linux-kernel - Re: [PATCH] mm/isolate: Drop pre-validating migrate type in undo_isolate_page

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Message-ID: <20190705091114.GG8231@dhcp22.suse.cz>
Date:   Fri, 5 Jul 2019 11:11:14 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Anshuman Khandual <anshuman.khandual@....com>
Cc:     linux-mm@...ck.org, Oscar Salvador <osalvador@...e.de>,
        Qian Cai <cai@....pw>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/isolate: Drop pre-validating migrate type in
 undo_isolate_page_range()

On Fri 05-07-19 11:42:41, Anshuman Khandual wrote:
> unset_migratetype_isolate() already validates under zone lock that a given
> page has already been isolated as MIGRATE_ISOLATE. There is no need for
> another check before. Hence just drop this redundant validation.

unset_migratetype_isolate take zone lock and it is always preferable to
skip not take this lock if we know it would be pointless. Besides that
undo_isolate_page_range is a slow path so a nano optimizing it is not
worth it.

> Cc: Oscar Salvador <osalvador@...e.de>
> Cc: Michal Hocko <mhocko@...e.com>
> Cc: Qian Cai <cai@....pw>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: linux-mm@...ck.org
> Cc: linux-kernel@...r.kernel.org
> 
> Signed-off-by: Anshuman Khandual <anshuman.khandual@....com>
> ---
> Is there any particular reason to do this migratetype pre-check without zone
> lock before calling unsert_migrate_isolate() ? If not this should be removed.
> 
>  mm/page_isolation.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/page_isolation.c b/mm/page_isolation.c
> index e3638a5bafff..f529d250c8a5 100644
> --- a/mm/page_isolation.c
> +++ b/mm/page_isolation.c
> @@ -243,7 +243,7 @@ int undo_isolate_page_range(unsigned long start_pfn, unsigned long end_pfn,
>  	     pfn < end_pfn;
>  	     pfn += pageblock_nr_pages) {
>  		page = __first_valid_page(pfn, pageblock_nr_pages);
> -		if (!page || !is_migrate_isolate_page(page))
> +		if (!page)
>  			continue;
>  		unset_migratetype_isolate(page, migratetype);
>  	}
> -- 
> 2.20.1

-- 
Michal Hocko
SUSE Labs