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Date:   Fri, 18 Oct 2019 13:52:27 +0200
From:   Michal Hocko <mhocko@kernel.org>
To:     Oscar Salvador <osalvador@suse.de>
Cc:     n-horiguchi@ah.jp.nec.com, mike.kravetz@oracle.com,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [RFC PATCH v2 02/16] mm,madvise: call soft_offline_page()
 without MF_COUNT_INCREASED
Message-ID: <20191018115227.GL5017@dhcp22.suse.cz>
References: <20191017142123.24245-1-osalvador@suse.de>
 <20191017142123.24245-3-osalvador@suse.de>
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On Thu 17-10-19 16:21:09, Oscar Salvador wrote:
> From: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
> 
> The call to get_user_pages_fast is only to get the pointer to a struct
> page of a given address, pinning it is memory-poisoning handler's job,
> so drop the refcount grabbed by get_user_pages_fast
> 
> Signed-off-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
> Signed-off-by: Oscar Salvador <osalvador@suse.de>
> ---
>  mm/madvise.c | 24 ++++++++++++------------
>  1 file changed, 12 insertions(+), 12 deletions(-)
> 
> diff --git a/mm/madvise.c b/mm/madvise.c
> index 2be9f3fdb05e..89ed9a22ff4f 100644
> --- a/mm/madvise.c
> +++ b/mm/madvise.c
> @@ -878,16 +878,24 @@ static int madvise_inject_error(int behavior,
>  		 */
>  		order = compound_order(compound_head(page));
>  
> -		if (PageHWPoison(page)) {
> -			put_page(page);
> +		/*
> +		 * The get_user_pages_fast() is just to get the pfn of the
> +		 * given address, and the refcount has nothing to do with
> +		 * what we try to test, so it should be released immediately.
> +		 * This is racy but it's intended because the real hardware
> +		 * errors could happen at any moment and memory error handlers
> +		 * must properly handle the race.
> +		 */
> +		put_page(page);
> +
> +		if (PageHWPoison(page))
>  			continue;
> -		}
>  
>  		if (behavior == MADV_SOFT_OFFLINE) {
>  			pr_info("Soft offlining pfn %#lx at process virtual address %#lx\n",
>  					pfn, start);
>  
> -			ret = soft_offline_page(page, MF_COUNT_INCREASED);
> +			ret = soft_offline_page(page, 0);

What does prevent this struct page to go away completely?

>  			if (ret)
>  				return ret;
>  			continue;
> @@ -895,14 +903,6 @@ static int madvise_inject_error(int behavior,
>  
>  		pr_info("Injecting memory failure for pfn %#lx at process virtual address %#lx\n",
>  				pfn, start);
> -
> -		/*
> -		 * Drop the page reference taken by get_user_pages_fast(). In
> -		 * the absence of MF_COUNT_INCREASED the memory_failure()
> -		 * routine is responsible for pinning the page to prevent it
> -		 * from being released back to the page allocator.
> -		 */
> -		put_page(page);
>  		ret = memory_failure(pfn, 0);
>  		if (ret)
>  			return ret;
> -- 
> 2.12.3
> 

-- 
Michal Hocko
SUSE Labs