linux-kernel - Re: [PATCH 02/10] mm,madvise: call soft_offline_page() without MF_COUNT

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Date:   Wed, 11 Sep 2019 12:23:24 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Oscar Salvador <osalvador@...e.de>, n-horiguchi@...jp.nec.com
Cc:     mhocko@...nel.org, mike.kravetz@...cle.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 02/10] mm,madvise: call soft_offline_page() without
 MF_COUNT_INCREASED

On 10.09.19 12:30, Oscar Salvador wrote:
> From: Naoya Horiguchi <n-horiguchi@...jp.nec.com>
> 
> Currently madvise_inject_error() pins the target via get_user_pages_fast.
> The call to get_user_pages_fast is only to get the respective page
> of a given address, but it is the job of the memory-poisoning handler
> to deal with races, so drop the refcount grabbed by get_user_pages_fast.
> 
> Signed-off-by: Naoya Horiguchi <n-horiguchi@...jp.nec.com>
> Signed-off-by: Oscar Salvador <osalvador@...e.de>
> ---
>  mm/madvise.c | 25 +++++++++++--------------
>  1 file changed, 11 insertions(+), 14 deletions(-)
> 
> diff --git a/mm/madvise.c b/mm/madvise.c
> index 6e023414f5c1..fbe6d402232c 100644
> --- a/mm/madvise.c
> +++ b/mm/madvise.c
> @@ -883,6 +883,16 @@ static int madvise_inject_error(int behavior,
>  		ret = get_user_pages_fast(start, 1, 0, &page);
>  		if (ret != 1)
>  			return ret;
> +		/*
> +		 * 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);
> +

I wonder if it would be clearer to do that after the page has been fully
used  - e.g. after getting the pfn and the order (and then e.g.,
symbolically setting the page pointer to 0).

I guess the important part of this patch is to not have an elevated
refcount while calling soft_offline_page().

>  		pfn = page_to_pfn(page);
>  
>  		/*
> @@ -892,16 +902,11 @@ static int madvise_inject_error(int behavior,
>  		 */
>  		order = compound_order(compound_head(page));
>  
> -		if (PageHWPoison(page)) {
> -			put_page(page);
> -			continue;
> -		}

This change is not reflected in the changelog. I would have expected
that only the put_page() would go. If this should go completely, I
suggest a separate patch.

> -
>  		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);
>  			if (ret)
>  				return ret;
>  			continue;
> @@ -909,14 +914,6 @@ 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;
> 


-- 

Thanks,

David / dhildenb