Message-Id: <20191028105458.28320-1-david@redhat.com>
Date:   Mon, 28 Oct 2019 11:54:58 +0100
From:   David Hildenbrand <david@...hat.com>
To:     linux-kernel@...r.kernel.org
Cc:     linux-mm@...ck.org, David Hildenbrand <david@...hat.com>,
        Tang Chen <tangchen@...fujitsu.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Keith Busch <keith.busch@...el.com>,
        Jiri Olsa <jolsa@...nel.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Jani Nikula <jani.nikula@...el.com>,
        Nayna Jain <nayna@...ux.ibm.com>,
        Michal Hocko <mhocko@...e.com>,
        Oscar Salvador <osalvador@...e.de>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Dan Williams <dan.j.williams@...el.com>,
        Pavel Tatashin <pasha.tatashin@...een.com>
Subject: [PATCH v1] mm/memory_hotplug: Fix try_offline_node()

try_offline_node() is pretty much broken right now:
- The node span is updated when onlining memory, not when adding it. We
  ignore memory that was mever onlined. Bad.
- We touch possible garbage memmaps. The pfn_to_nid(pfn) can easily
  trigger a kernel panic. Bad for memory that is offline but also bad
  for subsection hotadd with ZONE_DEVICE, whereby the memmap of the first
  PFN of a section might contain garbage.
- Sections belonging to mixed nodes are not properly considered.

As memory blocks might belong to multiple nodes, we would have to walk all
pageblocks (or at least subsections) within present sections. However,
we don't have a way to identify whether a memmap that is not online was
initialized (relevant for ZONE_DEVICE). This makes things more complicated.

Luckily, we can piggy pack on the node span and the sysfs links between
memory blocks and the node. Currently, the node span is grown when calling
move_pfn_range_to_zone() - e.g., when onlining memory, and shrunk when
removing memory, before calling try_offline_node(). Sysfs links are
created via link_mem_sections(), e.g., during boot or when adding memory.

If the node still spans memory or if any memory block is linked to the
node in sysfs, we don't set the node offline. Without CONFIG_NUMA, or
without CONFIG_SYSFS, we will simply always detect the node as being
linked to the memory block and not set the node offline.

Add a way to test if a sysfs link exists.

Note: We will soon stop shrinking the ZONE_DEVICE zone and the node span
when removing ZONE_DEVICE memory to fix similar issues (acess of garbage
memmaps) - until we have a reliable way to identify whether these memmaps
were properly initialized. This implies later, that once a node had
ZONE_DEVICE memory, we won't be able to set a node offline -
which should be acceptable.

Since commit f1dd2cd13c4b ("mm, memory_hotplug: do not associate hotadded
memory to zones until online") memory that is added is not assoziated
with a zone/node (memmap not initialized). The introducing commit
60a5a19e7419 ("memory-hotplug: remove sysfs file of node") already
missed that we could have multiple nodes for a section and that the
zone/node span is updated when onlining pages, not when adding them.

I tested this by hotplugging two DIMMs to a memory-less and cpu-less NUMA
node. The node is properly onlined when adding the DIMMs. When removing
the DIMMs, the node is properly offlined.

Fixes: 60a5a19e7419 ("memory-hotplug: remove sysfs file of node")
Fixes: f1dd2cd13c4b ("mm, memory_hotplug: do not associate hotadded memory to zones until online") # visiable after d0dc12e86b319
Cc: Tang Chen <tangchen@...fujitsu.com>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: "Rafael J. Wysocki" <rafael@...nel.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Keith Busch <keith.busch@...el.com>
Cc: Jiri Olsa <jolsa@...nel.org>
Cc: "Peter Zijlstra (Intel)" <peterz@...radead.org>
Cc: Jani Nikula <jani.nikula@...el.com>
Cc: Nayna Jain <nayna@...ux.ibm.com>
Cc: Michal Hocko <mhocko@...e.com>
Cc: Oscar Salvador <osalvador@...e.de>
Cc: Stephen Rothwell <sfr@...b.auug.org.au>
Cc: Dan Williams <dan.j.williams@...el.com>
Cc: Pavel Tatashin <pasha.tatashin@...een.com>
Signed-off-by: David Hildenbrand <david@...hat.com>
---

I just realized that we are are missing a try_online_node() call in the
memremap path. Bad when adding device memory to a memory-less and cpu-less
node - the pgdat is not initialized. Will also have to be fixed.

