mm: reference totalram_pages and managed_pages once per function

Patch series "mm: convert totalram_pages, totalhigh_pages and managed
pages to atomic", v5.

This series converts totalram_pages, totalhigh_pages and
zone->managed_pages to atomic variables.

totalram_pages, zone->managed_pages and totalhigh_pages updates are
protected by managed_page_count_lock, but readers never care about it.
Convert these variables to atomic to avoid readers potentially seeing a
store tear.

Main motivation was that managed_page_count_lock handling was complicating
things.  It was discussed in length here,
https://lore.kernel.org/patchwork/patch/995739/#1181785 It seemes better
to remove the lock and convert variables to atomic.  With the change,
preventing poteintial store-to-read tearing comes as a bonus.

This patch (of 4):

This is in preparation to a later patch which converts totalram_pages and
zone->managed_pages to atomic variables.  Please note that re-reading the
value might lead to a different value and as such it could lead to
unexpected behavior.  There are no known bugs as a result of the current
code but it is better to prevent from them in principle.

Link: http://lkml.kernel.org/r/1542090790-21750-2-git-send-email-arunks@codeaurora.org
Signed-off-by: Arun KS <arunks@codeaurora.org>
Reviewed-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Reviewed-by: David Hildenbrand <david@redhat.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Reviewed-by: Pavel Tatashin <pasha.tatashin@soleen.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 3 insertions, 2 deletions

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index c82c41edfe22..b79e79caea99 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -7257,6 +7257,7 @@ static void calculate_totalreserve_pages(void)
                 for (i = 0; i < MAX_NR_ZONES; i++) {
                         struct zone *zone = pgdat->node_zones + i;
                         long max = 0;
+                        unsigned long managed_pages = zone->managed_pages;
 
                         /* Find valid and maximum lowmem_reserve in the zone */
                         for (j = i; j < MAX_NR_ZONES; j++) {
@@ -7267,8 +7268,8 @@ static void calculate_totalreserve_pages(void)
                         /* we treat the high watermark as reserved pages. */
                         max += high_wmark_pages(zone);
 
-                        if (max > zone->managed_pages)
-                                max = zone->managed_pages;
+                        if (max > managed_pages)
+                                max = managed_pages;
 
                         pgdat->totalreserve_pages += max;