Grow: Make sure new data-offset is well-aligned

If we choose a new data-offset, make sure it is rounded to a largest
power of to possible, up to 1Meg

Signed-off-by: NeilBrown <neilb@suse.de>

diff --git a/Grow.c b/Grow.c
index afd40d302a5c8a331d9ad2348a4e9e3d702e0496..122ca0ec16d10c6a08b47962c109e4fb7c6a4493 100644 (file)
--- a/Grow.c
+++ b/Grow.c
@@ -2146,6 +2146,47 @@ static int verify_reshape_position(struct mdinfo *info, int level)
        return ret_val;
 }
 
+static unsigned long long choose_offset(unsigned long long lo,
+                                       unsigned long long hi,
+                                       unsigned long long min,
+                                       unsigned long long max)
+{
+       /* Choose a new offset between hi and lo.
+        * It must be between min and max, but
+        * we would prefer something near the middle of hi/lo, and also
+        * prefer to be aligned to a big power of 2.
+        *
+        * So we start with the middle, then for each bit,
+        * starting at '1' and increasing, if it is set, we either
+        * add it or subtract it if possible, preferring the option
+        * which is furthest from the boundary.
+        *
+        * We stop once we get a 1MB alignment. As units are in sectors,
+        * 1MB = 2*1024 sectors.
+        */
+       unsigned long long choice = (lo + hi) / 2;
+       unsigned long long bit = 1;
+
+       for (bit = 1; bit < 2*1024; bit = bit << 1) {
+               unsigned long long bigger, smaller;
+               if (! (bit & choice))
+                       continue;
+               bigger = choice + bit;
+               smaller = choice - bit;
+               if (bigger > max && smaller < min)
+                       break;
+               if (bigger > max)
+                       choice = smaller;
+               else if (smaller < min)
+                       choice = bigger;
+               else if (hi - bigger > smaller - lo)
+                       choice = bigger;
+               else
+                       choice = smaller;
+       }
+       return choice;
+}
+
 static int set_new_data_offset(struct mdinfo *sra, struct supertype *st,
                               char *devname, int delta_disks,
                               unsigned long long data_offset,
@@ -2285,14 +2326,11 @@ static int set_new_data_offset(struct mdinfo *sra, struct supertype *st,
                                }
                                if (data_offset != INVALID_SECTORS)
                                        new_data_offset = data_offset;
-                               else {
-                                       unsigned long long off = after / 2;
-                                       off &= ~7ULL;
-                                       if (off < min)
-                                               off = min;
-                                       new_data_offset =
-                                               sd->data_offset + off;
-                               }
+                               else
+                                       new_data_offset = choose_offset(sd->data_offset,
+                                                                       sd->data_offset + after,
+                                                                       sd->data_offset + min,
+                                                                       sd->data_offset + after);
                        } else {
                                /* Decrease data offset */
                                if (before < min) {
@@ -2308,14 +2346,11 @@ static int set_new_data_offset(struct mdinfo *sra, struct supertype *st,
                                }
                                if (data_offset != INVALID_SECTORS)
                                        new_data_offset = data_offset;
-                               else {
-                                       unsigned long long off = before / 2;
-                                       off &= ~7ULL;
-                                       if (off < min)
-                                               off = min;
-                                       new_data_offset =
-                                               sd->data_offset - off;
-                               }
+                               else
+                                       new_data_offset = choose_offset(sd->data_offset - before,
+                                                                       sd->data_offset,
+                                                                       sd->data_offset - before,
+                                                                       sd->data_offset - min);
                        }
                }
                if (sysfs_set_num(sra, sd, "new_offset",