linux-kernel - Re: [PATCH v1 1/6] mm/page_alloc: tweak comments in has_unmovable

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Date:   Tue, 28 Jul 2020 21:48:26 +0800
From:   Baoquan He <bhe@...hat.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>,
        "Michael S . Tsirkin" <mst@...hat.com>
Subject: Re: [PATCH v1 1/6] mm/page_alloc: tweak comments in
 has_unmovable_pages()

On 06/30/20 at 04:26pm, David Hildenbrand wrote:
> Let's move the split comment regarding bootmem allocations and memory
> holes, especially in the context of ZONE_MOVABLE, to the PageReserved()
> check.
> 
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Michal Hocko <mhocko@...e.com>
> Cc: Michael S. Tsirkin <mst@...hat.com>
> Signed-off-by: David Hildenbrand <david@...hat.com>
> ---
>  mm/page_alloc.c | 22 ++++++----------------
>  1 file changed, 6 insertions(+), 16 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 48eb0f1410d47..bd3ebf08f09b9 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -8207,14 +8207,6 @@ struct page *has_unmovable_pages(struct zone *zone, struct page *page,
>  	unsigned long iter = 0;
>  	unsigned long pfn = page_to_pfn(page);
>  
> -	/*
> -	 * TODO we could make this much more efficient by not checking every
> -	 * page in the range if we know all of them are in MOVABLE_ZONE and
> -	 * that the movable zone guarantees that pages are migratable but
> -	 * the later is not the case right now unfortunatelly. E.g. movablecore
> -	 * can still lead to having bootmem allocations in zone_movable.
> -	 */
> -
>  	if (is_migrate_cma_page(page)) {
>  		/*
>  		 * CMA allocations (alloc_contig_range) really need to mark
> @@ -8233,6 +8225,12 @@ struct page *has_unmovable_pages(struct zone *zone, struct page *page,
>  
>  		page = pfn_to_page(pfn + iter);
>  
> +		/*
> +		 * Both, bootmem allocations and memory holes are marked
> +		 * PG_reserved and are unmovable. We can even have unmovable
> +		 * allocations inside ZONE_MOVABLE, for example when
> +		 * specifying "movable_core".
                               ~~~~ should be 'movablecore', we don't
have kernel parameter 'movable_core'.

Otherwise, this looks good to me. Esp the code comment at below had been
added very long time ago and obsolete.

Reviewed-by: Baoquan He <bhe@...hat.com>

By the way, David, do you know what is the situation of having unmovable
allocations inside ZONE_MOVABLE when specifying 'movablecore'? I quickly
went through find_zone_movable_pfns_for_nodes(), but didn't get why.
Could you tell a little more detail about it?

Thanks
Baoquan

> +		 */
>  		if (PageReserved(page))
>  			return page;
>  
> @@ -8306,14 +8304,6 @@ struct page *has_unmovable_pages(struct zone *zone, struct page *page,
>  		 * it.  But now, memory offline itself doesn't call
>  		 * shrink_node_slabs() and it still to be fixed.
>  		 */
> -		/*
> -		 * If the page is not RAM, page_count()should be 0.
> -		 * we don't need more check. This is an _used_ not-movable page.
> -		 *
> -		 * The problematic thing here is PG_reserved pages. PG_reserved
> -		 * is set to both of a memory hole page and a _used_ kernel
> -		 * page at boot.
> -		 */
>  		return page;
>  	}
>  	return NULL;
> -- 
> 2.26.2
> 
>