xfs: calculate inode walk prefetch more carefully

The existing inode walk prefetch is based on the old bulkstat code,
which simply allocated 4 pages worth of memory and prefetched that many
inobt records, regardless of however many inodes the caller requested.
65536 inodes is a lot to prefetch (~32M on x64, ~512M on arm64) so let's
scale things down a little more intelligently based on the number of
inodes requested, etc.
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>

fs/xfs/xfs_iwalk.c

@@ -333,16 +333,58 @@ xfs_iwalk_ag(
 	return error;
 }
 
+/*
+ * We experimentally determined that the reduction in ioctl call overhead
+ * diminishes when userspace asks for more than 2048 inodes, so we'll cap
+ * prefetch at this point.
+ */
+#define IWALK_MAX_INODE_PREFETCH	(2048U)
+
 /*
  * Given the number of inodes to prefetch, set the number of inobt records that
  * we cache in memory, which controls the number of inodes we try to read
- * ahead.
+ * ahead.  Set the maximum if @inodes == 0.
  */
 static inline unsigned int
 xfs_iwalk_prefetch(
-	unsigned int		inode_records)
+	unsigned int		inodes)
 {
-	return PAGE_SIZE * 4 / sizeof(struct xfs_inobt_rec_incore);
+	unsigned int		inobt_records;
+
+	/*
+	 * If the caller didn't tell us the number of inodes they wanted,
+	 * assume the maximum prefetch possible for best performance.
+	 * Otherwise, cap prefetch at that maximum so that we don't start an
+	 * absurd amount of prefetch.
+	 */
+	if (inodes == 0)
+		inodes = IWALK_MAX_INODE_PREFETCH;
+	inodes = min(inodes, IWALK_MAX_INODE_PREFETCH);
+
+	/* Round the inode count up to a full chunk. */
+	inodes = round_up(inodes, XFS_INODES_PER_CHUNK);
+
+	/*
+	 * In order to convert the number of inodes to prefetch into an
+	 * estimate of the number of inobt records to cache, we require a
+	 * conversion factor that reflects our expectations of the average
+	 * loading factor of an inode chunk.  Based on data gathered, most
+	 * (but not all) filesystems manage to keep the inode chunks totally
+	 * full, so we'll underestimate slightly so that our readahead will
+	 * still deliver the performance we want on aging filesystems:
+	 *
+	 * inobt = inodes / (INODES_PER_CHUNK * (4 / 5));
+	 *
+	 * The funny math is to avoid integer division.
+	 */
+	inobt_records = (inodes * 5) / (4 * XFS_INODES_PER_CHUNK);
+
+	/*
+	 * Allocate enough space to prefetch at least two inobt records so that
+	 * we can cache both the record where the iwalk started and the next
+	 * record.  This simplifies the AG inode walk loop setup code.
+	 */
+	return max(inobt_records, 2U);
 }
 
 /*