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Memory management. Linked Lists structs and memory layout list.next list.prev list.next list.prev list.next list.prev fox.

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Presentation on theme: "Memory management. Linked Lists structs and memory layout list.next list.prev list.next list.prev list.next list.prev fox."— Presentation transcript:

1 Memory management

2 Linked Lists

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

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

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

6 List access methods struct list_head some_list; list_add(struct list_head * new_entry, struct list_head * list); list_del(struct list_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); }

7 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 struct address_space * mapping; // Used to define the data this page is holding }; pgoff_t index; // Our offset within mapping struct list_head lru; // Linked list node containing LRU ordering of pages void * virtual; // Kernel virtual address };

8 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?

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

10 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);

11 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);

12 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

13 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

14 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


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