Memory management

Memory management

Linked Lists

structs and memory layout fox fox fox list.next list.next list.next list.prev list.prev list.prev

Linked lists in Linux node fox fox fox list { .next .prev } list { .next .prev } list { .next .prev }

What about types? • Calculates a pointer to the containing struct structlist_headfox_list; struct fox * fox_ptr = list_entry(fox_list->next, struct fox, node);

List access methods structlist_headsome_list; list_add(structlist_head * new_entry, structlist_head * list); list_del(structlist_head * entry_to_remove); struct type * ptr; list_for_each_entry(ptr, &some_list, node){ … } struct type * ptr, * tmp_ptr; list_for_each_entry_safe(ptr, tmp_ptr, &some_list, node) { list_del(ptr); kfree(ptr); }

Page Frame Database /* Each physical page in the system has a struct page associated with * it to keep track of whatever it is we are using the page for at the * moment. Note that we have no way to track which tasks are using * a page */ struct page { unsigned long flags; // Atomic flags: locked, referenced, dirty, slab, disk atomic_t _count; // Usage count, atomic_t _mapcount; // Count of ptes mapping in this page struct { unsigned long private; // Used for managing page used in file I/O structaddress_space * mapping; // Used to define the data this page is holding }; pgoff_t index; // Our offset within mapping structlist_headlru; // Linked list node containing LRU ordering of pages void * virtual; // Kernel virtual address };

Memory Zones • Not all memory addresses are the same • ZONE_DMA: DMA memory (< 16MB) • Really old I/O devices that have constrained addresses • ZONE_DMA32: 32 bit DMA memory ( < 4GB) • Older I/O devices that only support 32 bit DMA • ZONE_NORMAL: Generic Kernel memory • Always directly addressable by the kernel • Linux groups memory into zones • Based on the use cases for memory • Allow allocations to occur in a given zone • How?

Buddy Allocator • Memory allocations are all backed by physical pages • Kernel allocations are persistent • Cannot be movedor swapped • Must find contiguous sets of pages • Allocations all come from free lists • Linked list of unallocated resources • Code example

Allocating pages • Return entry/entries from page list • Scans various lists for page(s) to allocate • struct page * alloc_pages(gfp_t flags, int order); • void * page_address(struct page * page) • unsigned long page_to_pfn(struct page * pg);

kmalloc • kernel version of malloc • manages global heap, accessible by all kernel threads • Returns kernel virtual addresses • void * kmalloc(size_t size, gfp_t flags);

gfp_t • What are these gfp_t flags? • Directions to allocator • Where to get the memory from • What steps allocator can take to find memory • Some Examples: • Zone • GFP_DMA, GFP_DMA32, GFP_NORMAL • Behavior • GFP_ATOMIC, GFP_KERNEL

vmalloc • Linux limits the number of contiguous pages you can allocate • MAX_ORDER typically is 11 (32MB) • 2^11 pages • What if you need to allocate more? • Must do the allocation in virtual memory • void * vmalloc(unsigned long size); • Allocates a virtually contiguous address region • Backed by physically discontinuous pages

Slab allocator • Optimization for kernel allocations • Provides a free list (or cache) of unused allocations of a certain type • Don’t have to search for a free region • Allocations become (almost) constant time • Create special caches for certain types of common allocations • i.e. network packets, inodes, process descriptors • Allocate those types using a special allocator • Slab subsystem dynamically ensures that enough memory is available • Allocates and frees pages behind the scenes

Memory management