linux-kernel - Re: mm: BUG in do_huge_pmd_wp

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Date:	Thu, 15 May 2014 13:31:48 -0400
From:	Sasha Levin <sasha.levin@...cle.com>
To:	"Kirill A. Shutemov" <kirill@...temov.name>
CC:	Hugh Dickins <hughd@...gle.com>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Rientjes <rientjes@...gle.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, Mel Gorman <mgorman@...e.de>,
	Dave Jones <davej@...hat.com>, linux-mm <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Josh Boyer <jwboyer@...oraproject.org>
Subject: Re: mm: BUG in do_huge_pmd_wp_page

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?


Thanks,
Sasha
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