Re: [RFC 08/11] ext4: Don't skip prefetching BLOCK_UNINIT groups

Jan Kara
2023-03-09T14:15:27Z

On Fri 27-01-23 18:07:35, Ojaswin Mujoo wrote:
> Currently, ext4_mb_prefetch() and ext4_mb_prefetch_fini() skip
> BLOCK_UNINIT groups since fetching their bitmaps doesn't need disk IO.
> As a consequence, we end not initializing the buddy structures and CR0/1
> lists for these BGs, even though it can be done without any disk IO
> overhead. Hence, don't skip such BGs during prefetch and prefetch_fini.
>
> This improves the accuracy of CR0/1 allocation as earlier, we could have
> essentially empty BLOCK_UNINIT groups being ignored by CR0/1 due to their buddy
> not being initialized, leading to slower CR2 allocations. With this patch CR0/1
> will be able to discover these groups as well, thus improving performance.
>
> Signed-off-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
> Reviewed-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>

The patch looks good. I just somewhat wonder - this change may result in
uninitialized groups being initialized and used earlier (previously we'd
rather search in other already initialized groups) which may spread
allocations more. But I suppose that's fine and uninit groups are not
really a feature meant to limit fragmentation and as the filesystem ages
the differences should be minimal. So feel free to add:

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

Honza

> ---
> fs/ext4/mballoc.c | 8 ++------
> 1 file changed, 2 insertions(+), 6 deletions(-)
>
> diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
> index 14529d2fe65f..48726a831264 100644
> --- a/fs/ext4/mballoc.c
> +++ b/fs/ext4/mballoc.c
> @@ -2557,9 +2557,7 @@ ext4_group_t ext4_mb_prefetch(struct super_block *sb, ext4_group_t group,
> */
> if (!EXT4_MB_GRP_TEST_AND_SET_READ(grp) &&
> EXT4_MB_GRP_NEED_INIT(grp) &&
> - ext4_free_group_clusters(sb, gdp) > 0 &&
> - !(ext4_has_group_desc_csum(sb) &&
> - (gdp->bg_flags & cpu_to_le16(EXT4_BG_BLOCK_UNINIT)))) {
> + ext4_free_group_clusters(sb, gdp) > 0 ) {
> bh = ext4_read_block_bitmap_nowait(sb, group, true);
> if (bh && !IS_ERR(bh)) {
> if (!buffer_uptodate(bh) && cnt)
> @@ -2600,9 +2598,7 @@ void ext4_mb_prefetch_fini(struct super_block *sb, ext4_group_t group,
> grp = ext4_get_group_info(sb, group);
>
> if (EXT4_MB_GRP_NEED_INIT(grp) &&
> - ext4_free_group_clusters(sb, gdp) > 0 &&
> - !(ext4_has_group_desc_csum(sb) &&
> - (gdp->bg_flags & cpu_to_le16(EXT4_BG_BLOCK_UNINIT)))) {
> + ext4_free_group_clusters(sb, gdp) > 0) {
> if (ext4_mb_init_group(sb, group, GFP_NOFS))
> break;
> }
> --
> 2.31.1
>
--
Jan Kara <jack@suse.com>
SUSE Labs, CR