linux-kernel - Re: [PATCH] mm/memory-failure.c: fix memory leak by race between poison and unpoison

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Date:	Wed, 14 May 2014 15:10:37 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
Cc:	Andi Kleen <andi@...stfloor.org>,
	Wu Fengguang <fengguang.wu@...el.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH] mm/memory-failure.c: fix memory leak by race between
 poison and unpoison

On Wed, 14 May 2014 11:21:31 -0400 Naoya Horiguchi <n-horiguchi@...jp.nec.com> wrote:

> When a memory error happens on an in-use page or (free and in-use) hugepage,
> the victim page is isolated with its refcount set to one. When you try to
> unpoison it later, unpoison_memory() calls put_page() for it twice in order to
> bring the page back to free page pool (buddy or free hugepage list.)
> However, if another memory error occurs on the page which we are unpoisoning,
> memory_failure() returns without releasing the refcount which was incremented
> in the same call at first, which results in memory leak and unconsistent
> num_poisoned_pages statistics. This patch fixes it.
> 
> ...
>
> --- next-20140512.orig/mm/memory-failure.c
> +++ next-20140512/mm/memory-failure.c
> @@ -1153,6 +1153,8 @@ int memory_failure(unsigned long pfn, int trapno, int flags)
>  	 */
>  	if (!PageHWPoison(p)) {
>  		printk(KERN_ERR "MCE %#lx: just unpoisoned\n", pfn);
> +		atomic_long_sub(nr_pages, &num_poisoned_pages);
> +		put_page(hpage);
>  		res = 0;
>  		goto out;
>  	}

Looking at the surrounding code...

	/*
	 * Lock the page and wait for writeback to finish.
	 * It's very difficult to mess with pages currently under IO
	 * and in many cases impossible, so we just avoid it here.
	 */
	lock_page(hpage);


lock_page() doesn't wait for writeback to finish -
wait_on_page_writeback() does that.  Either the code or the comment
could do with fixing.


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