linux-kernel - Re: [PATCH] mm/huge_memory: fix the memory leak due to the race

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Date:	Tue, 21 Jun 2016 17:37:01 +0300
From:	"Kirill A. Shutemov" <kirill@...temov.name>
To:	zhongjiang <zhongjiang@...wei.com>
Cc:	mhocko@...nel.org, akpm@...ux-foundation.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/huge_memory: fix the memory leak due to the race

On Tue, Jun 21, 2016 at 10:05:56PM +0800, zhongjiang wrote:
> From: zhong jiang <zhongjiang@...wei.com>
> 
> with great pressure, I run some test cases. As a result, I found
> that the THP is not freed, it is detected by check_mm().
> 
> BUG: Bad rss-counter state mm:ffff8827edb70000 idx:1 val:512
> 
> Consider the following race :
> 
> 	CPU0                               CPU1
>   __handle_mm_fault()
>         wp_huge_pmd()
>    	    do_huge_pmd_wp_page()
> 		pmdp_huge_clear_flush_notify()
>                 (pmd_none = true)
> 					exit_mmap()
> 					   unmap_vmas()
> 					     zap_pmd_range()
> 						pmd_none_or_trans_huge_or_clear_bad()
> 						   (result in memory leak)
>                 set_pmd_at()
> 
> because of CPU0 have allocated huge page before pmdp_huge_clear_notify,
> and it make the pmd entry to be null. Therefore, The memory leak can occur.
> 
> The patch fix the scenario that the pmd entry can lead to be null.

I don't think the scenario is possible.

exit_mmap() called when all mm users have gone, so no parallel threads
exist.

-- 
 Kirill A. Shutemov