linux-kernel - Re: [PATCH v2 2/4] mm/sparse: Optimize sparse_add_one

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Date:   Tue, 26 Mar 2019 10:29:36 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Baoquan He <bhe@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        akpm@...ux-foundation.org, rppt@...ux.ibm.com, osalvador@...e.de,
        willy@...radead.org, william.kucharski@...cle.com
Subject: Re: [PATCH v2 2/4] mm/sparse: Optimize sparse_add_one_section()

On Tue 26-03-19 17:02:25, Baoquan He wrote:
> Reorder the allocation of usemap and memmap since usemap allocation
> is much simpler and easier. Otherwise hard work is done to make
> memmap ready, then have to rollback just because of usemap allocation
> failure.

Is this really worth it? I can see that !VMEMMAP is doing memmap size
allocation which would be 2MB aka costly allocation but we do not do
__GFP_RETRY_MAYFAIL so the allocator backs off early.

> And also check if section is present earlier. Then don't bother to
> allocate usemap and memmap if yes.

Moving the check up makes some sense.

> Signed-off-by: Baoquan He <bhe@...hat.com>

The patch is not incorrect but I am wondering whether it is really worth
it for the current code base. Is it fixing anything real or it is a mere
code shuffling to please an eye?

> ---
> v1->v2:
>   Do section existence checking earlier to further optimize code.
> 
>  mm/sparse.c | 29 +++++++++++------------------
>  1 file changed, 11 insertions(+), 18 deletions(-)
> 
> diff --git a/mm/sparse.c b/mm/sparse.c
> index b2111f996aa6..f4f34d69131e 100644
> --- a/mm/sparse.c
> +++ b/mm/sparse.c
> @@ -714,20 +714,18 @@ int __meminit sparse_add_one_section(int nid, unsigned long start_pfn,
>  	ret = sparse_index_init(section_nr, nid);
>  	if (ret < 0 && ret != -EEXIST)
>  		return ret;
> -	ret = 0;
> -	memmap = kmalloc_section_memmap(section_nr, nid, altmap);
> -	if (!memmap)
> -		return -ENOMEM;
> -	usemap = __kmalloc_section_usemap();
> -	if (!usemap) {
> -		__kfree_section_memmap(memmap, altmap);
> -		return -ENOMEM;
> -	}
>  
>  	ms = __pfn_to_section(start_pfn);
> -	if (ms->section_mem_map & SECTION_MARKED_PRESENT) {
> -		ret = -EEXIST;
> -		goto out;
> +	if (ms->section_mem_map & SECTION_MARKED_PRESENT)
> +		return -EEXIST;
> +
> +	usemap = __kmalloc_section_usemap();
> +	if (!usemap)
> +		return -ENOMEM;
> +	memmap = kmalloc_section_memmap(section_nr, nid, altmap);
> +	if (!memmap) {
> +		kfree(usemap);
> +		return  -ENOMEM;
>  	}
>  
>  	/*
> @@ -739,12 +737,7 @@ int __meminit sparse_add_one_section(int nid, unsigned long start_pfn,
>  	section_mark_present(ms);
>  	sparse_init_one_section(ms, section_nr, memmap, usemap);
>  
> -out:
> -	if (ret < 0) {
> -		kfree(usemap);
> -		__kfree_section_memmap(memmap, altmap);
> -	}
> -	return ret;
> +	return 0;
>  }
>  
>  #ifdef CONFIG_MEMORY_HOTREMOVE
> -- 
> 2.17.2
> 

-- 
Michal Hocko
SUSE Labs