[PATCH] mm, page_alloc: only use per-cpu allocator for irq-safe requests -fix

From: Mel Gorman
Date: Wed Feb 08 2017 - 10:09:31 EST

preempt_enable_no_resched() was used based on review feedback that had no
strong objection at the time. It avoided introducing a preemption point
where one didn't exist before which was marginal at best.

However, it is hazardous to the RT tree according to Thomas Gleixner
and is a violation of its expected use according to Peter Zijlstra. In
Peter's own words "the only acceptable use of preempt_enable_no_resched()
is if the next statement is a schedule() variant".

The impact of using preempt_enable in this particular
fast path is negligible. This is a fix to the mmotm patch

Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
mm/page_alloc.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index eaecb4b145e6..2a36dad03dac 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -2520,7 +2520,7 @@ void free_hot_cold_page(struct page *page, bool cold)

- preempt_enable_no_resched();
+ preempt_enable();

@@ -2686,7 +2686,7 @@ static struct page *rmqueue_pcplist(struct zone *preferred_zone,
__count_zid_vm_events(PGALLOC, page_zonenum(page), 1 << order);
zone_statistics(preferred_zone, zone);
- preempt_enable_no_resched();
+ preempt_enable();
return page;