linux-kernel - Re: [PATCH] THP: Use explicit memory barrier

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Date:	Thu, 04 Apr 2013 10:45:25 +0800
From:	Simon Jeons <simon.jeons@...il.com>
To:	Minchan Kim <minchan@...nel.org>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Mel Gorman <mgorman@...e.de>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Hugh Dickins <hughd@...gle.com>
Subject: Re: [PATCH] THP: Use explicit memory barrier

Hi Minchan,
On 04/01/2013 07:45 AM, Minchan Kim wrote:
> __do_huge_pmd_anonymous_page depends on page_add_new_anon_rmap's
> spinlock for making sure that clear_huge_page write become visible
> after set set_pmd_at() write.

1. There are no pte modify, why take page_table_lock here?
2. What's the meaning of "clear_huge_page write become visible after set 
set_pmd_at() write"?

>
> But lru_cache_add_lru uses pagevec so it could miss spinlock
> easily so above rule was broken so user may see inconsistent data.
>
> This patch fixes it with using explict barrier rather than depending
> on lru spinlock.
>
> Cc: Mel Gorman <mgorman@...e.de>
> Cc: Andrea Arcangeli <aarcange@...hat.com>
> Cc: Hugh Dickins <hughd@...gle.com>
> Signed-off-by: Minchan Kim <minchan@...nel.org>
> ---
>   mm/huge_memory.c | 7 +++----
>   1 file changed, 3 insertions(+), 4 deletions(-)
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index bfa142e..fad800e 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -725,11 +725,10 @@ static int __do_huge_pmd_anonymous_page(struct mm_struct *mm,
>   		pmd_t entry;
>   		entry = mk_huge_pmd(page, vma);
>   		/*
> -		 * The spinlocking to take the lru_lock inside
> -		 * page_add_new_anon_rmap() acts as a full memory
> -		 * barrier to be sure clear_huge_page writes become
> -		 * visible after the set_pmd_at() write.
> +		 * clear_huge_page write become visible after the
> +		 * set_pmd_at() write.
>   		 */
> +		smp_wmb();
>   		page_add_new_anon_rmap(page, vma, haddr);
>   		set_pmd_at(mm, haddr, pmd, entry);
>   		pgtable_trans_huge_deposit(mm, pgtable);

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