xfs: factor out a xfs_growfs_rt_alloc_fake_mount helper

Split the code to set up a fake mount point to calculate new RT
geometry out of xfs_growfs_rt_bmblock so that it can be reused.

Note that this changes the rmblocks calculation method to be based
on the passed in rblocks and extsize and not the explicitly passed
one, but both methods will always lead to the same result.  The new
version just does a little bit more math while being more general.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>

Author: Darrick J. Wong

fs/xfs/xfs_rtalloc.c

@@ -734,6 +734,36 @@ xfs_rtginode_ensure(
 	return xfs_rtginode_create(rtg, type, true);
 }
 
+static struct xfs_mount *
+xfs_growfs_rt_alloc_fake_mount(
+	const struct xfs_mount	*mp,
+	xfs_rfsblock_t		rblocks,
+	xfs_agblock_t		rextsize)
+{
+	struct xfs_mount	*nmp;
+
+	nmp = kmemdup(mp, sizeof(*mp), GFP_KERNEL);
+	if (!nmp)
+		return NULL;
+	nmp->m_sb.sb_rextsize = rextsize;
+	xfs_mount_sb_set_rextsize(nmp, &nmp->m_sb);
+	nmp->m_sb.sb_rblocks = rblocks;
+	nmp->m_sb.sb_rextents = xfs_rtb_to_rtx(nmp, nmp->m_sb.sb_rblocks);
+	nmp->m_sb.sb_rbmblocks = xfs_rtbitmap_blockcount(nmp,
+			nmp->m_sb.sb_rextents);
+	nmp->m_sb.sb_rextslog = xfs_compute_rextslog(nmp->m_sb.sb_rextents);
+	nmp->m_rsumlevels = nmp->m_sb.sb_rextslog + 1;
+	nmp->m_rsumblocks = xfs_rtsummary_blockcount(nmp, nmp->m_rsumlevels,
+			nmp->m_sb.sb_rbmblocks);
+
+	if (rblocks > 0)
+		nmp->m_features |= XFS_FEAT_REALTIME;
+
+	/* recompute growfsrt reservation from new rsumsize */
+	xfs_trans_resv_calc(nmp, &nmp->m_resv);
+	return nmp;
+}
+
 static int
 xfs_growfs_rt_bmblock(
 	struct xfs_rtgroup	*rtg,
@@ -756,25 +786,15 @@ xfs_growfs_rt_bmblock(
 	xfs_rtbxlen_t		freed_rtx;
 	int			error;
 
-
+	/*
+	 * Calculate new sb and mount fields for this round.
+	 */
 	nrblocks_step = (bmbno + 1) * NBBY * mp->m_sb.sb_blocksize * rextsize;
-
-	nmp = nargs.mp = kmemdup(mp, sizeof(*mp), GFP_KERNEL);
+	nmp = nargs.mp = xfs_growfs_rt_alloc_fake_mount(mp,
+			min(nrblocks, nrblocks_step), rextsize);
 	if (!nmp)
 		return -ENOMEM;
 
-	/*
-	 * Calculate new sb and mount fields for this round.
-	 */
-	nmp->m_sb.sb_rextsize = rextsize;
-	xfs_mount_sb_set_rextsize(nmp, &nmp->m_sb);
-	nmp->m_sb.sb_rbmblocks = bmbno + 1;
-	nmp->m_sb.sb_rblocks = min(nrblocks, nrblocks_step);
-	nmp->m_sb.sb_rextents = xfs_rtb_to_rtx(nmp, nmp->m_sb.sb_rblocks);
-	nmp->m_sb.sb_rextslog = xfs_compute_rextslog(nmp->m_sb.sb_rextents);
-	nmp->m_rsumlevels = nmp->m_sb.sb_rextslog + 1;
-	nmp->m_rsumblocks = xfs_rtsummary_blockcount(mp, nmp->m_rsumlevels,
-			nmp->m_sb.sb_rbmblocks);
 	rtg->rtg_extents = xfs_rtgroup_extents(nmp, rtg_rgno(rtg));
 
 	/*