ufs: fix the logics for tail relocation

* original hysteresis loop got broken by typo back in 2002; now
it never switches out of OPTTIME state.  Fixed.
* critical levels for switching from OPTTIME to OPTSPACE and back
ought to be calculated once, at mount time.
* we should use mul_u64_u32_div() for those calculations, now that
->s_dsize is 64bit.
* to quote Kirk McKusick (in 1995 FreeBSD commit message):
    The threshold for switching from time-space and space-time is too small
    when minfree is 5%...so make it stay at space in this case.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/ufs/balloc.c

@@ -455,24 +455,14 @@ u64 ufs_new_fragments(struct inode *inode, void *p, u64 fragment,
 	/*
 	 * allocate new block and move data
 	 */
-	switch (fs32_to_cpu(sb, usb1->fs_optim)) {
-	    case UFS_OPTSPACE:
+	if (fs32_to_cpu(sb, usb1->fs_optim) == UFS_OPTSPACE) {
 		request = newcount;
-		if (uspi->s_minfree < 5 || uspi->cs_total.cs_nffree
-		    > uspi->s_dsize * uspi->s_minfree / (2 * 100))
-			break;
-		usb1->fs_optim = cpu_to_fs32(sb, UFS_OPTTIME);
-		break;
-	    default:
-		usb1->fs_optim = cpu_to_fs32(sb, UFS_OPTTIME);
-	
-	    case UFS_OPTTIME:
+		if (uspi->cs_total.cs_nffree < uspi->s_space_to_time)
+			usb1->fs_optim = cpu_to_fs32(sb, UFS_OPTTIME);
+	} else {
 		request = uspi->s_fpb;
-		if (uspi->cs_total.cs_nffree < uspi->s_dsize *
-		    (uspi->s_minfree - 2) / 100)
-			break;
-		usb1->fs_optim = cpu_to_fs32(sb, UFS_OPTTIME);
-		break;
+		if (uspi->cs_total.cs_nffree > uspi->s_time_to_space)
+			usb1->fs_optim = cpu_to_fs32(sb, UFS_OPTSPACE);
 	}
 	result = ufs_alloc_fragments (inode, cgno, goal, request, err);
 	if (result) {

fs/ufs/super.c

@@ -1211,6 +1211,15 @@ magic_found:
 
 	uspi->s_root_blocks = mul_u64_u32_div(uspi->s_dsize,
 					      uspi->s_minfree, 100);
+	if (uspi->s_minfree <= 5) {
+		uspi->s_time_to_space = ~0ULL;
+		uspi->s_space_to_time = 0;
+		usb1->fs_optim = cpu_to_fs32(sb, UFS_OPTSPACE);
+	} else {
+		uspi->s_time_to_space = (uspi->s_root_blocks / 2) + 1;
+		uspi->s_space_to_time = mul_u64_u32_div(uspi->s_dsize,
+					      uspi->s_minfree - 2, 100) - 1;
+	}
 
 	/*
 	 * Compute another frequently used values

fs/ufs/ufs_fs.h

@@ -792,6 +792,8 @@ struct ufs_sb_private_info {
 	__s32	fs_magic;       /* filesystem magic */
 	unsigned int s_dirblksize;
 	__u64   s_root_blocks;
+	__u64	s_time_to_space;
+	__u64	s_space_to_time;
 };
 
 /*