[PATCH 1/2] mm,memory_hotplug: Remove un-taken lock

[Brendan Jackman <jackmanb@google.com>]

It seems that [1] was acked, and the a v2 was written[2] which improved
upon it, but got bogged down in discussion of other topics, so the
improvements were not included. Then [1] got merged as commit
27cacaad16c5 ("mm,memory_hotplug: drop unneeded locking") and we ended
up with locks that get taken for read but never for write.

So, let's remove the read locking.

Compared to Oscar's original v2[2], I have added a READ_ONCE in
page_outside_zone_boundaries; this is a substitute for the compiler
barrier that was implied by read_seqretry(). I believe this is necessary
to insure against UB, although the value being read here is only used
for a printk so the stakes seem very low (and this is all debug code
anyway). I believe a compiler barrier is also needed in zone_spans_pfn,
but I'll address that in a separate patch.

That read_seqretry() also impleied a CPU-level memory barrier, which I
don't think needs replacing: page_outside_zone_boundaries() is used in
the alloc and free paths, but you can't allocate or free pages from
the span that is in the middle of being added/removed by hotplug.

In other words, page_outside_zone_boundaries() doesn't require a
strictly up-to-date view of spanned_pages, but I think it does require
a value that was once/will eventually be correct, hence READ_ONCE.

[1] https://lore.kernel.org/all/20210531093958.15021-1-osalvador@suse.de/T/#u
[2] https://lore.kernel.org/linux-mm/20210602091457.17772-3-osalvador@suse.de/#t

Cc: David Hildenbrand <david@redhat.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Anshuman Khandual <anshuman.khandual@arm.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Pavel Tatashin <pasha.tatashin@soleen.com>
Co-developed-by: Oscar Salvador <osalvador@suse.de>
Signed-off-by: Oscar Salvador <osalvador@suse.de>
Signed-off-by: Brendan Jackman <jackmanb@google.com>
---
 include/linux/memory_hotplug.h | 35 -----------------------------------
 include/linux/mmzone.h         | 23 +++++------------------
 mm/mm_init.c                   |  1 -
 mm/page_alloc.c                | 10 +++-------
 4 files changed, 8 insertions(+), 61 deletions(-)

diff --git a/include/linux/memory_hotplug.h b/include/linux/memory_hotplug.h
index 7a9ff464608d..f9577e67e5ee 100644
--- a/include/linux/memory_hotplug.h
+++ b/include/linux/memory_hotplug.h
@@ -141,31 +141,7 @@ bool mhp_supports_memmap_on_memory(void);
 
 /*
  * Zone resizing functions
- *
- * Note: any attempt to resize a zone should has pgdat_resize_lock()
- * zone_span_writelock() both held. This ensure the size of a zone
- * can't be changed while pgdat_resize_lock() held.
  */
-static inline unsigned zone_span_seqbegin(struct zone *zone)
-{
-	return read_seqbegin(&zone->span_seqlock);
-}
-static inline int zone_span_seqretry(struct zone *zone, unsigned iv)
-{
-	return read_seqretry(&zone->span_seqlock, iv);
-}
-static inline void zone_span_writelock(struct zone *zone)
-{
-	write_seqlock(&zone->span_seqlock);
-}
-static inline void zone_span_writeunlock(struct zone *zone)
-{
-	write_sequnlock(&zone->span_seqlock);
-}
-static inline void zone_seqlock_init(struct zone *zone)
-{
-	seqlock_init(&zone->span_seqlock);
-}
 extern void adjust_present_page_count(struct page *page,
 				      struct memory_group *group,
 				      long nr_pages);
@@ -251,17 +227,6 @@ static inline void pgdat_kswapd_lock_init(pg_data_t *pgdat)
 	___page;				\
  })
 
-static inline unsigned zone_span_seqbegin(struct zone *zone)
-{
-	return 0;
-}
-static inline int zone_span_seqretry(struct zone *zone, unsigned iv)
-{
-	return 0;
-}
-static inline void zone_span_writelock(struct zone *zone) {}
-static inline void zone_span_writeunlock(struct zone *zone) {}
-static inline void zone_seqlock_init(struct zone *zone) {}
 
 static inline int try_online_node(int nid)
 {
diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 8f9c9590a42c..194ef7fed9d6 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -14,7 +14,6 @@
 #include <linux/threads.h>
 #include <linux/numa.h>
 #include <linux/init.h>
-#include <linux/seqlock.h>
 #include <linux/nodemask.h>
 #include <linux/pageblock-flags.h>
 #include <linux/page-flags-layout.h>
@@ -896,18 +895,11 @@ struct zone {
 	 *
 	 * Locking rules:
 	 *
-	 * zone_start_pfn and spanned_pages are protected by span_seqlock.
-	 * It is a seqlock because it has to be read outside of zone->lock,
-	 * and it is done in the main allocator path.  But, it is written
-	 * quite infrequently.
-	 *
-	 * The span_seq lock is declared along with zone->lock because it is
-	 * frequently read in proximity to zone->lock.  It's good to
-	 * give them a chance of being in the same cacheline.
-	 *
-	 * Write access to present_pages at runtime should be protected by
-	 * mem_hotplug_begin/done(). Any reader who can't tolerant drift of
-	 * present_pages should use get_online_mems() to get a stable value.
+	 * Besides system initialization functions, memory-hotplug is the only
+	 * user that can change zone's {spanned,present} pages at runtime, and
+	 * it does so by holding the mem_hotplug_lock lock. Any readers who
+	 * can't tolerate drift values should use {get,put}_online_mems to get
+	 * a stable value.
 	 */
 	atomic_long_t		managed_pages;
 	unsigned long		spanned_pages;
@@ -930,11 +922,6 @@ struct zone {
 	unsigned long		nr_isolate_pageblock;
 #endif
 
-#ifdef CONFIG_MEMORY_HOTPLUG
-	/* see spanned/present_pages for more description */
-	seqlock_t		span_seqlock;
-#endif
-
 	int initialized;
 
 	/* Write-intensive fields used from the page allocator */
diff --git a/mm/mm_init.c b/mm/mm_init.c
index f72b852bd5b8..c725618aeb58 100644
--- a/mm/mm_init.c
+++ b/mm/mm_init.c
@@ -1383,7 +1383,6 @@ static void __meminit zone_init_internals(struct zone *zone, enum zone_type idx,
 	zone->name = zone_names[idx];
 	zone->zone_pgdat = NODE_DATA(nid);
 	spin_lock_init(&zone->lock);
-	zone_seqlock_init(zone);
 	zone_pcp_init(zone);
 }
 
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 2e22ce5675ca..5116a2b9ea6e 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -426,16 +426,12 @@ void set_pageblock_migratetype(struct page *page, int migratetype)
 static int page_outside_zone_boundaries(struct zone *zone, struct page *page)
 {
 	int ret;
-	unsigned seq;
 	unsigned long pfn = page_to_pfn(page);
 	unsigned long sp, start_pfn;
 
-	do {
-		seq = zone_span_seqbegin(zone);
-		start_pfn = zone->zone_start_pfn;
-		sp = zone->spanned_pages;
-		ret = !zone_spans_pfn(zone, pfn);
-	} while (zone_span_seqretry(zone, seq));
+	start_pfn = zone->zone_start_pfn;
+	sp = READ_ONCE(zone->spanned_pages);
+	ret = !zone_spans_pfn(zone, pfn);
 
 	if (ret)
 		pr_err("page 0x%lx outside node %d zone %s [ 0x%lx - 0x%lx ]\n",

-- 
2.45.0.rc1.225.g2a3ae87e7f-goog