linux-kernel - Re: [PATCH] mm,memory_hotplug: Explicitly pass the head to isolate_huge

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Date:   Tue, 12 Feb 2019 09:33:29 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Oscar Salvador <osalvador@...e.de>
Cc:     akpm@...ux-foundation.org, david@...hat.com,
        anthony.yznaga@...cle.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm,memory_hotplug: Explicitly pass the head to
 isolate_huge_page

On Fri 08-02-19 10:06:04, Oscar Salvador wrote:
> isolate_huge_page() expects we pass the head of hugetlb page to it:
> 
> bool isolate_huge_page(...)
> {
> 	...
> 	VM_BUG_ON_PAGE(!PageHead(page), page);
> 	...
> }
> 
> While I really cannot think of any situation where we end up with a
> non-head page between hands in do_migrate_range(), let us make sure
> the code is as sane as possible by explicitly passing the Head.
> Since we already got the pointer, it does not take us extra effort.
> 
> Signed-off-by: Oscar Salvador <osalvador@...e.de>

Acked-by: Michal Hocko <mhocko@...e.com>

Btw.

> ---
>  mm/memory_hotplug.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index 656ff386ac15..d5f7afda67db 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -1378,12 +1378,12 @@ do_migrate_range(unsigned long start_pfn, unsigned long end_pfn)
>  
>  		if (PageHuge(page)) {
>  			struct page *head = compound_head(page);
> -			pfn = page_to_pfn(head) + (1<<compound_order(head)) - 1;
>  			if (compound_order(head) > PFN_SECTION_SHIFT) {
>  				ret = -EBUSY;
>  				break;
>  			}

Why are we doing this, btw? 

> -			isolate_huge_page(page, &source);
> +			pfn = page_to_pfn(head) + (1<<compound_order(head)) - 1;
> +			isolate_huge_page(head, &source);
>  			continue;
>  		} else if (PageTransHuge(page))
>  			pfn = page_to_pfn(compound_head(page))
> -- 
> 2.13.7

-- 
Michal Hocko
SUSE Labs