On Thu, Nov 29, 2018 at 09:54:22AM +0100, Michal Hocko wrote:
>On Wed 28-11-18 17:12:43, Wei Yang wrote:
>> In function sparse_add/remove_one_section(), pgdat_resize_lock is used
>> to protect initialization/release of one mem_section. This looks not
>> necessary for current implementation.
>>
>> 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()
>>
>> which shows these functions is protected by the global mem_hotplug_lock.
>> It won't face contention when accessing the mem_section.
>
>Again there is no explanation _why_ we want this patch. The reason is
>that the lock doesn't really protect what the size of the pgdat. The
>comment above the lock also mentiones
>"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 I fail to see how this is helpful. I do not see any pfn walkers to
>take the lock so this looks like a relict from the past.
>
>The comment should go away in this patch.
>
Ok, let me try to address this.
>>
>> Since the information needed in sparse_add_one_section() is node id to
>> allocate proper memory. This patch also changes the prototype of
>> sparse_add_one_section() to pass node id directly. This is intended to
>> reduce misleading that sparse_add_one_section() would touch pgdat.
>
>I would do that in the separate patch because review would be slightly
>easier.
Oops, I thought the merged version is preferred.
Hmm... I would prepare v3 to separate them.
>
>> Signed-off-by: Wei Yang <richard.weiyang@xxxxxxxxx>
>
>With the comment removed
>Acked-by: Michal Hocko <mhocko@xxxxxxxx>
>
>>
>> ---
>> v2:
>> * adjust changelog to show this procedure is serialized by global
>> mem_hotplug_lock
>> ---
>> include/linux/memory_hotplug.h | 2 +-
>> mm/memory_hotplug.c | 2 +-
>> mm/sparse.c | 17 +++++------------
>> 3 files changed, 7 insertions(+), 14 deletions(-)
>>
>> diff --git a/include/linux/memory_hotplug.h b/include/linux/memory_hotplug.h
>> index 45a5affcab8a..3787d4e913e6 100644
>> --- a/include/linux/memory_hotplug.h
>> +++ b/include/linux/memory_hotplug.h
>> @@ -333,7 +333,7 @@ extern void move_pfn_range_to_zone(struct zone *zone, unsigned long start_pfn,
>> unsigned long nr_pages, struct vmem_altmap *altmap);
>> extern int offline_pages(unsigned long start_pfn, unsigned long nr_pages);
>> extern bool is_memblock_offlined(struct memory_block *mem);
>> -extern int sparse_add_one_section(struct pglist_data *pgdat,
>> +extern int sparse_add_one_section(int nid,
>> unsigned long start_pfn, struct vmem_altmap *altmap);
>> extern void sparse_remove_one_section(struct zone *zone, struct mem_section *ms,
>> unsigned long map_offset, struct vmem_altmap *altmap);
>> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
>> index f626e7e5f57b..5b3a3d7b4466 100644
>> --- a/mm/memory_hotplug.c
>> +++ b/mm/memory_hotplug.c
>> @@ -253,7 +253,7 @@ static int __meminit __add_section(int nid, unsigned long phys_start_pfn,
>> if (pfn_valid(phys_start_pfn))
>> return -EEXIST;
>>
>> - ret = sparse_add_one_section(NODE_DATA(nid), phys_start_pfn, altmap);
>> + ret = sparse_add_one_section(nid, phys_start_pfn, altmap);
>> if (ret < 0)
>> return ret;
>>
>> diff --git a/mm/sparse.c b/mm/sparse.c
>> index 33307fc05c4d..a4fdbcb21514 100644
>> --- a/mm/sparse.c
>> +++ b/mm/sparse.c
>> @@ -662,25 +662,24 @@ static void free_map_bootmem(struct page *memmap)
>> * set. If this is <=0, then that means that the passed-in
>> * map was not consumed and must be freed.
>> */
>> -int __meminit sparse_add_one_section(struct pglist_data *pgdat,
>> - unsigned long start_pfn, struct vmem_altmap *altmap)
>> +int __meminit sparse_add_one_section(int nid, unsigned long start_pfn,
>> + struct vmem_altmap *altmap)
>> {
>> unsigned long section_nr = pfn_to_section_nr(start_pfn);
>> struct mem_section *ms;
>> struct page *memmap;
>> unsigned long *usemap;
>> - unsigned long flags;
>> int ret;
>>
>> /*
>> * no locking for this, because it does its own
>> * plus, it does a kmalloc
>> */
>> - ret = sparse_index_init(section_nr, pgdat->node_id);
>> + ret = sparse_index_init(section_nr, nid);
>> if (ret < 0 && ret != -EEXIST)
>> return ret;
>> ret = 0;
>> - memmap = kmalloc_section_memmap(section_nr, pgdat->node_id, altmap);
>> + memmap = kmalloc_section_memmap(section_nr, nid, altmap);
>> if (!memmap)
>> return -ENOMEM;
>> usemap = __kmalloc_section_usemap();
>> @@ -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
>>
>
>--
>Michal Hocko
>SUSE Labs
--
Wei Yang
Help you, Help me
