xfs: use shifting and masking when converting rt extents, if possible

Avoid the costs of integer division (32-bit and 64-bit) if the realtime
extent size is a power of two.
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>

fs/xfs/libxfs/xfs_rtbitmap.h

@@ -11,6 +11,9 @@ xfs_rtx_to_rtb(
 	struct xfs_mount	*mp,
 	xfs_rtxnum_t		rtx)
 {
+	if (mp->m_rtxblklog >= 0)
+		return rtx << mp->m_rtxblklog;
+
 	return rtx * mp->m_sb.sb_rextsize;
 }
 
@@ -19,6 +22,9 @@ xfs_rtxlen_to_extlen(
 	struct xfs_mount	*mp,
 	xfs_rtxlen_t		rtxlen)
 {
+	if (mp->m_rtxblklog >= 0)
+		return rtxlen << mp->m_rtxblklog;
+
 	return rtxlen * mp->m_sb.sb_rextsize;
 }
 
@@ -28,6 +34,9 @@ xfs_extlen_to_rtxmod(
 	struct xfs_mount	*mp,
 	xfs_extlen_t		len)
 {
+	if (mp->m_rtxblklog >= 0)
+		return len & mp->m_rtxblkmask;
+
 	return len % mp->m_sb.sb_rextsize;
 }
 
@@ -36,6 +45,9 @@ xfs_extlen_to_rtxlen(
 	struct xfs_mount	*mp,
 	xfs_extlen_t		len)
 {
+	if (mp->m_rtxblklog >= 0)
+		return len >> mp->m_rtxblklog;
+
 	return len / mp->m_sb.sb_rextsize;
 }
 
@@ -45,6 +57,9 @@ xfs_rtb_to_rtx(
 	struct xfs_mount	*mp,
 	xfs_rtblock_t		rtbno)
 {
+	if (likely(mp->m_rtxblklog >= 0))
+		return rtbno >> mp->m_rtxblklog;
+
 	return div_u64(rtbno, mp->m_sb.sb_rextsize);
 }
 
@@ -54,6 +69,9 @@ xfs_rtb_to_rtxoff(
 	struct xfs_mount	*mp,
 	xfs_rtblock_t		rtbno)
 {
+	if (likely(mp->m_rtxblklog >= 0))
+		return rtbno & mp->m_rtxblkmask;
+
 	return do_div(rtbno, mp->m_sb.sb_rextsize);
 }
 
@@ -67,6 +85,11 @@ xfs_rtb_to_rtxrem(
 	xfs_rtblock_t		rtbno,
 	xfs_extlen_t		*off)
 {
+	if (likely(mp->m_rtxblklog >= 0)) {
+		*off = rtbno & mp->m_rtxblkmask;
+		return rtbno >> mp->m_rtxblklog;
+	}
+
 	return div_u64_rem(rtbno, mp->m_sb.sb_rextsize, off);
 }
 
@@ -79,6 +102,12 @@ xfs_rtb_to_rtxup(
 	struct xfs_mount	*mp,
 	xfs_rtblock_t		rtbno)
 {
+	if (likely(mp->m_rtxblklog >= 0)) {
+		if (rtbno & mp->m_rtxblkmask)
+			return (rtbno >> mp->m_rtxblklog) + 1;
+		return rtbno >> mp->m_rtxblklog;
+	}
+
 	if (do_div(rtbno, mp->m_sb.sb_rextsize))
 		rtbno++;
 	return rtbno;

fs/xfs/libxfs/xfs_sb.c

@@ -975,6 +975,8 @@ xfs_sb_mount_common(
 	mp->m_blockmask = sbp->sb_blocksize - 1;
 	mp->m_blockwsize = sbp->sb_blocksize >> XFS_WORDLOG;
 	mp->m_blockwmask = mp->m_blockwsize - 1;
+	mp->m_rtxblklog = log2_if_power2(sbp->sb_rextsize);
+	mp->m_rtxblkmask = mask64_if_power2(sbp->sb_rextsize);
 
 	mp->m_alloc_mxr[0] = xfs_allocbt_maxrecs(mp, sbp->sb_blocksize, 1);
 	mp->m_alloc_mxr[1] = xfs_allocbt_maxrecs(mp, sbp->sb_blocksize, 0);

fs/xfs/xfs_linux.h

@@ -198,6 +198,18 @@ static inline uint64_t howmany_64(uint64_t x, uint32_t y)
 	return x;
 }
 
+/* If @b is a power of 2, return log2(b).  Else return -1. */
+static inline int8_t log2_if_power2(unsigned long b)
+{
+	return is_power_of_2(b) ? ilog2(b) : -1;
+}
+
+/* If @b is a power of 2, return a mask of the lower bits, else return zero. */
+static inline unsigned long long mask64_if_power2(unsigned long b)
+{
+	return is_power_of_2(b) ? b - 1 : 0;
+}
+
 int xfs_rw_bdev(struct block_device *bdev, sector_t sector, unsigned int count,
 		char *data, enum req_op op);
 

fs/xfs/xfs_mount.h

@@ -119,6 +119,7 @@ typedef struct xfs_mount {
 	uint8_t			m_blkbb_log;	/* blocklog - BBSHIFT */
 	uint8_t			m_agno_log;	/* log #ag's */
 	uint8_t			m_sectbb_log;	/* sectlog - BBSHIFT */
+	int8_t			m_rtxblklog;	/* log2 of rextsize, if possible */
 	uint			m_blockmask;	/* sb_blocksize-1 */
 	uint			m_blockwsize;	/* sb_blocksize in words */
 	uint			m_blockwmask;	/* blockwsize-1 */
@@ -152,6 +153,7 @@ typedef struct xfs_mount {
 	uint64_t		m_features;	/* active filesystem features */
 	uint64_t		m_low_space[XFS_LOWSP_MAX];
 	uint64_t		m_low_rtexts[XFS_LOWSP_MAX];
+	uint64_t		m_rtxblkmask;	/* rt extent block mask */
 	struct xfs_ino_geometry	m_ino_geo;	/* inode geometry */
 	struct xfs_trans_resv	m_resv;		/* precomputed res values */
 						/* low free space thresholds */

fs/xfs/xfs_rtalloc.c

@@ -1054,6 +1054,7 @@ xfs_growfs_rt(
 		 * Calculate new sb and mount fields for this round.
 		 */
 		nsbp->sb_rextsize = in->extsize;
+		nmp->m_rtxblklog = -1; /* don't use shift or masking */
 		nsbp->sb_rbmblocks = bmbno + 1;
 		nrblocks_step = (bmbno + 1) * NBBY * nsbp->sb_blocksize *
 				nsbp->sb_rextsize;

fs/xfs/xfs_trans.c

@@ -656,6 +656,10 @@ xfs_trans_unreserve_and_mod_sb(
 	mp->m_sb.sb_agcount += tp->t_agcount_delta;
 	mp->m_sb.sb_imax_pct += tp->t_imaxpct_delta;
 	mp->m_sb.sb_rextsize += tp->t_rextsize_delta;
+	if (tp->t_rextsize_delta) {
+		mp->m_rtxblklog = log2_if_power2(mp->m_sb.sb_rextsize);
+		mp->m_rtxblkmask = mask64_if_power2(mp->m_sb.sb_rextsize);
+	}
 	mp->m_sb.sb_rbmblocks += tp->t_rbmblocks_delta;
 	mp->m_sb.sb_rblocks += tp->t_rblocks_delta;
 	mp->m_sb.sb_rextents += tp->t_rextents_delta;