[PATCH 4.19 6/7] mm: initialize deferred pages with interrupts enabled

[Pavel Tatashin <pasha.tatashin@xxxxxxxxxx>]

From: Pavel Tatashin <pasha.tatashin@xxxxxxxxxx>

commit da97f2d56bbd880b4138916a7ef96f9881a551b2 upstream.

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@xxxxxxxxxxxxxxxxx
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).

Let's 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")
Reported-by: Shile Zhang <shile.zhang@xxxxxxxxxxxxxxxxx>
Signed-off-by: Pavel Tatashin <pasha.tatashin@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Reviewed-by: Daniel Jordan <daniel.m.jordan@xxxxxxxxxx>
Reviewed-by: David Hildenbrand <david@xxxxxxxxxx>
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
Cc: James Morris <jmorris@xxxxxxxxx>
Cc: Kirill Tkhai <ktkhai@xxxxxxxxxxxxx>
Cc: Sasha Levin <sashal@xxxxxxxxxx>
Cc: Yiqian Wei <yiwei@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>	[4.17+]
Link: http://lkml.kernel.org/r/20200403140952.17177-3-pasha.tatashin@xxxxxxxxxx
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
---
 include/linux/mmzone.h |  6 ++++--
 mm/page_alloc.c        | 20 +++++++-------------
 2 files changed, 11 insertions(+), 15 deletions(-)

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index d6791e2df30a..fba0eee85392 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -636,8 +636,10 @@ typedef struct pglist_data {
 #endif
 #if defined(CONFIG_MEMORY_HOTPLUG) || defined(CONFIG_DEFERRED_STRUCT_PAGE_INIT)
 	/*
-	 * Must be held any time you expect node_start_pfn, node_present_pages
-	 * or node_spanned_pages stay constant.
+	 * 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 182f1198a406..05c27edbe076 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1644,6 +1644,13 @@ 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;
@@ -1666,8 +1673,6 @@ 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));
 
@@ -1709,17 +1714,6 @@ 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.
-- 
2.25.1