Re: [PATCH v2 09/12] ext4: Ensure ext4_mb_prefetch_fini() is called for all prefetched BGs

From: Guoqing Jiang <guoqing.jiang@linux.dev>
To: Ojaswin Mujoo <ojaswin@linux.ibm.com>,
	linux-ext4@vger.kernel.org, Theodore Ts'o <tytso@mit.edu>
Cc: Ritesh Harjani <riteshh@linux.ibm.com>,
	linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
	Jan Kara <jack@suse.cz>, Kemeng Shi <shikemeng@huaweicloud.com>,
	Ritesh Harjani <ritesh.list@gmail.com>
Subject: Re: [PATCH v2 09/12] ext4: Ensure ext4_mb_prefetch_fini() is called for all prefetched BGs
Date: Tue, 6 Jun 2023 22:00:57 +0800	[thread overview]
Message-ID: <c3173405-713d-d2eb-bd9c-af8b8c747533@linux.dev> (raw)
In-Reply-To: <05e648ae04ec5b754207032823e9c1de9a54f87a.1685449706.git.ojaswin@linux.ibm.com>

Hello,

On 5/30/23 20:33, 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.

Thanks for the series.

> 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.

Seems ext4_mb_prefetch_fini can't be called by ext4_mb_regular_allocator
if nr is 0.

https://elixir.bootlin.com/linux/v6.4-rc5/source/fs/ext4/mballoc.c#L2816

Am I miss something?

Thanks,
Guoqing

> Signed-off-by: Ojaswin Mujoo<ojaswin@linux.ibm.com>
> Reviewed-by: Ritesh Harjani (IBM)<ritesh.list@gmail.com>
> Reviewed-by: Jan Kara<jack@suse.cz>
> ---
>   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 79455c7e645b..6775d73dfc68 100644
> --- a/fs/ext4/mballoc.c
> +++ b/fs/ext4/mballoc.c
> @@ -2735,8 +2735,6 @@ 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;
> @@ -2745,8 +2743,6 @@ 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 2da5476fa48b..27c1dabacd43 100644
> --- a/fs/ext4/super.c
> +++ b/fs/ext4/super.c
> @@ -3692,16 +3692,13 @@ 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 &&