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 Fri, Mar 13, 2020 at 03:57:33PM +0100, Michal Hocko wrote:
>On Thu 12-03-20 14:18:26, Wei Yang wrote:
>> 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.
>
>I do not understand your concern to be honest. Even if a sub page memmap
>size was possible (I haven't checked), I fail to see why kmalloc would
>fail to allocate while vmalloc would have a bigger chance to succeed.
>

You are right. My concern is just at kvmalloc_node() level.

After I look deep into the implementation, __vmalloc_area_node(), it will
still allocate memory on PAGE_SIZE base and fill up kernel page table.

That's why kvmalloc_node() stops trying when size is less than PAGE_SIZE.

Learned one more point. Thanks

>-- 
>Michal Hocko
>SUSE Labs

-- 
Wei Yang
Help you, Help me