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Date:   Tue, 26 Mar 2019 15:57:07 +0200
From:   Mike Rapoport <rppt@...ux.ibm.com>
To:     Baoquan He <bhe@...hat.com>
Cc:     Michal Hocko <mhocko@...nel.org>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, akpm@...ux-foundation.org, 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, Mar 26, 2019 at 09:45:22PM +0800, Baoquan He wrote:
> On 03/26/19 at 11:17am, Michal Hocko wrote:
> > On Tue 26-03-19 18:08:17, Baoquan He wrote:
> > > On 03/26/19 at 10:29am, Michal Hocko wrote:
> > > > 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.
> > > 
> > > In !VMEMMAP case, it truly does simple allocation directly. surely
> > > usemap which size is 32 is smaller. So it doesn't matter that much who's
> > > ahead or who's behind. However, this benefit a little in VMEMMAP case.
> > 
> > How does it help there? The failure should be even much less probable
> > there because we simply fall back to a small 4kB pages and those
> > essentially never fail.
> 
> OK, I am fine to drop it. Or only put the section existence checking
> earlier to avoid unnecessary usemap/memmap allocation?
> 
> 
> From 7594b86ebf5d6fcc8146eca8fc5625f1961a15b1 Mon Sep 17 00:00:00 2001
> From: Baoquan He <bhe@...hat.com>
> Date: Tue, 26 Mar 2019 18:48:39 +0800
> Subject: [PATCH] mm/sparse: Check section's existence earlier in
>  sparse_add_one_section()
> 
> No need to allocate usemap and memmap if section has been present.
> And can clean up the handling on failure.
> 
> Signed-off-by: Baoquan He <bhe@...hat.com>
> ---
>  mm/sparse.c | 21 ++++++++-------------
>  1 file changed, 8 insertions(+), 13 deletions(-)
> 
> diff --git a/mm/sparse.c b/mm/sparse.c
> index 363f9d31b511..f564b531e0f7 100644
> --- a/mm/sparse.c
> +++ b/mm/sparse.c
> @@ -714,7 +714,13 @@ 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;
> +
> +	ms = __pfn_to_section(start_pfn);
> +	if (ms->section_mem_map & SECTION_MARKED_PRESENT) {
> +		ret = -EEXIST;
> +		goto out;

		return -EEXIST; ?

> +	}
> +
>  	memmap = kmalloc_section_memmap(section_nr, nid, altmap);
>  	if (!memmap)
>  		return -ENOMEM;
> @@ -724,12 +730,6 @@ int __meminit sparse_add_one_section(int nid, unsigned long start_pfn,
>  		return -ENOMEM;
>  	}
>  
> -	ms = __pfn_to_section(start_pfn);
> -	if (ms->section_mem_map & SECTION_MARKED_PRESENT) {
> -		ret = -EEXIST;
> -		goto out;
> -	}
> -
>  	/*
>  	 * Poison uninitialized struct pages in order to catch invalid flags
>  	 * combinations.
> @@ -739,12 +739,7 @@ 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
> 

-- 
Sincerely yours,
Mike.

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