linux-kernel - Re: [RFC 09/11] ext4: Ensure ext4_mb_prefetch

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Message-ID: <20230309142307.zkjnspjyc6ymqgeh@quack3>
Date:   Thu, 9 Mar 2023 15:23:07 +0100
From:   Jan Kara <jack@...e.cz>
To:     Ojaswin Mujoo <ojaswin@...ux.ibm.com>
Cc:     linux-ext4@...r.kernel.org, Theodore Ts'o <tytso@....edu>,
        Ritesh Harjani <riteshh@...ux.ibm.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        Jan Kara <jack@...e.cz>, Ritesh Harjani <ritesh.list@...il.com>
Subject: Re: [RFC 09/11] ext4: Ensure ext4_mb_prefetch_fini() is called for
 all prefetched BGs

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@...ux.ibm.com>
> Reviewed-by: Ritesh Harjani (IBM) <ritesh.list@...il.com>

Looks good to me. Feel free to add:

Reviewed-by: Jan Kara <jack@...e.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@...e.com>
SUSE Labs, CR