Function vfs_nonvmio_extend

freebsd/kern/vfs_bio.c:4214–4260

* Extend the backing for a non-VMIO buffer. */

 * Extend the backing for a non-VMIO buffer.
 */
static void
vfs_nonvmio_extend(struct buf *bp, int newbsize)
{
	caddr_t origbuf;
	int origbufsize;

	/*
	 * We only use malloced memory on the first allocation.
	 * and revert to page-allocated memory when the buffer
	 * grows.
	 *
	 * There is a potential smp race here that could lead
	 * to bufmallocspace slightly passing the max.  It
	 * is probably extremely rare and not worth worrying
	 * over.
	 */
	if (bp->b_bufsize == 0 && newbsize <= PAGE_SIZE/2 &&
	    bufmallocspace < maxbufmallocspace) {
		bp->b_data = malloc(newbsize, M_BIOBUF, M_WAITOK);
		bp->b_flags |= B_MALLOC;
		bufmallocadjust(bp, newbsize);
		return;
	}

	/*
	 * If the buffer is growing on its other-than-first
	 * allocation then we revert to the page-allocation
	 * scheme.
	 */
	origbuf = NULL;
	origbufsize = 0;
	if (bp->b_flags & B_MALLOC) {
		origbuf = bp->b_data;
		origbufsize = bp->b_bufsize;
		bp->b_data = bp->b_kvabase;
		bufmallocadjust(bp, 0);
		bp->b_flags &= ~B_MALLOC;
		newbsize = round_page(newbsize);
	}
	vm_hold_load_pages(bp, (vm_offset_t) bp->b_data + bp->b_bufsize,
	    (vm_offset_t) bp->b_data + newbsize);
	if (origbuf != NULL) {
		bcopy(origbuf, bp->b_data, origbufsize);
		free(origbuf, M_BIOBUF);
	}
	bufspace_adjust(bp, newbsize);
}

/*
 * This code constitutes the buffer memory from either anonymous system