ext4: fix off by one issue in ext4_mb_choose_next_group_best_avail()

In ext4_mb_choose_next_group_best_avail(), we want the start order to be
1 less than goal length and the min_order to be, at max, 1 more than the
original length. This commit fixes an off by one issue that arose due to
the fact that 1 << fls(n) > (n).

After all the processing:

order = 1 order below goal len
min_order = maximum of the three:-
             - order - trim_order
             - 1 order below B2C(s_stripe)
             - 1 order above original len

Cc: stable@kernel.org
Fixes: 33122aa930 ("ext4: Add allocation criteria 1.5 (CR1_5)")
Signed-off-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
Link: https://lore.kernel.org/r/20230609103403.112807-1-ojaswin@linux.ibm.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
index a2475b8..456150e 100644 (file)
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -1006,14 +1006,11 @@ static void ext4_mb_choose_next_group_best_avail(struct ext4_allocation_context
         * fls() instead since we need to know the actual length while modifying
         * goal length.
         */
-       order = fls(ac->ac_g_ex.fe_len);
+       order = fls(ac->ac_g_ex.fe_len) - 1;
        min_order = order - sbi->s_mb_best_avail_max_trim_order;
        if (min_order < 0)
                min_order = 0;
 
-       if (1 << min_order < ac->ac_o_ex.fe_len)
-               min_order = fls(ac->ac_o_ex.fe_len) + 1;
-
        if (sbi->s_stripe > 0) {
                /*
                 * We are assuming that stripe size is always a multiple of
@@ -1021,9 +1018,16 @@ static void ext4_mb_choose_next_group_best_avail(struct ext4_allocation_context
                 */
                num_stripe_clusters = EXT4_NUM_B2C(sbi, sbi->s_stripe);
                if (1 << min_order < num_stripe_clusters)
-                       min_order = fls(num_stripe_clusters);
+                       /*
+                        * We consider 1 order less because later we round
+                        * up the goal len to num_stripe_clusters
+                        */
+                       min_order = fls(num_stripe_clusters) - 1;
        }
 
+       if (1 << min_order < ac->ac_o_ex.fe_len)
+               min_order = fls(ac->ac_o_ex.fe_len);
+
        for (i = order; i >= min_order; i--) {
                int frag_order;
                /*