KMEM(9) NetBSD Kernel Developer's Manual KMEM(9)
NAME
kmem -- kernel wired memory allocator
SYNOPSIS
#include <sys/kmem.h> void * kmem_alloc(size_t size, km_flag_t kmflags); void * kmem_zalloc(size_t size, km_flag_t kmflags); void kmem_free(void *p, size_t size); void * kmem_intr_alloc(size_t size, km_flag_t kmflags); void * kmem_intr_zalloc(size_t size, km_flag_t kmflags); void kmem_intr_free(void *p, size_t size); char * kmem_asprintf(const char *fmt, ...); options KMEM_SIZE options KMEM_REDZONE options KMEM_GUARD
DESCRIPTION
kmem_alloc() allocates kernel wired memory. It takes the following argu- ments. size Specify the size of allocation in bytes. kmflags Either of the following: KM_SLEEP If the allocation cannot be satisfied immediately, sleep until enough memory is available. If KM_SLEEP is specified, then the allocation cannot fail. KM_NOSLEEP Don't sleep. Immediately return NULL if there is not enough memory available. It should only be used when failure to allocate will not have harmful, user-visible effects. Use of KM_NOSLEEP is strongly discouraged as it can create transient, hard to debug failures that occur when the system is under memory pressure. In situations where it is not possible to sleep, for example because locks are held by the caller, the code path should be restructured to allow the allo- cation to be made in another place. The contents of allocated memory are uninitialized. Unlike Solaris, kmem_alloc(0, flags) is illegal. kmem_zalloc() is the equivalent of kmem_alloc(), except that it initial- izes the memory to zero. kmem_asprintf() functions as the well known asprintf() function, but allocates memory using kmem_alloc(). This routine can sleep during allo- cation. The size of the allocated area is the length of the returned character string, plus one (for the NUL terminator). This must be taken into consideration when freeing the returned area with kmem_free(). kmem_free() frees kernel wired memory allocated by kmem_alloc() or kmem_zalloc() so that it can be used for other purposes. It takes the following arguments. p The pointer to the memory being freed. It must be the one returned by kmem_alloc() or kmem_zalloc(). size The size of the memory being freed, in bytes. It must be the same as the size argument used for kmem_alloc() or kmem_zalloc() when the memory was allocated. Freeing NULL is illegal. kmem_intr_alloc(), kmem_intr_zalloc() and kmem_intr_free() are the equiv- alents of the above kmem routines which can be called from the interrupt context. These routines are for the special cases. Normally, pool_cache(9) should be used for memory allocation from interrupt con- text.
NOTES
Making KM_SLEEP allocations while holding mutexes or reader/writer locks is discouraged, as the caller can sleep for an unbounded amount of time in order to satisfy the allocation. This can in turn block other threads that wish to acquire locks held by the caller. It should be noted that kmem_free() may also block. For some locks this is permissible or even unavoidable. For others, par- ticularly locks that may be taken from soft interrupt context, it is a serious problem. As a general rule it is better not to allow this type of situation to develop. One way to circumvent the problem is to make allocations speculative and part of a retryable sequence. For example: retry: /* speculative unlocked check */ if (need to allocate) { new_item = kmem_alloc(sizeof(*new_item), KM_SLEEP); } else { new_item = NULL; } mutex_enter(lock); /* check while holding lock for true status */ if (need to allocate) { if (new_item == NULL) { mutex_exit(lock); goto retry; } consume(new_item); new_item = NULL; } mutex_exit(lock); if (new_item != NULL) { /* did not use it after all */ kmem_free(new_item, sizeof(*new_item)); }
OPTIONS
KMEM_SIZE Kernels compiled with the KMEM_SIZE option ensure the size given in kmem_free() matches the actual allocated size. On kmem_alloc(), the ker- nel will allocate an additional contiguous kmem page of eight bytes in the buffer, will register the allocated size in the first kmem page of that buffer, and will return a pointer to the second kmem page in that same buffer. When freeing, the kernel reads the first page, and compares the size registered with the one given in kmem_free(). Any mismatch triggers a panic. KMEM_SIZE is enabled by default on DIAGNOSTIC and DEBUG. KMEM_REDZONE Kernels compiled with the KMEM_REDZONE option add a dynamic pattern of two bytes at the end of each allocated buffer, and check this pattern when freeing to ensure the caller hasn't written outside the requested area. This option does not introduce a significant performance impact, but has two drawbacks: it only catches write overflows, and catches them only on kmem_free(). KMEM_REDZONE is enabled by default on DIAGNOSTIC. KMEM_GUARD Kernels compiled with the KMEM_GUARD option perform CPU intensive sanity checks on kmem operations. It adds additional, very high overhead run- time verification to kmem operations. It must be enabled with KMEM_SIZE. KMEM_GUARD tries to catch the following types of bugs: · Overflow at time of occurrence, by means of a guard page. An unmapped guard page sits immediately after the requested area; a read/write overflow therefore triggers a page fault. · Underflow at kmem_free(), by using KMEM_SIZE's registered size. If an underflow occurs, the size stored by KMEM_SIZE will be overwrit- ten, which means that when freeing, the kernel will spot the mis- match. · Use-after-free at time of occurrence. When freeing, the memory is unmapped, and depending on the value of kmem_guard_depth, the kernel will more or less delay the recycling of that memory. Which means that any ulterior read/write access to the memory will trigger a page fault, given it hasn't been recycled yet. To enable it, boot the system with the -d option, which causes the debug- ger to be entered early during the kernel boot process. Issue commands such as the following: db> w kmem_guard_depth 0t30000 db> c This instructs kmem_guard to queue up to 60000 (30000*2) pages of unmapped KVA to catch use-after-free type errors. When kmem_free() is called, memory backing a freed item is unmapped and the kernel VA space pushed onto a FIFO. The VA space will not be reused until another 30k items have been freed. Until reused the kernel will catch invalid accesses and panic with a page fault. Limitations: · It has a severe impact on performance. · It is best used on a 64-bit machine with lots of RAM. KMEM_GUARD is enabled by default on DEBUG.
RETURN VALUES
On success, kmem_alloc() and kmem_zalloc() return a pointer to allocated memory. Otherwise, NULL is returned.
CODE REFERENCES
The kmem subsystem is implemented within the file sys/kern/subr_kmem.c.
SEE ALSO
intro(9), memoryallocators(9), percpu(9), pool_cache(9), uvm_km(9)
CAVEATS
Neither kmem_alloc() nor kmem_free() can be used from interrupt context, from a soft interrupt, or from a callout. Use pool_cache(9) in these situations.
SECURITY CONSIDERATIONS
As the memory allocated by kmem_alloc() is uninitialized, it can contain security-sensitive data left by its previous user. It is the caller's responsibility not to expose it to the world. NetBSD 8.3 February 28, 2016 NetBSD 8.3
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