Re: [Crash-utility] [PATCH] Support cpu_map/cpu_mask changes in 2.6.29

Michael, It's not clear to me how this function works: + * Get cpu map (possible, online, etc.) address from cpu mask (since 2.6.29) + */ +static ulong +get_cpu_map_addr_from_mask(const char *type) +{ + ulong cpu_mask_addr, cpu_map_addr; + char cpu_mask_symbol[32]; + char *cpu_mask_buf; + int cpu_mask_size; + + sprintf(cpu_mask_symbol, "cpu_%s_mask", type); + + if (!symbol_exists(cpu_mask_symbol)) + return 0; + + cpu_mask_addr = symbol_value(cpu_mask_symbol); + cpu_mask_size = STRUCT_SIZE("cpumask"); + cpu_mask_buf = malloc(cpu_mask_size); + if (!cpu_mask_buf) + error(FATAL, "get_cpu_map_addr_from_mask: out of memory\n"); + readmem(cpu_mask_addr, KVADDR, cpu_mask_buf, cpu_mask_size, + cpu_mask_symbol, FAULT_ON_ERROR); + cpu_map_addr = ULONG(cpu_mask_buf + MEMBER_OFFSET("cpumask", "bits")); + free(cpu_mask_buf); + + return cpu_map_addr; +} The caller to this function expects the address of a cpumask_t data structure. As I understand it, both before and after 2.6.29, the cpumask data structure looks like: typedef struct cpumask { unsigned long int bits[<variable-based-upon-NR_CPUS>]; } cpumask_t; And pre-2.6.29, the 3 "cpu_xxxx_map" symbols were all statically defined cpumask structures: cpumask_t cpu_online_mask; cpumask_t cpu_possible_mask; cpumask_t cpu_present_mask; Whereas now the 3 symbols don't exist except as #define's like so: #define cpu_possible_map (*(cpumask_t *)cpu_possible_mask) #define cpu_online_map (*(cpumask_t *)cpu_online_mask) #define cpu_present_map (*(cpumask_t *)cpu_present_mask) and -- if I understand the code correctly -- each cpu_xxxx_mask is just a pointer to a cpumask data structure. So when the caller needs the address of the relevant cpumask data structure why doesn't your cpu_map_addr_from_mask() function simply do something like: get_symbol_data(cpu_mask_symbol, sizeof(void *), &tmp); return tmp; i.e., which would read the contents of the cpu_mask_symbol, which is the pointer that the caller wants. Instead, your function reads in the whole data structure, where the cpumask.bits[] array typically contains several items, i.e., enough longs to contain NR_CPUS bits. Then, with ULONG(), it is essentially reading the unsigned long value found in the first bit[] array item, or: cpumask.bits[0]; and therefore returning the *bitmask* contained in the first unsigned long in the bit[] array -- instead of the *address* of the cpumask data structure. How can that work? Or am I missing something?