Re: [PATCH 09/14] mm: __first_valid_page skip over offline pages

[Vlastimil Babka <vbabka@suse.cz>]

On 05/15/2017 10:58 AM, Michal Hocko wrote:
> From: Michal Hocko <mhocko@suse.com>
> 
> __first_valid_page skips over invalid pfns in the range but it might
> still stumble over offline pages. At least start_isolate_page_range
> will mark those set_migratetype_isolate. This doesn't represent
> any immediate AFAICS because alloc_contig_range will fail to isolate
> those pages but it relies on not fully initialized page which will
> become a problem later when we stop associating offline pages to zones.
> Use pfn_to_online_page to handle this.
> 
> This is more a preparatory patch than a fix.
> 
> Signed-off-by: Michal Hocko <mhocko@suse.com>

Iterating over single pages when the whole section is offline seems
rather wasteful, but it should be really rare, so whatever.

Acked-by: Vlastimil Babka <vbabka@suse.cz>

> ---
>  mm/page_isolation.c | 26 ++++++++++++++++++--------
>  1 file changed, 18 insertions(+), 8 deletions(-)
> 
> diff --git a/mm/page_isolation.c b/mm/page_isolation.c
> index 5092e4ef00c8..3606104893e0 100644
> --- a/mm/page_isolation.c
> +++ b/mm/page_isolation.c
> @@ -138,12 +138,18 @@ static inline struct page *
>  __first_valid_page(unsigned long pfn, unsigned long nr_pages)
>  {
>  	int i;
> -	for (i = 0; i < nr_pages; i++)
> -		if (pfn_valid_within(pfn + i))
> -			break;
> -	if (unlikely(i == nr_pages))
> -		return NULL;
> -	return pfn_to_page(pfn + i);
> +
> +	for (i = 0; i < nr_pages; i++) {
> +		struct page *page;
> +
> +		if (!pfn_valid_within(pfn + i))
> +			continue;
> +		page = pfn_to_online_page(pfn + i);
> +		if (!page)
> +			continue;
> +		return page;
> +	}
> +	return NULL;
>  }
>  
>  /*
> @@ -184,8 +190,12 @@ int start_isolate_page_range(unsigned long start_pfn, unsigned long end_pfn,
>  undo:
>  	for (pfn = start_pfn;
>  	     pfn < undo_pfn;
> -	     pfn += pageblock_nr_pages)
> -		unset_migratetype_isolate(pfn_to_page(pfn), migratetype);
> +	     pfn += pageblock_nr_pages) {
> +		struct page *page = pfn_to_online_page(pfn);
> +		if (!page)
> +			continue;
> +		unset_migratetype_isolate(page, migratetype);
> +	}
>  
>  	return -EBUSY;
>  }
>