[PATCH v4 1/2] mm, sparse: drop pgdat_resize_lock in sparse_add/remove_one_section()

[Wei Yang <richard.weiyang@gmail.com>]

pgdat_resize_lock is used to protect pgdat's memory region information
like: node_start_pfn, node_present_pages, etc. While in function
sparse_add/remove_one_section(), pgdat_resize_lock is used to protect
initialization/release of one mem_section. This looks not proper.

These code paths are currently protected by mem_hotplug_lock currently
but should there ever be any reason for locking at the sparse layer a
dedicated lock should be introduced.

Following is the current call trace of sparse_add/remove_one_section()

    mem_hotplug_begin()
    arch_add_memory()
       add_pages()
           __add_pages()
               __add_section()
                   sparse_add_one_section()
    mem_hotplug_done()

    mem_hotplug_begin()
    arch_remove_memory()
        __remove_pages()
            __remove_section()
                sparse_remove_one_section()
    mem_hotplug_done()

The comment above the pgdat_resize_lock also mentions "Holding this will
also guarantee that any pfn_valid() stays that way.", which is true with
the current implementation and false after this patch. But current
implementation doesn't meet this comment. There isn't any pfn walkers
to take the lock so this looks like a relict from the past. This patch
also removes this comment.

[Michal: changelog suggestion]

Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
Acked-by: Michal Hocko <mhocko@suse.com>

---
v4:
   * fix typo in changelog
   * adjust second paragraph of changelog
v3:
   * adjust the changelog with the reason for this change
   * remove a comment for pgdat_resize_lock
   * separate the prototype change of sparse_add_one_section() to
     another one
v2:
   * adjust changelog to show this procedure is serialized by global
     mem_hotplug_lock
---
 include/linux/mmzone.h | 2 --
 mm/sparse.c            | 9 +--------
 2 files changed, 1 insertion(+), 10 deletions(-)

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index d76177cb8436..be126113b499 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -639,8 +639,6 @@ typedef struct pglist_data {
 	/*
 	 * Must be held any time you expect node_start_pfn,
 	 * node_present_pages, node_spanned_pages or nr_zones stay constant.
-	 * Holding this will also guarantee that any pfn_valid() stays that
-	 * way.
 	 *
 	 * pgdat_resize_lock() and pgdat_resize_unlock() are provided to
 	 * manipulate node_size_lock without checking for CONFIG_MEMORY_HOTPLUG
diff --git a/mm/sparse.c b/mm/sparse.c
index 33307fc05c4d..5825f276485f 100644
--- a/mm/sparse.c
+++ b/mm/sparse.c
@@ -669,7 +669,6 @@ int __meminit sparse_add_one_section(struct pglist_data *pgdat,
 	struct mem_section *ms;
 	struct page *memmap;
 	unsigned long *usemap;
-	unsigned long flags;
 	int ret;
 
 	/*
@@ -689,8 +688,6 @@ int __meminit sparse_add_one_section(struct pglist_data *pgdat,
 		return -ENOMEM;
 	}
 
-	pgdat_resize_lock(pgdat, &flags);
-
 	ms = __pfn_to_section(start_pfn);
 	if (ms->section_mem_map & SECTION_MARKED_PRESENT) {
 		ret = -EEXIST;
@@ -707,7 +704,6 @@ int __meminit sparse_add_one_section(struct pglist_data *pgdat,
 	sparse_init_one_section(ms, section_nr, memmap, usemap);
 
 out:
-	pgdat_resize_unlock(pgdat, &flags);
 	if (ret < 0) {
 		kfree(usemap);
 		__kfree_section_memmap(memmap, altmap);
@@ -769,10 +765,8 @@ void sparse_remove_one_section(struct zone *zone, struct mem_section *ms,
 		unsigned long map_offset, struct vmem_altmap *altmap)
 {
 	struct page *memmap = NULL;
-	unsigned long *usemap = NULL, flags;
-	struct pglist_data *pgdat = zone->zone_pgdat;
+	unsigned long *usemap = NULL;
 
-	pgdat_resize_lock(pgdat, &flags);
 	if (ms->section_mem_map) {
 		usemap = ms->pageblock_flags;
 		memmap = sparse_decode_mem_map(ms->section_mem_map,
@@ -780,7 +774,6 @@ void sparse_remove_one_section(struct zone *zone, struct mem_section *ms,
 		ms->section_mem_map = 0;
 		ms->pageblock_flags = NULL;
 	}
-	pgdat_resize_unlock(pgdat, &flags);
 
 	clear_hwpoisoned_pages(memmap + map_offset,
 			PAGES_PER_SECTION - map_offset);
-- 
2.15.1