linux-kernel - [PATCH 10/10] mm/page-flags: change the implementation of the PageHighMem()

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Message-Id: <1587369582-3882-11-git-send-email-iamjoonsoo.kim@lge.com>
Date:   Mon, 20 Apr 2020 16:59:42 +0900
From:   js1304@...il.com
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     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>,
        Joonsoo Kim <iamjoonsoo.kim@....com>
Subject: [PATCH 10/10] mm/page-flags: change the implementation of the PageHighMem()

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.

Previous patches introduce PageHighMemZone() macro and separates both
cases strictly. So, now, PageHighMem() is used just for checking if
there is a direct mapping for this page or not.

In the following patchset, ZONE_MOVABLE which could be considered as
the highmem type zone in some configuration could have both types of
pages, direct mapped pages and unmapped pages. So, current implementation
of PageHighMem() that checks the zone rather than checks the page in order
to check if a direct mapping exists will be invalid. This patch prepares
that case by implementing PageHighMem() with the max_low_pfn.

Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@....com>
---
 include/linux/page-flags.h | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index fca0cce..7ac5fc8 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -375,6 +375,8 @@ PAGEFLAG(Readahead, reclaim, PF_NO_COMPOUND)
 	TESTCLEARFLAG(Readahead, reclaim, PF_NO_COMPOUND)
 
 #ifdef CONFIG_HIGHMEM
+extern unsigned long max_low_pfn;
+
 /*
  * Must use a macro here due to header dependency issues. page_zone() is not
  * available at this point.
@@ -383,7 +385,7 @@ PAGEFLAG(Readahead, reclaim, PF_NO_COMPOUND)
  * in order to predict previous gfp_flags or to count something for system
  * memory management.
  */
-#define PageHighMem(__p) is_highmem_idx(page_zonenum(__p))
+#define PageHighMem(__p) (page_to_pfn(__p) >= max_low_pfn)
 #define PageHighMemZone(__p) is_highmem_idx(page_zonenum(__p))
 #else
 PAGEFLAG_FALSE(HighMem)
-- 
2.7.4