Messages in this thread Patch in this message | | | Subject | [RFC][PATCH 2/2] sparsemem: fix boot when SECTIONS_PER_ROOT is not power-of-2 | From | Dave Hansen <> | Date | Mon, 21 May 2012 13:30:24 -0700 |
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I was getting some interesting oopses at boot after adding a field to 'struct mem_section'. I tracked it down to __nr_to_section(). In my case, SECTIONS_PER_ROOT got set to 73, which leads to an interesting bitmask:
#define SECTIONS_PER_ROOT (PAGE_SIZE / sizeof (struct mem_section)) ... #define SECTION_ROOT_MASK (SECTIONS_PER_ROOT - 1)
We only use SECTION_ROOT_MASK in one place. If we replace it with some modulo arithmetic, it compiles down to the same thing for power-of-2 SECTIONS_PER_ROOT values, but it also actually *works* instead of just failing to boot at some random point for the other case.
This also adds some requisite comments in the structure for future hapless kernel developers, plus a bonus WARN_ON_ONCE() just in case they miss the big fat comment.
Granted, this patch is not fixing a bug that anyone will really hit in practice, but it will surely save future developers a headache or two, plus it removes a #define!
Signed-off-by: Dave Hansen <dave@linux.vnet.ibm.com> ---
linux-2.6.git-dave/include/linux/mmzone.h | 9 +++++++-- linux-2.6.git-dave/mm/sparse.c | 5 +++++ 2 files changed, 12 insertions(+), 2 deletions(-)
diff -puN include/linux/mmzone.h~sparsemem-fix-boot-when-SECTIONS_PER_ROOT-is-not-power_of_2 include/linux/mmzone.h --- linux-2.6.git/include/linux/mmzone.h~sparsemem-fix-boot-when-SECTIONS_PER_ROOT-is-not-power_of_2 2012-05-21 13:29:43.777274223 -0700 +++ linux-2.6.git-dave/include/linux/mmzone.h 2012-05-21 13:29:43.789274356 -0700 @@ -1018,6 +1018,12 @@ struct mem_section { struct page_cgroup *page_cgroup; unsigned long pad; #endif + /* + * WARNING: Do not put any fields here that could cause + * this structure to become a non-power-of-2 size. + * Operations like pfn_to_page() will end up doing + * division in hot paths for CONFIG_SPARSEMEM_EXTREME. + */ }; #ifdef CONFIG_SPARSEMEM_EXTREME @@ -1028,7 +1034,6 @@ struct mem_section { #define SECTION_NR_TO_ROOT(sec) ((sec) / SECTIONS_PER_ROOT) #define NR_SECTION_ROOTS DIV_ROUND_UP(NR_MEM_SECTIONS, SECTIONS_PER_ROOT) -#define SECTION_ROOT_MASK (SECTIONS_PER_ROOT - 1) #ifdef CONFIG_SPARSEMEM_EXTREME extern struct mem_section *mem_section[NR_SECTION_ROOTS]; @@ -1040,7 +1045,7 @@ static inline struct mem_section *__nr_t { if (!mem_section[SECTION_NR_TO_ROOT(nr)]) return NULL; - return &mem_section[SECTION_NR_TO_ROOT(nr)][nr & SECTION_ROOT_MASK]; + return &mem_section[SECTION_NR_TO_ROOT(nr)][nr % SECTIONS_PER_ROOT]; } extern int __section_nr(struct mem_section* ms); extern unsigned long usemap_size(void); diff -puN mm/sparse.c~sparsemem-fix-boot-when-SECTIONS_PER_ROOT-is-not-power_of_2 mm/sparse.c --- linux-2.6.git/mm/sparse.c~sparsemem-fix-boot-when-SECTIONS_PER_ROOT-is-not-power_of_2 2012-05-21 13:29:43.781274268 -0700 +++ linux-2.6.git-dave/mm/sparse.c 2012-05-21 13:29:43.789274356 -0700 @@ -63,6 +63,11 @@ static struct mem_section noinline __ini unsigned long array_size = SECTIONS_PER_ROOT * sizeof(struct mem_section); + /* + * See note in 'struct mem_section' definition + */ + WARN_ON_ONCE(!is_power_of_2(sizeof(struct mem_section))); + if (slab_is_available()) { if (node_state(nid, N_HIGH_MEMORY)) section = kmalloc_node(array_size, GFP_KERNEL, nid); _
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