xfs: calculate inode walk prefetch more carefully

	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);

}

/*