Re: [PATCH] mm, hugetlb, soft_offline: save compound page order before page migration

[Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>]

Hi Alexandru,

On Tue, Sep 12, 2017 at 10:43:06PM +0200, Alexandru Moise wrote:
> This fixes a bug in madvise() where if you'd try to soft offline a
> hugepage via madvise(), while walking the address range you'd end up,
> using the wrong page offset due to attempting to get the compound
> order of a former but presently not compound page, due to dissolving
> the huge page (since c3114a8).
> 
> Signed-off-by: Alexandru Moise <00moses.alexander00@gmail.com>

There was a similar discussion in https://marc.info/?l=linux-kernel&m=150354919510631&w=2
over thp. As I stated there, if we give multi-page range into the parameters
[start, end), we expect that memory errors are injected to every single page
within the range. 

So I start to feel that we should revert the following patch which introduced
the multi-page stepping.

   commit 20cb6cab52a21b46e3c0dc7bd23f004f810fb421
   Author: Wanpeng Li <liwanp@linux.vnet.ibm.com>
   Date:   Mon Sep 30 13:45:21 2013 -0700
   
       mm/hwpoison: fix traversal of hugetlbfs pages to avoid printk flood

In order to suppress the printk flood, we can use ratelimit mechanism, or
just s/pr_info/pr_debug/ might be ok.

Thanks,
Naoya Horiguchi

> ---
>  mm/madvise.c | 12 ++++++++++--
>  1 file changed, 10 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/madvise.c b/mm/madvise.c
> index 21261ff0466f..25bade36e9ca 100644
> --- a/mm/madvise.c
> +++ b/mm/madvise.c
> @@ -625,18 +625,26 @@ static int madvise_inject_error(int behavior,
>  {
>  	struct page *page;
>  	struct zone *zone;
> +	unsigned int order;
>  
>  	if (!capable(CAP_SYS_ADMIN))
>  		return -EPERM;
>  
> -	for (; start < end; start += PAGE_SIZE <<
> -				compound_order(compound_head(page))) {
> +
> +	for (; start < end; start += PAGE_SIZE << order) {
>  		int ret;
>  
>  		ret = get_user_pages_fast(start, 1, 0, &page);
>  		if (ret != 1)
>  			return ret;
>  
> +		/*
> +		 * When soft offlining hugepages, after migrating the page
> +		 * we dissolve it, therefore in the second loop "page" will
> +		 * no longer be a compound page, and order will be 0.
> +		 */
> +		order = compound_order(compound_head(page));
> +
>  		if (PageHWPoison(page)) {
>  			put_page(page);
>  			continue;
> -- 
> 2.14.1
> 
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