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Subject[PATCH v4 2/3] mm: initialize deferred pages with interrupts enabled
Date
Initializing struct pages is a long task and keeping interrupts disabled
for the duration of this operation introduces a number of problems.

1. jiffies are not updated for long period of time, and thus incorrect time
is reported. See proposed solution and discussion here:
lkml/20200311123848.118638-1-shile.zhang@linux.alibaba.com
2. It prevents farther improving deferred page initialization by allowing
intra-node multi-threading.

We are keeping interrupts disabled to solve a rather theoretical problem
that was never observed in real world (See 3a2d7fa8a3d5).

Lets keep interrupts enabled. In case we ever encounter a scenario where
an interrupt thread wants to allocate large amount of memory this early in
boot we can deal with that by growing zone (see deferred_grow_zone()) by
the needed amount before starting deferred_init_memmap() threads.

Before:
[ 1.232459] node 0 initialised, 12058412 pages in 1ms

After:
[ 1.632580] node 0 initialised, 12051227 pages in 436ms

Fixes: 3a2d7fa8a3d5 ("mm: disable interrupts while initializing deferred pages")
Cc: stable@vger.kernel.org # 4.17+

Reported-by: Shile Zhang <shile.zhang@linux.alibaba.com>
Signed-off-by: Pavel Tatashin <pasha.tatashin@soleen.com>
Reviewed-by: Daniel Jordan <daniel.m.jordan@oracle.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
---
include/linux/mmzone.h | 2 ++
mm/page_alloc.c | 20 +++++++-------------
2 files changed, 9 insertions(+), 13 deletions(-)

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index e84d448988b6..ac6a8245f063 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -723,6 +723,8 @@ typedef struct pglist_data {
/*
* Must be held any time you expect node_start_pfn,
* node_present_pages, node_spanned_pages or nr_zones to stay constant.
+ * Also synchronizes pgdat->first_deferred_pfn during deferred page
+ * init.
*
* pgdat_resize_lock() and pgdat_resize_unlock() are provided to
* manipulate node_size_lock without checking for CONFIG_MEMORY_HOTPLUG
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index d95bfd328107..5ffa8d7e5545 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1784,6 +1784,13 @@ static int __init deferred_init_memmap(void *data)
BUG_ON(pgdat->first_deferred_pfn > pgdat_end_pfn(pgdat));
pgdat->first_deferred_pfn = ULONG_MAX;

+ /*
+ * Once we unlock here, the zone cannot be grown anymore, thus if an
+ * interrupt thread must allocate this early in boot, zone must be
+ * pre-grown prior to start of deferred page initialization.
+ */
+ pgdat_resize_unlock(pgdat, &flags);
+
/* Only the highest zone is deferred so find it */
for (zid = 0; zid < MAX_NR_ZONES; zid++) {
zone = pgdat->node_zones + zid;
@@ -1806,8 +1813,6 @@ static int __init deferred_init_memmap(void *data)
touch_nmi_watchdog();
}
zone_empty:
- pgdat_resize_unlock(pgdat, &flags);
-
/* Sanity check that the next zone really is unpopulated */
WARN_ON(++zid < MAX_NR_ZONES && populated_zone(++zone));

@@ -1849,17 +1854,6 @@ deferred_grow_zone(struct zone *zone, unsigned int order)

pgdat_resize_lock(pgdat, &flags);

- /*
- * If deferred pages have been initialized while we were waiting for
- * the lock, return true, as the zone was grown. The caller will retry
- * this zone. We won't return to this function since the caller also
- * has this static branch.
- */
- if (!static_branch_unlikely(&deferred_pages)) {
- pgdat_resize_unlock(pgdat, &flags);
- return true;
- }
-
/*
* If someone grew this zone while we were waiting for spinlock, return
* true, as there might be enough pages already.
--
2.17.1
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