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

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Date:   Thu, 12 Mar 2020 22:50:55 +0000
From:   Wei Yang <richard.weiyang@...il.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Wei Yang <richard.weiyang@...il.com>, Baoquan He <bhe@...hat.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org, mhocko@...e.com,
        akpm@...ux-foundation.org, david@...hat.com
Subject: Re: [PATCH v2] mm/sparse.c: Use kvmalloc_node/kvfree to alloc/free
 memmap for the classic sparse

On Thu, Mar 12, 2020 at 07:25:35AM -0700, Matthew Wilcox wrote:
>On Thu, Mar 12, 2020 at 02:18:26PM +0000, 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@...nel.org>
>> >> Signed-off-by: Baoquan He <bhe@...hat.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.
>
>Yes, I thought about that.  I decided it wasn't a problem, as long as
>the struct page remains aligned, and we now have a guarantee that allocations
>above 512 bytes in size are aligned.  With a 64 byte struct page, as long

Where is this 512 bytes condition comes from?

>as we're allocating at least 8 pages, we know it'll be naturally aligned.
>
>Your calculation doesn't take into account the size of struct page.
>128M / 64k is indeed 2k, but you forgot to multiply by 64, which takes
>us to 128kB.

You are right. While would there be other combination? Or in the future?

For example, there are definitions of

#define SECTION_SIZE_BITS       26
#define SECTION_SIZE_BITS       24

Are we sure it won't break some thing?

-- 
Wei Yang
Help you, Help me