Messages in this thread Patch in this message | | | Date | Fri, 19 Jan 2018 08:09:08 +0100 | From | Petr Tesarik <> | Subject | [PATCH] Fix explanation of lower bits in the SPARSEMEM mem_map pointer |
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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.
Signed-off-by: Petr Tesarik <ptesarik@suse.com> --- include/linux/mmzone.h | 12 ++++++++++-- 1 file changed, 10 insertions(+), 2 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) -- 2.13.6
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