[Crash-utility] More fixes for kmem on slabs

More testing revealed a machine in our stable that either failed to initialize kmem: please wait... (gathering kmem slab cache data) crash-6.0.3: page excluded: kernel virtual address: ffff8801263d6000 type: "kmem_cache buffer" crash-6.0.3: unable to initialize kmem slab cache subsystem Or succeeded on initialize and then failed on a kmem -s command: crash-6.0.3> kmem -s CACHE NAME OBJSIZE ALLOCATED TOTAL SLABS SSIZE Segmentation fault The problem is that the array struct at the end of kmem_cache remains declared as 32 elements, but for all dynamically allocated copies, is actually trimmed down to nr_cpu_ids in length. crash-6.0.3.best> struct kmem_cache struct kmem_cache { unsigned int batchcount; ... struct list_head next; struct kmem_list3 **nodelists; struct array_cache *array[32]; } SIZE: 368 On my normal play machine, nr_cpu_ids = 32 and actual cpus = 16. On the failing machine, nr_cpus_ids and actual cpus are both 2. Two problems occur: 1) max_cpudata_limit traverses the array until it finds a 0x0 or reaches the real size. On the 2-cpu system, the "third" element in the array belonged elsewhere, was non-zero, and pointed to data that caused the apparent limit to be 0xffffffffffff8801, which didn't work well as a length in a memcopy. 2) kmem_cache structs can be allocated near enough to the edge of a page that the old incorrect length crosses the page boundary, even though the real smaller structure fits in the page. That caused a readmem of the structure to cross into a coincidentally missing page in the dump. This patch fixes both of those (after wrestling ARRAY_LENGTH to the ground), but *does not* fix the similar page crossing problem when I try to use a "struct kmem_cache" command on the particular structure at the end of the page. Reference this unfortunate comment in include/linux/slab_def.h: /* 6) per-cpu/per-node data, touched during every alloc/free */ /* * We put array[] at the end of kmem_cache, because we want to size * this array to nr_cpu_ids slots instead of NR_CPUS * (see kmem_cache_init()) * We still use [NR_CPUS] and not [1] or [0] because cache_cache * is statically defined, so we reserve the max number of cpus. */ struct kmem_list3 **nodelists; struct array_cache *array[NR_CPUS]; /* * Do not add fields after array[] */ }; Bob Montgomery