We stop shrinking the ZONE_DEVICE zone after the following patch:
	[PATCH v6 04/10] mm/memory_hotplug: Don't access uninitialized memmaps
	in shrink_zone_span()
This implies, the above note regarding ZONE_DEVICE on a node blocking a
node from getting offlined until we sorted out how to properly shrink
the ZONE_DEVICE zone.

This patch is especially important for:
	[PATCH v6 05/10] mm/memory_hotplug: Shrink zones when offlining
	memory
As the BUG fixed with this patch becomes now easier to observe when memory
is offlined (in contrast to when memory would never have been onlined
before).

As both patches are stable fixes and in next/master for a long time, we
should probably pull this patch in front of both and also backport this
patch at least to
	Cc: stable@...r.kernel.org # v4.13+
I have not checked yet if there are real blockers to do that. I guess not.

---
 drivers/base/node.c   |  9 +++++++++
 fs/sysfs/symlink.c    | 21 +++++++++++++++++++++
 include/linux/node.h  |  7 +++++++
 include/linux/sysfs.h |  6 ++++++
 mm/memory_hotplug.c   | 42 ++++++++++++++++++++++++++----------------
 5 files changed, 69 insertions(+), 16 deletions(-)

diff --git a/drivers/base/node.c b/drivers/base/node.c
index 98a31bafc8a2..32aeb85f1d4a 100644
--- a/drivers/base/node.c
+++ b/drivers/base/node.c
@@ -833,6 +833,15 @@ int link_mem_sections(int nid, unsigned long start_pfn, unsigned long end_pfn)
 				  register_mem_sect_under_node);
 }
 
+bool memory_block_registered_under_node(struct memory_block *mem, int nid)
+{
+	if (mem->nid == nid)
+		return true;
+	/* memory blocks can span multiple nodes. Check against the link. */
+	return sysfs_link_exists(&mem->dev.kobj,
+				 kobject_name(&node_devices[nid]->dev.kobj));
+}
+
 #ifdef CONFIG_HUGETLBFS
 /*
  * Handle per node hstate attribute [un]registration on transistions
diff --git a/fs/sysfs/symlink.c b/fs/sysfs/symlink.c
index c4deecc80f67..b99697a9dae6 100644
--- a/fs/sysfs/symlink.c
+++ b/fs/sysfs/symlink.c
@@ -153,6 +153,27 @@ void sysfs_remove_link(struct kobject *kobj, const char *name)
 }
 EXPORT_SYMBOL_GPL(sysfs_remove_link);
 
+/**
+ *	sysfs_link_exists - test if a symlink exists in object's directory.
+ *	@kobj:	object we're acting for.
+ *	@name:	name of the symlink to test.
+ */
+bool sysfs_link_exists(struct kobject *kobj, const char *name)
+{
+	struct kernfs_node *parent = NULL, *kn;
+
+	if (!kobj)
+		parent = sysfs_root_kn;
+	else
+		parent = kobj->sd;
+
+	kn = kernfs_find_and_get(parent, name);
+	kernfs_put(kn);
+
+	return kn != NULL;
+}
+EXPORT_SYMBOL_GPL(sysfs_link_exists);
+
 /**
  *	sysfs_rename_link_ns - rename symlink in object's directory.
  *	@kobj:	object we're acting for.
diff --git a/include/linux/node.h b/include/linux/node.h
index 4866f32a02d8..353853018689 100644
--- a/include/linux/node.h
+++ b/include/linux/node.h
@@ -138,6 +138,8 @@ extern void unregister_one_node(int nid);
 extern int register_cpu_under_node(unsigned int cpu, unsigned int nid);
 extern int unregister_cpu_under_node(unsigned int cpu, unsigned int nid);
 extern void unregister_memory_block_under_nodes(struct memory_block *mem_blk);
+extern bool memory_block_registered_under_node(struct memory_block *mem,
+					       int nid);
 
