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

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Date:   Wed, 30 Oct 2019 14:31:48 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Vincent Whitchurch <vincent.whitchurch@...s.com>,
        akpm@...ux-foundation.org
Cc:     osalvador@...e.de, mhocko@...e.com, 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 30.10.19 14:11, 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.

I agree

Acked-by: David Hildenbrand <david@...hat.com>

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


-- 

Thanks,

David / dhildenb