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CONTIGMALLOC(9) Kernel Developer's Manual CONTIGMALLOC(9)


contigmalloc, contigfreemanage contiguous kernel physical memory


#include <sys/types.h>
#include <sys/malloc.h>

void *
contigmalloc(unsigned long size, struct malloc_type *type, int flags, vm_paddr_t low, vm_paddr_t high, unsigned long alignment, vm_paddr_t boundary);

contigfree(void *addr, unsigned long size, struct malloc_type *type);

#include <sys/param.h>
#include <sys/domainset.h>

void *
contigmalloc_domainset(unsigned long size, struct malloc_type *type, struct domainset *ds, int flags, vm_paddr_t low, vm_paddr_t high, unsigned long alignment, vm_paddr_t boundary);


The () function allocates size bytes of contiguous physical memory that is aligned to alignment bytes, and which does not cross a boundary of boundary bytes. If successful, the allocation will reside between physical addresses low and high. The returned pointer points to a wired kernel virtual address range of size bytes allocated from the kernel virtual address (KVA) map.

The () variant allows the caller to additionally specify a numa(4) domain selection policy. See domainset(9) for some example policies.

The flags parameter modifies ()'s behaviour as follows:

Causes the allocated physical memory to be zero filled.
Causes contigmalloc() to return NULL if the request cannot be immediately fulfilled due to resource shortage.

Other flags (if present) are ignored.

The () function deallocates memory allocated by a previous call to contigmalloc() or contigmalloc_domainset().


The contigmalloc() function does not sleep waiting for memory resources to be freed up, but instead actively reclaims pages before giving up. However, unless M_NOWAIT is specified, it may select a page for reclamation that must first be written to backing storage, causing it to sleep.

The contigfree() function does not accept NULL as an address input, unlike free(9).


The contigmalloc() function returns a kernel virtual address if allocation succeeds, or NULL otherwise.


void *p;
p = contigmalloc(8192, M_DEVBUF, M_ZERO, 0, (1L << 22),
    32 * 1024, 1024 * 1024);

Ask for 8192 bytes of zero-filled memory residing between physical address 0 and 4194303 inclusive, aligned to a 32K boundary and not crossing a 1M address boundary.


The contigmalloc() function will panic if size is zero, or if alignment or boundary is not a power of two.


malloc(9), memguard(9)

October 30, 2018 Debian