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Date: Thu, 15 May 2014 10:37:14 -0700 (PDT)
From: Hugh Dickins <hughd@google.com>
Subject: Re: mm: BUG in do_huge_pmd_wp_page
In-Reply-To: <5374FA04.5@oracle.com>
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To: Sasha Levin <sasha.levin@oracle.com>
Cc: "Kirill A. Shutemov" <kirill@shutemov.name>, Hugh Dickins <hughd@google.com>, "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>

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.

Hugh

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