linux-kernel - Re: [PATCH] mm/sparse: Consistently do not zero memmap

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Message-ID: <20191030132958.GD31513@dhcp22.suse.cz>
Date:   Wed, 30 Oct 2019 14:29:58 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Vincent Whitchurch <vincent.whitchurch@...s.com>
Cc:     akpm@...ux-foundation.org, osalvador@...e.de,
        pasha.tatashin@...cle.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Vincent Whitchurch <rabinv@...s.com>
Subject: Re: [PATCH] mm/sparse: Consistently do not zero memmap

On Wed 30-10-19 14:11:22, Vincent Whitchurch wrote:
> sparsemem without VMEMMAP has two allocation paths to allocate the
> memory needed for its memmap (done in sparse_mem_map_populate()).
> 
> In one allocation path (sparse_buffer_alloc() succeeds), the memory is
> not zeroed (since it was previously allocated with
> memblock_alloc_try_nid_raw()).
> 
> In the other allocation path (sparse_buffer_alloc() fails and
> sparse_mem_map_populate() falls back to memblock_alloc_try_nid()), the
> memory is zeroed.
> 
> AFAICS this difference does not appear to be on purpose.  If the code is
> supposed to work with non-initialized memory (__init_single_page() takes
> care of zeroing the struct pages which are actually used), we should
> consistently not zero the memory, to avoid masking bugs.

You are right that this is not intentional.

> (I noticed this because on my ARM64 platform, with 1 GiB of memory the
>  first [and only] section is allocated from the zeroing path while with
>  2 GiB of memory the first 1 GiB section is allocated from the
>  non-zeroing path.)

Do I get it right that sparse_buffer_init couldn't allocate memmap for
the full node for some reason and so sparse_init_nid would have to
allocate one for each memory section?

> Signed-off-by: Vincent Whitchurch <vincent.whitchurch@...s.com>

Anyway the patch is OK. Even though this is not a bug strictly speaking
it is certainly a suboptimal behavior because zeroying takes time so
I would flag this for a stable tree 4.19+. There is no clear Fixes tag
to apply (35fd1eb1e8212 would get closest I guess).

Acked-by: Michal Hocko <mhocko@...e.com>

> ---
>  mm/sparse.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/sparse.c b/mm/sparse.c
> index f6891c1992b1..01e467adc219 100644
> --- a/mm/sparse.c
> +++ b/mm/sparse.c
> @@ -458,7 +458,7 @@ struct page __init *__populate_section_memmap(unsigned long pfn,
>  	if (map)
>  		return map;
>  
> -	map = memblock_alloc_try_nid(size,
> +	map = memblock_alloc_try_nid_raw(size,
>  					  PAGE_SIZE, addr,
>  					  MEMBLOCK_ALLOC_ACCESSIBLE, nid);
>  	if (!map)
> -- 
> 2.20.0

-- 
Michal Hocko
SUSE Labs