[PATCH] ext4: fix off by one issue in ext4_mb_choose_next_group_best_avail()

[PATCH] ext4: fix off by one issue in ext4_mb_choose_next_group_best_avail()

[Ojaswin Mujoo]

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

Fixes: 33122aa930 ("ext4: Add allocation criteria 1.5 (CR1_5)")
Signed-off-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
---
 fs/ext4/mballoc.c | 17 +++++++++--------
 1 file changed, 9 insertions(+), 8 deletions(-)

diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
index 4f2a1df98141..d890495127d8 100644
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -1007,14 +1007,11 @@ 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
@@ -1022,9 +1019,16 @@ 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;
 		/*
@@ -1038,9 +1042,6 @@ static void ext4_mb_choose_next_group_best_avail(struct ext4_allocation_context
 			/*
 			 * Try to round up the adjusted goal to stripe size
 			 * (in cluster units) multiple for efficiency.
-			 *
-			 * XXX: Is s->stripe always a power of 2? In that case
-			 * we can use the faster round_up() variant.
 			 */
 			ac->ac_g_ex.fe_len = roundup(ac->ac_g_ex.fe_len,
 						     num_stripe_clusters);
-- 
2.31.1

Re: [PATCH] ext4: fix off by one issue in ext4_mb_choose_next_group_best_avail()

[Theodore Ts'o]

On Fri, 09 Jun 2023 16:04:03 +0530, Ojaswin Mujoo wrote:
> 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:
> 
> [...]

Applied, thanks!

[1/1] ext4: fix off by one issue in ext4_mb_choose_next_group_best_avail()
      commit: 7c79210b15ef69a5702d1fb817bad9a30bffb097

Best regards,
-- 
Theodore Ts'o <tytso@mit.edu>