Function buf_alloc

freebsd/kern/vfs_bio.c:1647–1713

* buf_alloc: * * Allocate an empty buffer header. */

 *	Allocate an empty buffer header.
 */
static struct buf *
buf_alloc(struct bufdomain *bd)
{
	struct buf *bp;
	int freebufs, error;

	/*
	 * We can only run out of bufs in the buf zone if the average buf
	 * is less than BKVASIZE.  In this case the actual wait/block will
	 * come from buf_reycle() failing to flush one of these small bufs.
	 */
	bp = NULL;
	freebufs = atomic_fetchadd_int(&bd->bd_freebuffers, -1);
	if (freebufs > 0)
		bp = uma_zalloc(buf_zone, M_NOWAIT);
	if (bp == NULL) {
		atomic_add_int(&bd->bd_freebuffers, 1);
		bufspace_daemon_wakeup(bd);
		counter_u64_add(numbufallocfails, 1);
		return (NULL);
	}
	/*
	 * Wake-up the bufspace daemon on transition below threshold.
	 */
	if (freebufs == bd->bd_lofreebuffers)
		bufspace_daemon_wakeup(bd);

	error = BUF_LOCK(bp, LK_EXCLUSIVE, NULL);
	KASSERT(error == 0, ("%s: BUF_LOCK on free buf %p: %d.", __func__, bp,
	    error));
	(void)error;

	KASSERT(bp->b_vp == NULL,
	    ("bp: %p still has vnode %p.", bp, bp->b_vp));
	KASSERT((bp->b_flags & (B_DELWRI | B_NOREUSE)) == 0,
	    ("invalid buffer %p flags %#x", bp, bp->b_flags));
	KASSERT((bp->b_xflags & (BX_VNCLEAN|BX_VNDIRTY)) == 0,
	    ("bp: %p still on a buffer list. xflags %X", bp, bp->b_xflags));
	KASSERT(bp->b_npages == 0,
	    ("bp: %p still has %d vm pages\n", bp, bp->b_npages));
	KASSERT(bp->b_kvasize == 0, ("bp: %p still has kva\n", bp));
	KASSERT(bp->b_bufsize == 0, ("bp: %p still has bufspace\n", bp));
	MPASS((bp->b_flags & B_MAXPHYS) == 0);

	bp->b_domain = BD_DOMAIN(bd);
	bp->b_flags = 0;
	bp->b_ioflags = 0;
	bp->b_xflags = 0;
	bp->b_vflags = 0;
	bp->b_vp = NULL;
	bp->b_blkno = bp->b_lblkno = 0;
	bp->b_offset = NOOFFSET;
	bp->b_iodone = 0;
	bp->b_error = 0;
	bp->b_resid = 0;
	bp->b_bcount = 0;
	bp->b_npages = 0;
	bp->b_dirtyoff = bp->b_dirtyend = 0;