linux-kernel - Re: [PATCH] mm, sparse: do not waste pre allocated memmap space

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Date:   Tue, 19 Nov 2019 11:03:58 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Michal Hocko <mhocko@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     Pavel Tatashin <pasha.tatashin@...een.com>,
        Vincent Whitchurch <vincent.whitchurch@...s.com>,
        Oscar Salvador <OSalvador@...e.com>, linux-mm@...ck.org,
        LKML <linux-kernel@...r.kernel.org>,
        Michal Hocko <mhocko@...e.com>
Subject: Re: [PATCH] mm, sparse: do not waste pre allocated memmap space

On 19.11.19 10:26, Michal Hocko wrote:
> From: Michal Hocko <mhocko@...e.com>
> 
> Vincent has noticed [1] that there is something unusual with the memmap
> allocations going on on his platform
> : 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.
> 
> The underlying problem is that although sparse_buffer_init allocates enough
> memory for all sections on the node sparse_buffer_alloc is not able to
> consume them due to mismatch in the expected allocation alignement.
> While sparse_buffer_init preallocation uses the PAGE_SIZE alignment the
> real memmap has to be aligned to section_map_size() this results in a
> wasted initial chunk of the preallocated memmap and unnecessary fallback
> allocation for a section.
> 
> While we are at it also change __populate_section_memmap to align to the
> requested size because at least VMEMMAP has constrains to have memmap
> properly aligned.
> 
> [1] http://lkml.kernel.org/r/20191030131122.8256-1-vincent.whitchurch@axis.com
> Reported-and-debugged-by: Vincent Whitchurch <vincent.whitchurch@...s.com>
> Fixes: 35fd1eb1e821 ("mm/sparse: abstract sparse buffer allocations")
> Signed-off-by: Michal Hocko <mhocko@...e.com>
> ---
>   mm/sparse.c | 10 +++++++---
>   1 file changed, 7 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/sparse.c b/mm/sparse.c
> index f6891c1992b1..079f3e3c4cab 100644
> --- a/mm/sparse.c
> +++ b/mm/sparse.c
> @@ -458,8 +458,7 @@ struct page __init *__populate_section_memmap(unsigned long pfn,
>   	if (map)
>   		return map;
>   
> -	map = memblock_alloc_try_nid(size,
> -					  PAGE_SIZE, addr,
> +	map = memblock_alloc_try_nid(size, size, addr,
>   					  MEMBLOCK_ALLOC_ACCESSIBLE, nid);
>   	if (!map)
>   		panic("%s: Failed to allocate %lu bytes align=0x%lx nid=%d from=%pa\n",
> @@ -482,8 +481,13 @@ static void __init sparse_buffer_init(unsigned long size, int nid)
>   {
>   	phys_addr_t addr = __pa(MAX_DMA_ADDRESS);
>   	WARN_ON(sparsemap_buf);	/* forgot to call sparse_buffer_fini()? */
> +	/*
> +	 * Pre-allocated buffer is mainly used by __populate_section_memmap
> +	 * and we want it to be properly aligned to the section size - this is
> +	 * especially the case for VMEMMAP which maps memmap to PMDs
> +	 */
>   	sparsemap_buf =
> -		memblock_alloc_try_nid_raw(size, PAGE_SIZE,
> +		memblock_alloc_try_nid_raw(size, section_map_size(),
>   						addr,
>   						MEMBLOCK_ALLOC_ACCESSIBLE, nid);

Wow, that alignment/layout gives me nightmares  ^

None of your business, though :)

>   	sparsemap_buf_end = sparsemap_buf + size;
> 

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

-- 

Thanks,

David / dhildenb