Kernel Memory Chris Gill, David Ferry, Brian Kocoloski CSE 422S - Operating Systems Organization Washington University in St. Louis St. Louis, MO 63130

Kernel vs User Virtual Memory Kernel Virtual Memory Physical Memory Mappings created at runtime via memory allocation (e.g., mmap()) and page fault handling Mappings created at system boot time CSE 422S – Operating Systems Organization

Traditional Memory Mapping Strategy in Linux (on x86_64) Virtual Memory Mappings created at system boot time Kernel address space Physical Memory CSE 422S – Operating Systems Organization

Traditional Memory Mapping Strategy in Linux (on x86_64) Virtual Memory Mappings created at system boot time User address space Mappings created at runtime via memory allocation (e.g., mmap()) and page fault handling Kernel address space Physical Memory CSE 422S – Operating Systems Organization

Traditional Memory Mapping Strategy in Linux (on x86_64) Virtual Memory Green VA are free Blue VA are allocated But, both area always mapped in the kernel’s page tables User address space Kernel address space Physical Memory CSE 422S – Operating Systems Organization

Kernel vs User Virtual Memory Given a physical address, how do I calculate the kernel virtual address to access it? __va(unsigned long pa); On arm 7 (R Pi 3): ((unsigned long)((x) - PHYS_OFFSET + PAGE_OFFSET)) PHYS_OFFSET: first address of physical memory PAGE_OFFSET: (On our 32-bit R Pis) 0x80000000 The point is, reverse PA->VA translation can be done in O(1) time (You do this in studio 12) Given a user virtual address, how would the kernel calculate its physical address? Given a physical address, how do I calculate the user virtual address to access it? CSE 422S – Operating Systems Organization

CSE 422S – Operating Systems Organization Memory Pages All physical memory is divided into pages Pages are mapped to virtual memory Virtual addresses require translation Virtual Address Translation: Process A Process B Virtual Address Page # Offset 4KB 4KB 4KB 4KB 4KB 4KB Physical Page Phys. Addr CSE 422S – Operating Systems Organization

Dynamic Management of Kernel Memory Page level management Use alloc_pages() and free_pages() etc. Object level management Use kmalloc() and kfree() etc. (virtually and physically contiguous) Use vmalloc() and vfree() etc. (only virtually contiguous ) Slab allocation Use kmem_cache_create() and then kmem_cache_alloc() and kmem_cache_free() etc. for each distinct type of memory object CSE 422S – Operating Systems Organization

CSE 422S – Operating Systems Organization Vmalloc vs Kmalloc Kmalloc: virtually and physically contiguous Vmalloc: only virtually contiguous What does virtually contiguous mean? int * ar = kmalloc(sizeof(int) * 100, GFP_KERNEL) printf(“%p”, ar); If value of ar is X, then the virtual address range [X, X+sizeof(int)*100) stores the array contents CSE 422S – Operating Systems Organization

CSE 422S – Operating Systems Organization Vmalloc vs Kmalloc Kmalloc: virtually and physically contiguous Vmalloc: only virtually contiguous What does physically contiguous mean? int * ar = kmalloc(sizeof(int) * 100, GFP_KERNEL) printf(“%p”, ar); If VA of ar is X, what is the physical address (PA)? X+Y, where Y is some known constant value (recall slides 2 through 5) CSE 422S – Operating Systems Organization

CSE 422S – Operating Systems Organization Vmalloc vs Kmalloc kmalloc() returns virtually and physically contiguous addresses i.e., Assume we want to allocate S bytes of memory Assume X is the return value (virtual address) from a call to kmalloc(S) Assume pa(X) = Y Then, for all every byte x from 0 to S, PA(x) = Y+x CSE 422S – Operating Systems Organization

Traditional Memory Mapping Strategy in Linux (on x86_64) Virtual Memory Assume green VA are free Assume blue VA are allocated Kernel address space Physical Memory Calls to kmalloc() effectively search through green VA space to find free virtual address space CSE 422S – Operating Systems Organization

CSE 422S – Operating Systems Organization Vmalloc vs Kmalloc vmalloc() returns only virtually contiguous addresses i.e Assume we want to allocate S bytes of memory Assume X is the return value (virtual address) from a call to vmalloc(S) Assume pa(X) = Y Then, for all every byte x from 0 to PAGE_SIZE-1, PA(X+x) = Y+x But what about byte x=PAGE_SIZE? Assume PA(X+PAGE_SIZE) = Z Could be that Z=Y+PAGE_SIZE But, could be a completely different physical address Z CSE 422S – Operating Systems Organization

Traditional Memory Mapping Strategy in Linux (on x86_64) Virtual Memory Assume blue VA are allocated Calls to vmalloc() effectively allocate new, previously unused virtual address space, and map it to possibly discontiguous memory Kernel address space Physical Memory New vmalloc()’d memory CSE 422S – Operating Systems Organization

CSE 422S – Operating Systems Organization Page Level Management (Pre-)allocating larger chunks of memory E.g., 2n contiguous pages: alloc_pages(GFP_KERNEL, n) Each page is fixed size (usually 4Kb or 8Kb) with enough room for hundreds of individual objects Can request a page with filled-in values Use get_zeroed_page() to hide previous contents Must free pages once finished using them Use free_page() if a single page was allocated Use free_pages() if multiple were allocated at once CSE 422S – Operating Systems Organization

Object Level Management Size of each allocation is in bytes I.e., the size of the object being allocated Rather than the number of fixed-sized pages Kernel version of malloc: kmalloc() and kfree() Action modifiers (can/cannot sleep, etc.) Zone modifiers (DMA, DMA32, HIGH, NORMAL) Type flags: combine action and zone modifiers Allocated memory is physically contiguous Virtually contiguous memory Though not necessarily physically contiguous Use vmalloc() and vfree() etc. CSE 422S – Operating Systems Organization

Kernel Memory Trade-Offs Page level management Coarse granularity, low performance overhead Object level management Finer granularity, higher performance overhead By default, prefer the kmalloc() version over the vmalloc() version, for performance Slab allocation Optimizes performance if allocating and de-allocating large numbers of the same kind of object CSE 422S – Operating Systems Organization

Other Kernel Memory Options Static allocation on the stack Can be risky, as stacks are limited to 1 or 2 pages In general, minimize stack usage by design By default, use the kernel allocation techniques previously discussed instead of stack allocation Per-CPU allocation Think of this as “core-specific storage” Can improve cache performance, reduce contention Must disable kernel preemption (per-CPU interface does) Must “alloc” and “put” per-CPU data (like alloc/free) CSE 422S – Operating Systems Organization