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

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