linux-kernel - Re: [PATCH v2 05/10] mm/gup: separate PageHighMem() and PageHighMemZone() use case

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Message-ID: <20200501122427.GB21897@infradead.org>
Date:   Fri, 1 May 2020 05:24:27 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     js1304@...il.com
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Vlastimil Babka <vbabka@...e.cz>,
        Laura Abbott <labbott@...hat.com>,
        "Aneesh Kumar K . V" <aneesh.kumar@...ux.ibm.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Michal Hocko <mhocko@...e.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Roman Gushchin <guro@...com>, Minchan Kim <minchan@...nel.org>,
        Rik van Riel <riel@...riel.com>,
        Christian Koenig <christian.koenig@....com>,
        Huang Rui <ray.huang@....com>,
        Eric Biederman <ebiederm@...ssion.com>,
        "Rafael J . Wysocki" <rjw@...ysocki.net>,
        Pavel Machek <pavel@....cz>, kernel-team@....com,
        Christoph Hellwig <hch@...radead.org>,
        Joonsoo Kim <iamjoonsoo.kim@....com>
Subject: Re: [PATCH v2 05/10] mm/gup: separate PageHighMem() and
 PageHighMemZone() use case

On Wed, Apr 29, 2020 at 12:26:38PM +0900, js1304@...il.com wrote:
> From: Joonsoo Kim <iamjoonsoo.kim@....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.
> 
> Acked-by: Roman Gushchin <guro@...com>
> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@....com>
> ---
>  mm/gup.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/gup.c b/mm/gup.c
> index 11fda53..9652eed 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -1608,7 +1608,7 @@ static struct page *new_non_cma_page(struct page *page, unsigned long private)
>  	 */
>  	gfp_t gfp_mask = GFP_USER | __GFP_NOWARN;
>  
> -	if (PageHighMem(page))
> +	if (PageHighMemZone(page))
>  		gfp_mask |= __GFP_HIGHMEM;

I think this wants to stay PageHighMem.  This migrates CMA pages to
other places before doing a long term pin.  Anything that didn't have
a direct mapping before won't need one for the new page, which could
also include non-highmem zones without a highmem mapping.