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SubjectRe: [PATCH v3 4/4] mm/memory_hotplug: optimize memory hotplug
On Tue 13-02-18 14:31:59, Pavel Tatashin wrote:
[...]
> @@ -201,21 +202,24 @@ static bool pages_correctly_reserved(unsigned long start_pfn)
> * SPARSEMEM_VMEMMAP. We lookup the page once per section
> * and assume memmap is contiguous within each section
> */
> - for (i = 0; i < sections_per_block; i++, pfn += PAGES_PER_SECTION) {
> + for (; section_nr < section_nr_end; section_nr++) {
> if (WARN_ON_ONCE(!pfn_valid(pfn)))
> return false;
> - page = pfn_to_page(pfn);
> -
> - for (j = 0; j < PAGES_PER_SECTION; j++) {
> - if (PageReserved(page + j))
> - continue;
> -
> - printk(KERN_WARNING "section number %ld page number %d "
> - "not reserved, was it already online?\n",
> - pfn_to_section_nr(pfn), j);
>
> + if (!present_section_nr(section_nr)) {
> + pr_warn("section %ld pfn[%lx, %lx) not present",
> + section_nr, pfn, pfn + PAGES_PER_SECTION);
> + return false;
> + } else if (!valid_section_nr(section_nr)) {
> + pr_warn("section %ld pfn[%lx, %lx) no valid memmap",
> + section_nr, pfn, pfn + PAGES_PER_SECTION);
> + return false;
> + } else if (online_section_nr(section_nr)) {
> + pr_warn("section %ld pfn[%lx, %lx) is already online",
> + section_nr, pfn, pfn + PAGES_PER_SECTION);
> return false;
> }
> + pfn += PAGES_PER_SECTION;
> }

This should be a separate patch IMHO. It is an optimization on its
own. The original code tries to be sparse neutral but we do depend on
sparse anyway.

[...]
> /* register memory section under specified node if it spans that node */
> -int register_mem_sect_under_node(struct memory_block *mem_blk, int nid)
> +int register_mem_sect_under_node(struct memory_block *mem_blk, int nid,
> + bool check_nid)

This check_nid begs for a documentation. When do we need to set it? I
can see that register_new_memory path doesn't check node id. It is quite
reasonable to expect that a new memblock doesn't span multiple numa
nodes which can be the case for register_one_node but a word or two are
really due.

> {
> int ret;
> unsigned long pfn, sect_start_pfn, sect_end_pfn;
> @@ -423,11 +424,13 @@ int register_mem_sect_under_node(struct memory_block *mem_blk, int nid)
> continue;
> }
>
> - page_nid = get_nid_for_pfn(pfn);
> - if (page_nid < 0)
> - continue;
> - if (page_nid != nid)
> - continue;
> + if (check_nid) {
> + page_nid = get_nid_for_pfn(pfn);
> + if (page_nid < 0)
> + continue;
> + if (page_nid != nid)
> + continue;
> + }
> ret = sysfs_create_link_nowarn(&node_devices[nid]->dev.kobj,
> &mem_blk->dev.kobj,
> kobject_name(&mem_blk->dev.kobj));
> @@ -502,7 +505,7 @@ int link_mem_sections(int nid, unsigned long start_pfn, unsigned long nr_pages)
>
> mem_blk = find_memory_block_hinted(mem_sect, mem_blk);
>
> - ret = register_mem_sect_under_node(mem_blk, nid);
> + ret = register_mem_sect_under_node(mem_blk, nid, true);
> if (!err)
> err = ret;
>

I would be tempted to split this into a separate patch as well. The
review will be much easier.

[...]
> diff --git a/mm/sparse.c b/mm/sparse.c
> index 7af5e7a92528..d7808307023b 100644
> --- a/mm/sparse.c
> +++ b/mm/sparse.c
> @@ -30,11 +30,14 @@ struct mem_section mem_section[NR_SECTION_ROOTS][SECTIONS_PER_ROOT]
> #endif
> EXPORT_SYMBOL(mem_section);
>
> -#ifdef NODE_NOT_IN_PAGE_FLAGS
> +#if defined(NODE_NOT_IN_PAGE_FLAGS) || defined(CONFIG_MEMORY_HOTPLUG)
> /*
> * If we did not store the node number in the page then we have to
> * do a lookup in the section_to_node_table in order to find which
> * node the page belongs to.
> + *
> + * We also use this data in case memory hotplugging is enabled to be
> + * able to determine nid while struct pages are not yet initialized.
> */
> #if MAX_NUMNODES <= 256
> static u8 section_to_node_table[NR_MEM_SECTIONS] __cacheline_aligned;
> @@ -42,17 +45,28 @@ static u8 section_to_node_table[NR_MEM_SECTIONS] __cacheline_aligned;
> static u16 section_to_node_table[NR_MEM_SECTIONS] __cacheline_aligned;
> #endif
>
> +#ifdef NODE_NOT_IN_PAGE_FLAGS
> int page_to_nid(const struct page *page)
> {
> return section_to_node_table[page_to_section(page)];
> }
> EXPORT_SYMBOL(page_to_nid);
> +#endif /* NODE_NOT_IN_PAGE_FLAGS */

This is quite ugly. You allocate 256MB for small numa systems and 512MB
for larger NUMAs unconditionally for MEMORY_HOTPLUG. I see you need it
to safely replace page_to_nid by get_section_nid but this is just too
high of the price. Please note that this shouldn't be really needed. At
least not for onlining. We already _do_ know the node association with
the pfn range. So we should be able to get the nid from memblock.

[...]
--
Michal Hocko
SUSE Labs

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