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
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