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

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Date:	Mon, 29 Sep 2014 21:52:24 -0700 (PDT)
From:	Hugh Dickins <hughd@...gle.com>
To:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Hugh Dickins <hughd@...gle.com>,
	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, 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.

>  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).
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