Re: [RFC 09/11] ext4: Ensure ext4_mb_prefetch_fini() is called for all prefetched BGs

Jan Kara
2023-03-09T14:23:31Z

On Fri 27-01-23 18:07:36, Ojaswin Mujoo wrote:
> Before this patch, the call stack in ext4_run_li_request is as follows:
>
> /*
> * nr = no. of BGs we want to fetch (=s_mb_prefetch)
> * prefetch_ios = no. of BGs not uptodate after
> * ext4_read_block_bitmap_nowait()
> */
> next_group = ext4_mb_prefetch(sb, group, nr, prefetch_ios);
> ext4_mb_prefetch_fini(sb, next_group prefetch_ios);
>
> ext4_mb_prefetch_fini() will only try to initialize buddies for BGs in
> range [next_group - prefetch_ios, next_group). This is incorrect since
> sometimes (prefetch_ios < nr), which causes ext4_mb_prefetch_fini() to
> incorrectly ignore some of the BGs that might need initialization. This
> issue is more notable now with the previous patch enabling "fetching" of
> BLOCK_UNINIT BGs which are marked buffer_uptodate by default.
>
> Fix this by passing nr to ext4_mb_prefetch_fini() instead of
> prefetch_ios so that it considers the right range of groups.
>
> Similarly, make sure we don't pass nr=0 to ext4_mb_prefetch_fini() in
> ext4_mb_regular_allocator() since we might have prefetched BLOCK_UNINIT
> groups that would need buddy initialization.
>
> Signed-off-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
> Reviewed-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>

Looks good to me. Feel free to add:

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

Honza

> ---
> fs/ext4/mballoc.c | 4 ----
> fs/ext4/super.c | 11 ++++-------
> 2 files changed, 4 insertions(+), 11 deletions(-)
>
> diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
> index 48726a831264..410c9636907b 100644
> --- a/fs/ext4/mballoc.c
> +++ b/fs/ext4/mballoc.c
> @@ -2702,8 +2702,6 @@ ext4_mb_regular_allocator(struct ext4_allocation_context *ac)
> if ((prefetch_grp == group) &&
> (cr > CR1 ||
> prefetch_ios < sbi->s_mb_prefetch_limit)) {
> - unsigned int curr_ios = prefetch_ios;
> -
> nr = sbi->s_mb_prefetch;
> if (ext4_has_feature_flex_bg(sb)) {
> nr = 1 << sbi->s_log_groups_per_flex;
> @@ -2712,8 +2710,6 @@ ext4_mb_regular_allocator(struct ext4_allocation_context *ac)
> }
> prefetch_grp = ext4_mb_prefetch(sb, group,
> nr, &prefetch_ios);
> - if (prefetch_ios == curr_ios)
> - nr = 0;
> }
>
> /* This now checks without needing the buddy page */
> diff --git a/fs/ext4/super.c b/fs/ext4/super.c
> index 72ead3b56706..9dbb09cfc8f7 100644
> --- a/fs/ext4/super.c
> +++ b/fs/ext4/super.c
> @@ -3636,16 +3636,13 @@ static int ext4_run_li_request(struct ext4_li_request *elr)
> ext4_group_t group = elr->lr_next_group;
> unsigned int prefetch_ios = 0;
> int ret = 0;
> + int nr = EXT4_SB(sb)->s_mb_prefetch;
> u64 start_time;
>
> if (elr->lr_mode == EXT4_LI_MODE_PREFETCH_BBITMAP) {
> - elr->lr_next_group = ext4_mb_prefetch(sb, group,
> - EXT4_SB(sb)->s_mb_prefetch, &prefetch_ios);
> - if (prefetch_ios)
> - ext4_mb_prefetch_fini(sb, elr->lr_next_group,
> - prefetch_ios);
> - trace_ext4_prefetch_bitmaps(sb, group, elr->lr_next_group,
> - prefetch_ios);
> + elr->lr_next_group = ext4_mb_prefetch(sb, group, nr, &prefetch_ios);
> + ext4_mb_prefetch_fini(sb, elr->lr_next_group, nr);
> + trace_ext4_prefetch_bitmaps(sb, group, elr->lr_next_group, nr);
> if (group >= elr->lr_next_group) {
> ret = 1;
> if (elr->lr_first_not_zeroed != ngroups &&
> --
> 2.31.1
>
--
Jan Kara <jack@suse.com>
SUSE Labs, CR