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

[cyc <soldier.cyc81@gmail.com>]

a?? 2014-05-14a,?c?? 11:21 -0400i 1/4 ?Naoya Horiguchia??e??i 1/4 ?
> 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.

We assume that a new memory error occurs on the hugepage which we are
unpoisoning. 

          A   unpoisoned  B    poisoned    C          
hugepage: |---------------+++++++++++++++++|

There are two cases, so shown.
  1. the victim page belongs to A-B, the memory_failure will be blocked
by lock_page() until unlock_page() invoked by unpoison_memory().
  2. the victim page belongs to B-C, the memory_failure() will return
very soon at the beginning of this function.

So the new memory error will have no effect what you say so.

thx!
cyc 

> 
> Signed-off-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
> Cc: <stable@vger.kernel.org>    [2.6.32+]
> ---
>  mm/memory-failure.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git next-20140512.orig/mm/memory-failure.c next-20140512/mm/memory-failure.c
> index 9872af1b1e9d..93a08bd78c78 100644
> --- 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;
>  	}


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