linux-kernel - Re: [PATCH v3 3/5] mm/hugetlb: fix getting refcount 0 page in hugetlb

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Message-ID: <20141007165657.GB24093@nhori.bos.redhat.com>
Date:	Tue, 7 Oct 2014 12:56:57 -0400
From:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
To:	Hugh Dickins <hughd@...gle.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	David Rientjes <rientjes@...gle.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Naoya Horiguchi <nao.horiguchi@...il.com>,
	stable@...r.kernel.org
Subject: Re: [PATCH v3 3/5] mm/hugetlb: fix getting refcount 0 page in
 hugetlb_fault()

On Mon, Sep 29, 2014 at 09:52:24PM -0700, Hugh Dickins wrote:
> On Mon, 15 Sep 2014, Naoya Horiguchi wrote:
> > When running the test which causes the race as shown in the previous patch,
> > we can hit the BUG "get_page() on refcount 0 page" in hugetlb_fault().
> 
> Two minor comments...
> 
> > @@ -3192,22 +3208,19 @@ int hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
> > 	 * Note that locking order is always pagecache_page -> page,
> >  	 * so no worry about deadlock.
> 
> That sentence of comment is stale and should be deleted,
> now that you're only doing a trylock_page(page) here.

OK, I'll delete it.

> >  out_mutex:
> >  	mutex_unlock(&htlb_fault_mutex_table[hash]);
> > +	if (need_wait_lock)
> > +		wait_on_page_locked(page);
> >  	return ret;
> >  }
> 
> It will be hard to trigger any problem from this (I guess it would
> need memory hotremove), but you ought really to hold a reference to
> page while doing a wait_on_page_locked(page).

I'll do that.

Thanks,
Naoya Horiguchi
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