Re: [RFC][PATCH v1 04/11] mm: madvise: call soft_offline_page() without MF_COUNT_INCREASED

[Anshuman Khandual <anshuman.khandual@arm.com>]

On 11/09/2018 12:17 PM, Naoya Horiguchi wrote:
> Currently madvise_inject_error() pins the target page when calling
> memory error handler, but it's not good because the refcount is just
> an artifact of error injector and mock nothing about hw error itself.
> IOW, pinning the error page is part of error handler's task, so
> let's stop doing it.

Did not get that. Could you please kindly explain how an incremented
ref count through get_user_pages_fast() was a mocking the HW error
previously ? Though I might be missing the some context here.

> 
> Signed-off-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
> ---
>  mm/madvise.c | 25 +++++++++++--------------
>  1 file changed, 11 insertions(+), 14 deletions(-)
> 
> diff --git v4.19-mmotm-2018-10-30-16-08/mm/madvise.c v4.19-mmotm-2018-10-30-16-08_patched/mm/madvise.c
> index 6cb1ca9..9fa0225 100644
> --- v4.19-mmotm-2018-10-30-16-08/mm/madvise.c
> +++ v4.19-mmotm-2018-10-30-16-08_patched/mm/madvise.c
> @@ -637,6 +637,16 @@ 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);
> +
>  		pfn = page_to_pfn(page);
>  
>  		/*
> @@ -646,16 +656,11 @@ static int madvise_inject_error(int behavior,
>  		 */
>  		order = compound_order(compound_head(page));
>  
> -		if (PageHWPoison(page)) {
> -			put_page(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);

Probably something defined as a new "ignored" in the memory faults flag
enumeration instead of passing '0' directly.