Function allocbuf

freebsd/kern/vfs_bio.c:4276–4323

* This code constitutes the buffer memory from either anonymous system * memory (in the case of non-VMIO operations) or from an associated * VM object (in the case of VMIO operations). This code is able to * resize a buffer up or down. * * Note that this code is tricky, and has many complications to resolve * deadlock or inconsistent data situations. Tread lightly!!! * There are B_CACHE

 * B_CACHE for the non-VMIO case.
 */
int
allocbuf(struct buf *bp, int size)
{
	int newbsize;

	if (bp->b_bcount == size)
		return (1);

	if (bp->b_kvasize != 0 && bp->b_kvasize < size)
		panic("allocbuf: buffer too small");

	newbsize = roundup2(size, DEV_BSIZE);
	if ((bp->b_flags & B_VMIO) == 0) {
		if ((bp->b_flags & B_MALLOC) == 0)
			newbsize = round_page(newbsize);
		/*
		 * Just get anonymous memory from the kernel.  Don't
		 * mess with B_CACHE.
		 */
		if (newbsize < bp->b_bufsize)
			vfs_nonvmio_truncate(bp, newbsize);
		else if (newbsize > bp->b_bufsize)
			vfs_nonvmio_extend(bp, newbsize);
	} else {
		int desiredpages;

		desiredpages = (size == 0) ? 0 :
		    num_pages((bp->b_offset & PAGE_MASK) + newbsize);

		if (bp->b_flags & B_MALLOC)
			panic("allocbuf: VMIO buffer can't be malloced");
		/*
		 * Set B_CACHE initially if buffer is 0 length or will become
		 * 0-length.
		 */
		if (size == 0 || bp->b_bufsize == 0)
			bp->b_flags |= B_CACHE;

		if (newbsize < bp->b_bufsize)
			vfs_vmio_truncate(bp, desiredpages);
		/* XXX This looks as if it should be newbsize > b_bufsize */
		else if (size > bp->b_bcount)
			vfs_vmio_extend(bp, desiredpages, size);
		bufspace_adjust(bp, newbsize);
	}
	bp->b_bcount = size;		/* requested buffer size. */
	return (1);
}

extern int inflight_transient_maps;