Function sf_buf_alloc

freebsd/kern/subr_sfbuf.c:113–168

* Get an sf_buf from the freelist. May block if none are available. */

 * Get an sf_buf from the freelist.  May block if none are available.
 */
struct sf_buf *
sf_buf_alloc(struct vm_page *m, int flags)
{
	struct sf_head *hash_list;
	struct sf_buf *sf;
	int error;

	if (PMAP_HAS_DMAP)
		return ((struct sf_buf *)m);

	KASSERT(curthread->td_pinned > 0 || (flags & SFB_CPUPRIVATE) == 0,
	    ("sf_buf_alloc(SFB_CPUPRIVATE): curthread not pinned"));
	hash_list = &sf_buf_active[SF_BUF_HASH(m)];
	mtx_lock(&sf_buf_lock);
	LIST_FOREACH(sf, hash_list, list_entry) {
		if (sf->m == m) {
			sf->ref_count++;
			if (sf->ref_count == 1) {
				TAILQ_REMOVE(&sf_buf_freelist, sf, free_entry);
				nsfbufsused++;
				nsfbufspeak = imax(nsfbufspeak, nsfbufsused);
			}
#if defined(SMP) && defined(SFBUF_CPUSET)
			sf_buf_shootdown(sf, flags);
#endif
			goto done;
		}
	}
	while ((sf = TAILQ_FIRST(&sf_buf_freelist)) == NULL) {
		if (flags & SFB_NOWAIT)
			goto done;
		sf_buf_alloc_want++;
		SFSTAT_INC(sf_allocwait);
		error = msleep(&sf_buf_freelist, &sf_buf_lock,
		    (flags & SFB_CATCH) ? PCATCH | PVM : PVM, "sfbufa", 0);
		sf_buf_alloc_want--;

		/*
		 * If we got a signal, don't risk going back to sleep. 
		 */
		if (error)
			goto done;
	}
	TAILQ_REMOVE(&sf_buf_freelist, sf, free_entry);
	if (sf->m != NULL)
		LIST_REMOVE(sf, list_entry);
	LIST_INSERT_HEAD(hash_list, sf, list_entry);
	sf->ref_count = 1;
	sf->m = m;
	nsfbufsused++;
	nsfbufspeak = imax(nsfbufspeak, nsfbufsused);
	sf_buf_map(sf, flags);
done:
	mtx_unlock(&sf_buf_lock);
	return (sf);
}

/*