linux-kernel - Re: [PATCH 3/6] mm/hwpoison: fix num_poisoned

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Date:	Thu, 22 Aug 2013 12:43:08 -0400
From:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
To:	Wanpeng Li <liwanp@...ux.vnet.ibm.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Andi Kleen <andi@...stfloor.org>,
	Fengguang Wu <fengguang.wu@...el.com>,
	Tony Luck <tony.luck@...el.com>, gong.chen@...ux.intel.com,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/6] mm/hwpoison: fix num_poisoned_pages error statistics
 for thp

On Thu, Aug 22, 2013 at 05:48:24PM +0800, Wanpeng Li wrote:
> There is a race between hwpoison page and unpoison page, memory_failure 
> set the page hwpoison and increase num_poisoned_pages without hold page 
> lock, and one page count will be accounted against thp for num_poisoned_pages.
> However, unpoison can occur before memory_failure hold page lock and 
> split transparent hugepage, unpoison will decrease num_poisoned_pages 
> by 1 << compound_order since memory_failure has not yet split transparent 
> hugepage with page lock held. That means we account one page for hwpoison
> and 1 << compound_order for unpoison. This patch fix it by decrease one 
> account for num_poisoned_pages against no hugetlbfs pages case.
> 
> Signed-off-by: Wanpeng Li <liwanp@...ux.vnet.ibm.com>

I think that a thp never becomes hwpoisoned without splitting, so "trying
to unpoison thp" never happens (I think that this implicit fact should be
commented somewhere or asserted with VM_BUG_ON().)
And nr_pages in unpoison_memory() can be greater than 1 for hugetlbfs page.
So does this patch break counting when unpoisoning free hugetlbfs pages?

Thanks,
Naoya Horiguchi

> ---
>  mm/memory-failure.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/memory-failure.c b/mm/memory-failure.c
> index 5092e06..6bfd51e 100644
> --- a/mm/memory-failure.c
> +++ b/mm/memory-failure.c
> @@ -1350,7 +1350,7 @@ int unpoison_memory(unsigned long pfn)
>  			return 0;
>  		}
>  		if (TestClearPageHWPoison(p))
> -			atomic_long_sub(nr_pages, &num_poisoned_pages);
> +			atomic_long_dec(&num_poisoned_pages);
>  		pr_info("MCE: Software-unpoisoned free page %#lx\n", pfn);
>  		return 0;
>  	}
> -- 
> 1.8.1.2
>
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