From: Petr Tesarik <ptesarik@xxxxxxxx> Subject: include/linux/mmzone.h: fix explanation of lower bits in the SPARSEMEM mem_map pointer 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. [ptesarik@xxxxxxxx: v2] Link: http://lkml.kernel.org/r/20180125100516.589ea6af@xxxxxxxxxxxxxxx Link: http://lkml.kernel.org/r/20180119080908.3a662e6f@xxxxxxxxxxxxxxx Signed-off-by: Petr Tesarik <ptesarik@xxxxxxxx> Acked-by: Michal Hocko <mhocko@xxxxxxxx> Cc: Vlastimil Babka <vbabka@xxxxxxx> Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx> Cc: Johannes Weiner <hannes@xxxxxxxxxxx> Cc: Kemi Wang <kemi.wang@xxxxxxxxx> Cc: YASUAKI ISHIMATSU <yasu.isimatu@xxxxxxxxx> Cc: Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- include/linux/mmzone.h | 12 ++++++++++-- mm/sparse.c | 6 +++++- 2 files changed, 15 insertions(+), 3 deletions(-) diff -puN include/linux/mmzone.h~fix-explanation-of-lower-bits-in-the-sparsemem-mem_map-pointer include/linux/mmzone.h --- a/include/linux/mmzone.h~fix-explanation-of-lower-bits-in-the-sparsemem-mem_map-pointer +++ a/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 -puN mm/sparse.c~fix-explanation-of-lower-bits-in-the-sparsemem-mem_map-pointer mm/sparse.c --- a/mm/sparse.c~fix-explanation-of-lower-bits-in-the-sparsemem-mem_map-pointer +++ a/mm/sparse.c @@ -264,7 +264,11 @@ unsigned long __init node_memmap_size_by */ 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; } /* _ -- To unsubscribe from this list: send the line "unsubscribe mm-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
