Re: [PATCH v2] mm/sparse.c: Use kvmalloc_node/kvfree to alloc/free memmap for the classic sparse

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

On Thu, Mar 12, 2020 at 06:34:16AM -0700, Matthew Wilcox wrote:
>On Thu, Mar 12, 2020 at 09:08:22PM +0800, Baoquan He wrote:
>> This change makes populate_section_memmap()/depopulate_section_memmap
>> much simpler.
>> 
>> Suggested-by: Michal Hocko <mhocko@kernel.org>
>> Signed-off-by: Baoquan He <bhe@redhat.com>
>> ---
>> v1->v2:
>>   The old version only used __get_free_pages() to replace alloc_pages()
>>   in populate_section_memmap().
>>   http://lkml.kernel.org/r/20200307084229.28251-8-bhe@redhat.com
>> 
>>  mm/sparse.c | 27 +++------------------------
>>  1 file changed, 3 insertions(+), 24 deletions(-)
>> 
>> diff --git a/mm/sparse.c b/mm/sparse.c
>> index bf6c00a28045..362018e82e22 100644
>> --- a/mm/sparse.c
>> +++ b/mm/sparse.c
>> @@ -734,35 +734,14 @@ static void free_map_bootmem(struct page *memmap)
>>  struct page * __meminit populate_section_memmap(unsigned long pfn,
>>  		unsigned long nr_pages, int nid, struct vmem_altmap *altmap)
>>  {
>> -	struct page *page, *ret;
>> -	unsigned long memmap_size = sizeof(struct page) * PAGES_PER_SECTION;
>> -
>> -	page = alloc_pages(GFP_KERNEL|__GFP_NOWARN, get_order(memmap_size));
>> -	if (page)
>> -		goto got_map_page;
>> -
>> -	ret = vmalloc(memmap_size);
>> -	if (ret)
>> -		goto got_map_ptr;
>> -
>> -	return NULL;
>> -got_map_page:
>> -	ret = (struct page *)pfn_to_kaddr(page_to_pfn(page));
>> -got_map_ptr:
>> -
>> -	return ret;
>> +	return kvmalloc_node(sizeof(struct page) * PAGES_PER_SECTION,
>> +			     GFP_KERNEL|__GFP_NOWARN, nid);
>
>Use of NOWARN here is inappropriate, because there's no fallback.

Hmm... this replacement is a little tricky.

When you look into kvmalloc_node(), it will do the fallback if the size is
bigger than PAGE_SIZE. This means the change here may not be equivalent as
before if memmap_size is less than PAGE_SIZE.

For example if :
  PAGE_SIZE = 64K 
  SECTION_SIZE = 128M

would lead to memmap_size = 2K, which is less than PAGE_SIZE.

Not sure this combination would happen?

>Also, I'd use array_size(sizeof(struct page), PAGES_PER_SECTION).

-- 
Wei Yang
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