Re: [PATCH v2 09/16] ext4: fix zombie groups in average fragment size lists

Jan Kara
2025-06-27T19:14:33Z

On Mon 23-06-25 15:32:57, Baokun Li wrote:
> Groups with no free blocks shouldn't be in any average fragment size list.
> However, when all blocks in a group are allocated(i.e., bb_fragments or
> bb_free is 0), we currently skip updating the average fragment size, which
> means the group isn't removed from its previous s_mb_avg_fragment_size[old]
> list.
>
> This created "zombie" groups that were always skipped during traversal as
> they couldn't satisfy any block allocation requests, negatively impacting
> traversal efficiency.
>
> Therefore, when a group becomes completely free, bb_avg_fragment_size_order
^^^ full

> is now set to -1. If the old order was not -1, a removal operation is
> performed; if the new order is not -1, an insertion is performed.
>
> Fixes: 196e402adf2e ("ext4: improve cr 0 / cr 1 group scanning")
> CC: stable@vger.kernel.org
> Signed-off-by: Baokun Li <libaokun1@huawei.com>

Good catch! The patch looks good. Feel free to add:

Reviewed-by: Jan Kara <jack@suse.cz>

Honza

> ---
> fs/ext4/mballoc.c | 36 ++++++++++++++++++------------------
> 1 file changed, 18 insertions(+), 18 deletions(-)
>
> diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
> index 94950b07a577..e6d6c2da3c6e 100644
> --- a/fs/ext4/mballoc.c
> +++ b/fs/ext4/mballoc.c
> @@ -841,30 +841,30 @@ static void
> mb_update_avg_fragment_size(struct super_block *sb, struct ext4_group_info *grp)
> {
> struct ext4_sb_info *sbi = EXT4_SB(sb);
> - int new_order;
> + int new, old;
>
> - if (!test_opt2(sb, MB_OPTIMIZE_SCAN) || grp->bb_fragments == 0)
> + if (!test_opt2(sb, MB_OPTIMIZE_SCAN))
> return;
>
> - new_order = mb_avg_fragment_size_order(sb,
> - grp->bb_free / grp->bb_fragments);
> - if (new_order == grp->bb_avg_fragment_size_order)
> + old = grp->bb_avg_fragment_size_order;
> + new = grp->bb_fragments == 0 ? -1 :
> + mb_avg_fragment_size_order(sb, grp->bb_free / grp->bb_fragments);
> + if (new == old)
> return;
>
> - if (grp->bb_avg_fragment_size_order != -1) {
> - write_lock(&sbi->s_mb_avg_fragment_size_locks[
> - grp->bb_avg_fragment_size_order]);
> + if (old >= 0) {
> + write_lock(&sbi->s_mb_avg_fragment_size_locks[old]);
> list_del(&grp->bb_avg_fragment_size_node);
> - write_unlock(&sbi->s_mb_avg_fragment_size_locks[
> - grp->bb_avg_fragment_size_order]);
> - }
> - grp->bb_avg_fragment_size_order = new_order;
> - write_lock(&sbi->s_mb_avg_fragment_size_locks[
> - grp->bb_avg_fragment_size_order]);
> - list_add_tail(&grp->bb_avg_fragment_size_node,
> - &sbi->s_mb_avg_fragment_size[grp->bb_avg_fragment_size_order]);
> - write_unlock(&sbi->s_mb_avg_fragment_size_locks[
> - grp->bb_avg_fragment_size_order]);
> + write_unlock(&sbi->s_mb_avg_fragment_size_locks[old]);
> + }
> +
> + grp->bb_avg_fragment_size_order = new;
> + if (new >= 0) {
> + write_lock(&sbi->s_mb_avg_fragment_size_locks[new]);
> + list_add_tail(&grp->bb_avg_fragment_size_node,
> + &sbi->s_mb_avg_fragment_size[new]);
> + write_unlock(&sbi->s_mb_avg_fragment_size_locks[new]);
> + }
> }
>
> /*
> --
> 2.46.1
>
--
Jan Kara <jack@suse.com>
SUSE Labs, CR