Index: head/share/man/man9/malloc.9
===================================================================
--- head/share/man/man9/malloc.9
+++ head/share/man/man9/malloc.9
@@ -29,7 +29,7 @@
 .\" $NetBSD: malloc.9,v 1.3 1996/11/11 00:05:11 lukem Exp $
 .\" $FreeBSD$
 .\"
-.Dd August 3, 2020
+.Dd August 28, 2020
 .Dt MALLOC 9
 .Os
 .Sh NAME
@@ -55,6 +55,8 @@
 .Fn realloc "void *addr" "size_t size" "struct malloc_type *type" "int flags"
 .Ft void *
 .Fn reallocf "void *addr" "size_t size" "struct malloc_type *type" "int flags"
+.Ft size_t
+.Fn malloc_usable_size "const void *addr"
 .Fn MALLOC_DECLARE type
 .In sys/param.h
 .In sys/malloc.h
@@ -149,6 +151,13 @@
 .Fn realloc
 except that it
 will free the passed pointer when the requested memory cannot be allocated.
+.Pp
+The
+.Fn malloc_usable_size
+function returns the usable size of the allocation pointed to by
+.Fa addr .
+The return value may be larger than the size that was requested during
+allocation.
 .Pp
 Unlike its standard C library counterpart
 .Pq Xr malloc 3 ,
Index: head/sys/compat/linuxkpi/common/include/linux/slab.h
===================================================================
--- head/sys/compat/linuxkpi/common/include/linux/slab.h
+++ head/sys/compat/linuxkpi/common/include/linux/slab.h
@@ -154,6 +154,12 @@
 	free(__DECONST(void *, ptr), M_KMALLOC);
 }
 
+static inline size_t
+ksize(const void *ptr)
+{
+	return (malloc_usable_size(ptr));
+}
+
 extern struct linux_kmem_cache *linux_kmem_cache_create(const char *name,
     size_t size, size_t align, unsigned flags, linux_kmem_ctor_t *ctor);
 
Index: head/sys/kern/kern_malloc.c
===================================================================
--- head/sys/kern/kern_malloc.c
+++ head/sys/kern/kern_malloc.c
@@ -938,6 +938,42 @@
 	return (mem);
 }
 
+/*
+ *	malloc_usable_size: returns the usable size of the allocation.
+ */
+size_t
+malloc_usable_size(const void *addr)
+{
+#ifndef DEBUG_REDZONE
+	uma_zone_t zone;
+	uma_slab_t slab;
+#endif
+	u_long size;
+
+	if (addr == NULL)
+		return (0);
+
+#ifdef DEBUG_MEMGUARD
+	if (is_memguard_addr(__DECONST(void *, addr)))
+		return (memguard_get_req_size(addr));
+#endif
+
+#ifdef DEBUG_REDZONE
+	size = redzone_get_size(__DECONST(void *, addr));
+#else
+	vtozoneslab((vm_offset_t)addr & (~UMA_SLAB_MASK), &zone, &slab);
+	if (slab == NULL)
+		panic("malloc_usable_size: address %p(%p) is not allocated.\n",
+		    addr, (void *)((u_long)addr & (~UMA_SLAB_MASK)));
+
+	if (!malloc_large_slab(slab))
+		size = zone->uz_size;
+	else
+		size = malloc_large_size(slab);
+#endif
+	return (size);
+}
+
 CTASSERT(VM_KMEM_SIZE_SCALE >= 1);
 
 /*
Index: head/sys/sys/malloc.h
===================================================================
--- head/sys/sys/malloc.h
+++ head/sys/sys/malloc.h
@@ -244,6 +244,7 @@
 void	malloc_type_freed(struct malloc_type *type, unsigned long size);
 void	malloc_type_list(malloc_type_list_func_t *, void *);
 void	malloc_uninit(void *);
+size_t	malloc_usable_size(const void *);
 void	*realloc(void *addr, size_t size, struct malloc_type *type, int flags)
 	    __result_use_check __alloc_size(2);
 void	*reallocf(void *addr, size_t size, struct malloc_type *type, int flags)
Index: head/sys/vm/memguard.h
===================================================================
--- head/sys/vm/memguard.h
+++ head/sys/vm/memguard.h
@@ -43,6 +43,7 @@
 void 	*memguard_alloc(unsigned long, int);
 void	*memguard_realloc(void *, unsigned long, struct malloc_type *, int);
 void	memguard_free(void *);
+unsigned long	memguard_get_req_size(const void *);
 int	memguard_cmp_mtp(struct malloc_type *, unsigned long);
 int	memguard_cmp_zone(uma_zone_t);
 int	is_memguard_addr(void *);
@@ -52,6 +53,7 @@
 #define	memguard_alloc(size, flags)	NULL
 #define	memguard_realloc(a, s, mtp, f)	NULL
 #define	memguard_free(addr)		do { } while (0)
+#define	memguard_get_req_size(addr)	0
 #define	memguard_cmp_mtp(mtp, size)	0
 #define	memguard_cmp_zone(zone)		0
 #define	is_memguard_addr(addr)		0
Index: head/sys/vm/memguard.c
===================================================================
--- head/sys/vm/memguard.c
+++ head/sys/vm/memguard.c
@@ -504,3 +504,9 @@
 	 */
 	return (strcmp(zone->uz_name, vm_memguard_desc) == 0);
 }
+
+unsigned long
+memguard_get_req_size(const void *addr)
+{
+	return (*v2sizep(trunc_page((uintptr_t)addr)));
+}