 extern int register_memory_node_under_compute_node(unsigned int mem_nid,
 						   unsigned int cpu_nid,
@@ -171,6 +173,11 @@ static inline int unregister_cpu_under_node(unsigned int cpu, unsigned int nid)
 static inline void unregister_memory_block_under_nodes(struct memory_block *mem_blk)
 {
 }
+static inline bool memory_block_registered_under_node(struct memory_block *mem,
+						      int nid)
+{
+	return true;
+}
 
 static inline void register_hugetlbfs_with_node(node_registration_func_t reg,
 						node_registration_func_t unreg)
diff --git a/include/linux/sysfs.h b/include/linux/sysfs.h
index fa7ee503fb76..3ae9a69a0786 100644
--- a/include/linux/sysfs.h
+++ b/include/linux/sysfs.h
@@ -265,6 +265,7 @@ int __must_check sysfs_create_link_nowarn(struct kobject *kobj,
 					  struct kobject *target,
 					  const char *name);
 void sysfs_remove_link(struct kobject *kobj, const char *name);
+bool sysfs_link_exists(struct kobject *kobj, const char *name);
 
 int sysfs_rename_link_ns(struct kobject *kobj, struct kobject *target,
 			 const char *old_name, const char *new_name,
@@ -420,6 +421,11 @@ static inline void sysfs_remove_link(struct kobject *kobj, const char *name)
 {
 }
 
+static inline bool sysfs_link_exists(struct kobject *kobj, const char *name)
+{
+	return true;
+}
+
 static inline int sysfs_rename_link_ns(struct kobject *k, struct kobject *t,
 				       const char *old_name,
 				       const char *new_name, const void *ns)
diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index 0140c20837b6..c75e99b292cd 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -1634,6 +1634,15 @@ static int check_cpu_on_node(pg_data_t *pgdat)
 	return 0;
 }
 
+static int check_no_memblock_registered_under_node_cb(struct memory_block *mem,
+						      void *arg)
+{
+
+	const int nid = *(int *)arg;
+
+	return memory_block_registered_under_node(mem, nid) ? -EEXIST : 0;
+}
+
 /**
  * try_offline_node
  * @nid: the node ID
@@ -1645,26 +1654,27 @@ static int check_cpu_on_node(pg_data_t *pgdat)
  */
 void try_offline_node(int nid)
 {
+	unsigned long end_pfn = section_nr_to_pfn(__highest_present_section_nr);
 	pg_data_t *pgdat = NODE_DATA(nid);
-	unsigned long start_pfn = pgdat->node_start_pfn;
-	unsigned long end_pfn = start_pfn + pgdat->node_spanned_pages;
-	unsigned long pfn;
-
-	for (pfn = start_pfn; pfn < end_pfn; pfn += PAGES_PER_SECTION) {
-		unsigned long section_nr = pfn_to_section_nr(pfn);
-
-		if (!present_section_nr(section_nr))
-			continue;
+	int rc;
 
-		if (pfn_to_nid(pfn) != nid)
-			continue;
+	/*
+	 * If the node still spans pages (especially ZONE_DEVICE), don't
+	 * offline it. A node spans memory after move_pfn_range_to_zone(),
+	 * e.g., after the memory block was onlined.
+	 */
+	if (pgdat->node_spanned_pages)
+		return;
 
-		/*
-		 * some memory sections of this node are not removed, and we
-		 * can't offline node now.
-		 */
+	/*
+	 * Especially offline memory blocks might not be spanned by the
+	 * node. They will get spanned by the node once they get onlined.
+	 * However, they link to the node in sysfs and can get onlined later.
+	 */
+	rc = walk_memory_blocks(0, PFN_PHYS(end_pfn), &nid,
+				check_no_memblock_registered_under_node_cb);
+	if (rc)
 		return;
-	}
 
 	if (check_cpu_on_node(pgdat))
 		return;
-- 
2.21.0

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