This post is the first in a series in which I discuss the basics of a component or concept in the Linux kernel. The goal is to explain it in such a way that a beginner C or C++ programmer unfamiliar with the Linux kernel can comprehend it. There will be at least 2 posts in this series, including this one.<\/p>\n\n\n\n
This week, the topic is When programming in kernel space<\/a>, you cannot use the C standard library memory allocation functions available in user space, such as The An array of several memory regions of the same type can be represented with The memory region types are memory<\/strong>, reserved<\/strong>, and physmem<\/strong>. <\/p>\n\n\n\n The The memory region attributes are defined in These functions add or remove a memblock region with the given base address and size. They return an int indicating whether the operation succeeded. The These functions allocate a memory block and return the physical address of the memory.<\/p>\n\n\n\n These functions allocate a memory block and return the virtual address of the memory.<\/p>\n\n\n\n The functions <\/p>\n","protected":false},"excerpt":{"rendered":" This post is the first in a series in which I discuss the basics of a component or concept in the Linux kernel. The goal is to explain it in such a way that a beginner C or C++ programmer … Continue reading memblock<\/code>.<\/p>\n\n\n\nWhat is
memblock<\/code>?<\/h2>\n\n\n\nmalloc<\/code>, free<\/code>, calloc<\/code>, realloc<\/code>, or reallocarray<\/code>.[1]<\/a><\/sup> There are kernel space equivalents, such as kmalloc<\/code> or kzalloc<\/code>, kfree<\/code>, kcalloc<\/code>, krealloc_array<\/code>, krealloc<\/code>,[2]<\/a><\/sup> and more<\/a>. However, these memory allocators are set up during the booting process. In the period of time before these allocators are available, the system still needs to allocate memory. During that period of time, a specialized allocator called memblock<\/code> is used to allocate memory.[3]<\/a><\/sup><\/p>\n\n\n\nmemblock<\/code> views the system memory as a collection of contiguous regions. memblock<\/code> is made up of memblock types, which are themselves made up of multiple memory regions. There are several structures and functions for working with memblock<\/code>.[4]<\/a><\/sup><\/p>\n\n\n\nStructures<\/h2>\n\n\n\n
struct memblock<\/h3>\n\n\n\n
struct memblock<\/code> wraps the metadata for the memory<\/strong> and reserved<\/strong> memblock types, along with current_limit<\/code>, the physical address of the current allocation limit, and bottom_up<\/code>, the allocation direction. The allocation limit limits allocations to memory that is currently accessible and can be set with memblock_set_current_limit(phys_addr_t limit)<\/code>. The memblock<\/code> structure is statically initialized at build time.<\/p>\n\n\n\n[5]<\/a><\/sup>\nstruct memblock {\n\tbool bottom_up;\n\tphys_addr_t current_limit;\n\tstruct memblock_type memory;\n\tstruct memblock_type reserved;\n};<\/code><\/pre>\n\n\n\nstruct memblock_type<\/h3>\n\n\n\n
struct memblock_type<\/code>. This struct defines an array of memory regions regions<\/code> of type name<\/code> with cnt<\/code> number of regions. The total size of the array is max<\/code>, and the size of all the regions is total_size<\/code>.<\/p>\n\n\n\n[5]<\/a><\/sup>\nstruct memblock_type {\n\tunsigned long cnt;\n\tunsigned long max;\n\tphys_addr_t total_size;\n\tstruct memblock_region *regions;\n\tchar *name;\n};<\/code><\/pre>\n\n\n\nstruct memblock_region<\/h3>\n\n\n\n
struct memblock_region<\/code> represents a memory region with base address base<\/code>, size size<\/code>, memory region attributes flags<\/code>, and NUMA node id nid<\/code>. NUMA is a system with non-uniform memory access<\/a>. <\/p>\n\n\n\n[5]<\/a><\/sup>\nstruct memblock_region {\n\tphys_addr_t base;\n\tphys_addr_t size;\n\tenum memblock_flags flags;\n#ifdef CONFIG_NUMA\n\tint nid;\n#endif\n};<\/code><\/pre>\n\n\n\nenum memblock_flags<\/code>.<\/p>\n\n\n\n[5]<\/a><\/sup>\nenum memblock_flags {\n\tMEMBLOCK_NONE\t\t= 0x0,\t\/* No special request *\/\n\tMEMBLOCK_HOTPLUG\t= 0x1,\t\/* hotpluggable region *\/\n\tMEMBLOCK_MIRROR\t\t= 0x2,\t\/* mirrored region *\/\n\tMEMBLOCK_NOMAP\t\t= 0x4,\t\/* don't add to kernel direct mapping *\/\n\tMEMBLOCK_DRIVER_MANAGED = 0x8,\t\/* always detected via a driver *\/\n};<\/code><\/pre>\n\n\n\nFunctions<\/h2>\n\n\n\n
memblock<\/code> is initialized in setup_arch<\/code>() and taken down in mem_init<\/code>().<\/p>\n\n\n\nmemblock<\/code> has many useful APIs. These APIs include functions for adding and removing memory regions, and allocating and freeing memory, among many other functions<\/a>.<\/p>\n\n\n\nAdding and removing<\/h3>\n\n\n\n
memblock_add_node()<\/code> version adds the region within a NUMA node and includes parameters for the node id and memblock flags.<\/p>\n\n\n\nmemblock_add(base, size)<\/code>memblock_add_node(base, size, nid, flags)\nmemblock_remove(base, size)<\/code><\/pre>\n\n\n\nAllocating<\/h3>\n\n\n\n
memblock_phys_alloc(size, align)\nmemblock_phys_alloc_range(size, align, start, end)\nmemblock_phys_alloc_try_nid(size, align, nid)<\/code><\/pre>\n\n\n\nmemblock_alloc(size, align)\nmemblock_alloc_from(size, align, min_addr)\nmemblock_alloc_low(size, align)\nmemblock_alloc_node(size, align, nid)<\/code><\/pre>\n\n\n\nFreeing<\/h3>\n\n\n\n
memblock_free()<\/code> and memblock_phys_free()<\/code> free a boot memory block that was previously allocated; it is not released to the buddy allocator (which handles allocation after boot process completes). The *ptr<\/code> parameter in memblock_free()<\/code> is the starting address of the boot memory block. The functions memblock_free_all()<\/code> and memblock_free_late()<\/code> free pages to the buddy allocator.<\/p>\n\n\n\nmemblock_free(*ptr, size)\nmemblock_phys_free(base, size)\nmemblock_free_all(void)\nmemblock_free_late(base, size)<\/code><\/pre>\n\n\n\nReferences<\/h2>\n\n\n\n