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>

3 files changed, 17 insertions, 16 deletions

diff --git a/fs/ufs/balloc.c b/fs/ufs/balloc.c
index 0315fea1d589..f80be4c5df9d 100644
--- a/fs/ufs/balloc.c
+++ b/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) {
diff --git a/fs/ufs/super.c b/fs/ufs/super.c
index 34656c7a8e22..f211b662dd92 100644
--- a/fs/ufs/super.c
+++ b/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
diff --git a/fs/ufs/ufs_fs.h b/fs/ufs/ufs_fs.h
index 823d55a37586..150eef6f1233 100644
--- a/fs/ufs/ufs_fs.h
+++ b/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;
 };
 
 /*