjes <rientjes@google.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>, Mel Gorman <mgorman@suse.de>,
        Dave Jones <davej@redhat.com>, linux-mm <linux-mm@kvack.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Josh Boyer <jwboyer@fedoraproject.org>
Subject: Re: mm: BUG in do_huge_pmd_wp_page
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On 05/15/2014 01:37 PM, Hugh Dickins wrote:
> On Thu, 15 May 2014, Sasha Levin wrote:
>> On 04/07/2014 04:11 PM, Kirill A. Shutemov wrote:
>>> On Mon, Apr 07, 2014 at 03:40:46PM -0400, Sasha Levin wrote:
>>>>> It also breaks fairly quickly under testing because:
>>>>>
>>>>> On 04/07/2014 10:48 AM, Kirill A. Shutemov wrote:
>>>>>>> +	if (IS_ENABLED(CONFIG_DEBUG_PAGEALLOC)) {
>>>>>>> +		spin_lock(ptl);
>>>>>
>>>>> ^ We go into atomic
>>>>>
>>>>>>> +		if (unlikely(!pmd_same(*pmd, orig_pmd)))
>>>>>>> +			goto out_race;
>>>>>>> +	}
>>>>>>> +
>>>>>>>  	if (!page)
>>>>>>>  		clear_huge_page(new_page, haddr, HPAGE_PMD_NR);
>>>>>>>  	else
>>>>>>>  		copy_user_huge_page(new_page, page, haddr, vma, HPAGE_PMD_NR);
>>>>>
>>>>> copy_user_huge_page() doesn't like running in atomic state,
>>>>> and asserts might_sleep().
>>> Okay, I'll try something else.
>>
>> I've Cc'ed Josh Boyer to this since it just occurred to me that Fedora
>> is running with CONFIG_DEBUG_VM set, where this bug is rather easy to
>> trigger.
>>
>> This issue was neglected because it triggers only on CONFIG_DEBUG_VM builds,
>> but with Fedora running that, maybe it shouldn't be?
> 
> But it triggers only on CONFIG_DEBUG_PAGEALLOC builds, doesn't it?
> I hope Fedora doesn't go out with that enabled.

Ow, it needs DEBUG_PAGEALLOC too? I forgot about that one.

No problem for Fedora then, sorry for the noise :(


Thanks,
Sasha