Re: [PATCH v2] Fix explanation of lower bits in the SPARSEMEM mem_map pointer

[Michal Hocko]

On Thu 25-01-18 10:05:16, Petr Tesarik wrote:
> The comment is confusing. On the one hand, it refers to 32-bit
> alignment (struct page alignment on 32-bit platforms), but this
> would only guarantee that the 2 lowest bits must be zero. On the
> other hand, it claims that at least 3 bits are available, and 3 bits
> are actually used.
> 
> This is not broken, because there is a stronger alignment guarantee,
> just less obvious. Let's fix the comment to make it clear how many
> bits are available and why.
> 
> Although memmap arrays are allocated in various places, the
> resulting pointer is encoded eventually, so I am adding a BUG_ON()
> here to enforce at runtime that all expected bits are indeed
> available.
> 
> I have also added a BUILD_BUG_ON to check that PFN_SECTION_SHIFT is
> sufficient, because this part of the calculation can be easily
> checked at build time.
> 
> Signed-off-by: Petr Tesarik <ptesarik@xxxxxxxx>

Thank you. The check is much simpler than I originally thought.
Acked-by: Michal Hocko <mhocko@xxxxxxxx>

> ---
>  include/linux/mmzone.h | 12 ++++++++++--
>  mm/sparse.c            |  6 +++++-
>  2 files changed, 15 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 67f2e3c38939..7522a6987595 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -1166,8 +1166,16 @@ extern unsigned long usemap_size(void);
>  
>  /*
>   * We use the lower bits of the mem_map pointer to store
> - * a little bit of information.  There should be at least
> - * 3 bits here due to 32-bit alignment.
> + * a little bit of information.  The pointer is calculated
> + * as mem_map - section_nr_to_pfn(pnum).  The result is
> + * aligned to the minimum alignment of the two values:
> + *   1. All mem_map arrays are page-aligned.
> + *   2. section_nr_to_pfn() always clears PFN_SECTION_SHIFT
> + *      lowest bits.  PFN_SECTION_SHIFT is arch-specific
> + *      (equal SECTION_SIZE_BITS - PAGE_SHIFT), and the
> + *      worst combination is powerpc with 256k pages,
> + *      which results in PFN_SECTION_SHIFT equal 6.
> + * To sum it up, at least 6 bits are available.
>   */
>  #define	SECTION_MARKED_PRESENT	(1UL<<0)
>  #define SECTION_HAS_MEM_MAP	(1UL<<1)
> diff --git a/mm/sparse.c b/mm/sparse.c
> index 2609aba121e8..6b8b5e91ceef 100644
> --- a/mm/sparse.c
> +++ b/mm/sparse.c
> @@ -264,7 +264,11 @@ unsigned long __init node_memmap_size_bytes(int nid, unsigned long start_pfn,
>   */
>  static unsigned long sparse_encode_mem_map(struct page *mem_map, unsigned long pnum)
>  {
> -	return (unsigned long)(mem_map - (section_nr_to_pfn(pnum)));
> +	unsigned long coded_mem_map =
> +		(unsigned long)(mem_map - (section_nr_to_pfn(pnum)));
> +	BUILD_BUG_ON(SECTION_MAP_LAST_BIT > (1UL<<PFN_SECTION_SHIFT));
> +	BUG_ON(coded_mem_map & ~SECTION_MAP_MASK);
> +	return coded_mem_map;
>  }
>  
>  /*
> -- 
> 2.13.6

-- 
Michal Hocko
SUSE Labs