linux-kernel - Re: hugepage related lockdep trace.

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Date:	Tue, 23 Jul 2013 16:01:20 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	Minchan Kim <minchan@...nel.org>
Cc:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
	Dave Jones <davej@...hat.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	linux-mm@...ck.org, Rik van Riel <riel@...hat.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Hillf Danton <dhillf@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: hugepage related lockdep trace.

On Fri 19-07-13 09:13:03, Minchan Kim wrote:
> On Thu, Jul 18, 2013 at 11:12:24PM +0530, Aneesh Kumar K.V wrote:
[...]
> > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > index 83aff0a..2cb1be3 100644
> > --- a/mm/hugetlb.c
> > +++ b/mm/hugetlb.c
> > @@ -3266,8 +3266,8 @@ pte_t *huge_pmd_share(struct mm_struct *mm, unsigned long addr, pud_t *pud)
> >  		put_page(virt_to_page(spte));
> >  	spin_unlock(&mm->page_table_lock);
> >  out:
> > -	pte = (pte_t *)pmd_alloc(mm, pud, addr);
> >  	mutex_unlock(&mapping->i_mmap_mutex);
> > +	pte = (pte_t *)pmd_alloc(mm, pud, addr);
> >  	return pte;
> 
> I am blind on hugetlb but not sure it doesn't break eb48c071.
> Michal?

Well, it is some time since I debugged the race and all the details
vanished in the meantime. But this part of the changelog suggests that
this indeed breaks the fix:
"
    This patch addresses the issue by moving pmd_alloc into huge_pmd_share
    which guarantees that the shared pud is populated in the same critical
    section as pmd.  This also means that huge_pte_offset test in
    huge_pmd_share is serialized correctly now which in turn means that the
    success of the sharing will be higher as the racing tasks see the pud
    and pmd populated together.
"

Besides that I fail to see how moving pmd_alloc down changes anything.
Even if pmd_alloc triggered reclaim then we cannot trip over the same
i_mmap_mutex as hugetlb pages are not reclaimable because they are not
on the LRU.
-- 
Michal Hocko
SUSE Labs
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