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Date:   Wed, 22 Apr 2020 00:44:23 -0700
From:   Christoph Hellwig <hch@infradead.org>
To:     js1304@gmail.com
Cc:     Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, Vlastimil Babka <vbabka@suse.cz>,
        Laura Abbott <labbott@redhat.com>,
        "Aneesh Kumar K . V" <aneesh.kumar@linux.ibm.com>,
        Mel Gorman <mgorman@techsingularity.net>,
        Michal Hocko <mhocko@suse.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Roman Gushchin <guro@fb.com>, Minchan Kim <minchan@kernel.org>,
        Rik van Riel <riel@surriel.com>,
        Christian Koenig <christian.koenig@amd.com>,
        Huang Rui <ray.huang@amd.com>,
        Eric Biederman <ebiederm@xmission.com>,
        "Rafael J . Wysocki" <rjw@rjwysocki.net>,
        Pavel Machek <pavel@ucw.cz>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>
Subject: Re: [PATCH 03/10] mm/migrate: separate PageHighMem() and
 PageHighMemZone() use case
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On Mon, Apr 20, 2020 at 04:59:35PM +0900, js1304@gmail.com wrote:
> From: Joonsoo Kim <iamjoonsoo.kim@lge.com>
> 
> Until now, PageHighMem() is used for two different cases. One is to check
> if there is a direct mapping for this page or not. The other is to check
> the zone of this page, that is, weather it is the highmem type zone or not.
> 
> Now, we have separate functions, PageHighMem() and PageHighMemZone() for
> each cases. Use appropriate one.
> 
> Note that there are some rules to determine the proper macro.
> 
> 1. If PageHighMem() is called for checking if the direct mapping exists
> or not, use PageHighMem().
> 2. If PageHighMem() is used to predict the previous gfp_flags for
> this page, use PageHighMemZone(). The zone of the page is related to
> the gfp_flags.
> 3. If purpose of calling PageHighMem() is to count highmem page and
> to interact with the system by using this count, use PageHighMemZone().
> This counter is usually used to calculate the available memory for an
> kernel allocation and pages on the highmem zone cannot be available
> for an kernel allocation.
> 4. Otherwise, use PageHighMemZone(). It's safe since it's implementation
> is just copy of the previous PageHighMem() implementation and won't
> be changed.
> 
> I apply the rule #2 for this patch.
> 
> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
> ---
>  include/linux/migrate.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/linux/migrate.h b/include/linux/migrate.h
> index 3e546cb..9a57766 100644
> --- a/include/linux/migrate.h
> +++ b/include/linux/migrate.h
> @@ -47,7 +47,7 @@ static inline struct page *new_page_nodemask(struct page *page,
>  		order = HPAGE_PMD_ORDER;
>  	}
>  
> -	if (PageHighMem(page) || (zone_idx(page_zone(page)) == ZONE_MOVABLE))
> +	if (PageHighMemZone(page) || zone_idx(page_zone(page)) == ZONE_MOVABLE)
>  		gfp_mask |= __GFP_HIGHMEM;

I think this would be much cleaner if you open coded PageHighMemZone
here.

E.g.

	int zone = page_zone(page);

	if (is_highmem_idx(zone) || zone_idx(zone) == ZONE_MOVABLE)