Re: [PATCH v2 1/5] mm: memory_hotplug: remove head variable in do_migrate_range()

[Jonathan Cameron <Jonathan.Cameron@Huawei.com>]

On Sat, 17 Aug 2024 16:49:37 +0800
Kefeng Wang <wangkefeng.wang@huawei.com> wrote:

> Directly use a folio for HugeTLB and THP when calculate the next pfn,
> then remove unused head variable.
> 
> Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>

Hi,

I was curious so had a read and have a quick comment
on the comment.


> ---
>  mm/memory_hotplug.c | 22 ++++++++++++++--------
>  1 file changed, 14 insertions(+), 8 deletions(-)
> 
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index df291f2e509d..dc19b0e28fbc 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -1773,7 +1773,7 @@ static int scan_movable_pages(unsigned long start, unsigned long end,
>  static void do_migrate_range(unsigned long start_pfn, unsigned long end_pfn)
>  {
>  	unsigned long pfn;
> -	struct page *page, *head;
> +	struct page *page;
>  	LIST_HEAD(source);
>  	static DEFINE_RATELIMIT_STATE(migrate_rs, DEFAULT_RATELIMIT_INTERVAL,
>  				      DEFAULT_RATELIMIT_BURST);
> @@ -1786,14 +1786,20 @@ static void do_migrate_range(unsigned long start_pfn, unsigned long end_pfn)
>  			continue;
>  		page = pfn_to_page(pfn);
>  		folio = page_folio(page);
> -		head = &folio->page;
>  
> -		if (PageHuge(page)) {
> -			pfn = page_to_pfn(head) + compound_nr(head) - 1;
> -			isolate_hugetlb(folio, &source);
> -			continue;
> -		} else if (PageTransHuge(page))
> -			pfn = page_to_pfn(head) + thp_nr_pages(page) - 1;
> +		/*
> +		 * The folio hold no reference or lock, it might be changed
> +		 * concurrently(eg, split), folio_nr_pages() may read garbage,
> +		 * but out loop could handle that as it revisits the split
> +		 * folio later.
> +		 */
Perhaps reword this (it confused me a bit) I'm not sure I've retained
the intended meaning!

		* No reference or lock is held on the folio, so it might
		* be modified concurrently (e.g. split).  As such,
		* folio_nr_pages() may read garbage.  This is fine as the
		* outer loop will revisit the split folio later.


> +		if (folio_test_large(folio)) {
> +			pfn = folio_pfn(folio) + folio_nr_pages(folio) - 1;
> +			if (folio_test_hugetlb(folio)) {
> +				isolate_hugetlb(folio, &source);
> +				continue;
> +			}
> +		}
>  
>  		/*
>  		 * HWPoison pages have elevated reference counts so the migration would