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Date:   Wed, 11 May 2022 13:18:53 +0530
From:   Ritesh Harjani <ritesh.list@gmail.com>
To:     Ojaswin Mujoo <ojaswin@linux.ibm.com>
Cc:     linux-ext4@vger.kernel.org, Theodore Ts'o <tytso@mit.edu>,
        Ritesh Harjani <riteshh@linux.ibm.com>,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] ext4: Fix journal_ioprio mount option handling
Message-ID: <20220511074853.4xgdzagwmkp4ejuz@riteshh-domain>
References: <20220418083545.45778-1-ojaswin@linux.ibm.com>
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In-Reply-To: <20220418083545.45778-1-ojaswin@linux.ibm.com>
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On 22/04/18 02:05PM, Ojaswin Mujoo wrote:
> In __ext4_super() we always overwrote the user specified journal_ioprio
> value with a default value, expecting  parse_apply_sb_mount_options() to
> later correctly set ctx->journal_ioprio to the user specified value.
> However, if parse_apply_sb_mount_options() returned early because of
> empty sbi->es_s->s_mount_opts, the correct journal_ioprio value was
> never set.

>
> This patch fixes __ext4_super() to only use the default value if the
					^^^ __ext4_fill_super
> user has not specified any value for journal_ioprio.

Also the problem is that ext4_parse_param() is called before
__ext4_fill_super(). Hence when we overwrite ctx->journal_ioprio to default
value in __ext4_fill_super(), that will end up ignoring the user passed
journal_ioprio value via mount opts (which was passed earlier in
ext4_parse_param()).


>
> Similarly, the remount behavior was to either use journal_ioprio
> value specified during initial mount, or use the default value
> irrespective of the journal_ioprio value specified during remount.
> This patch modifies this to first check if a new value for ioprio
> has been passed during remount and apply it. Incase, no new value is
> passed, use the value specified during initial mount.

Yup, here also ext4_parse_param() is called before __ext4_remount().
Hence we should check if the user has passed it's value in mount opts, if not,
only then we should use the task original ioprio.


I tested this patch and with the patch applied, the task ioprio can be correctly
set using "journal_ioprio" mount option.

"Mount test"
=============
qemu-> sudo perf record -e probe:* -aR mount -o journal_ioprio=1 /dev/loop2 /mnt
qemu-> ps -eaf |grep -E "jbd2|loop2"
root        3506       2  0 07:41 ?        00:00:00 [jbd2/loop2-8]
qemu-> sudo perf script
           mount  3504 [000]  2503.106871: probe:ext4_parse_param_L222: (ffffffff8147817f) journal_ioprio=16385 spec=32
           mount  3504 [000]  2503.106908: probe:__ext4_fill_super_L26: (ffffffff8147a650) journal_ioprio=16385 spec=32
qemu-> ionice -p 3506
best-effort: prio 1

"remount test"
=================
qemu-> sudo perf record -e probe:* -aR mount -o remount,journal_ioprio=0 /dev/loop2 /mnt
qemu-> sudo perf script
           mount  3519 [000]  2544.958850: probe:ext4_parse_param_L222: (ffffffff8147817f) journal_ioprio=16384 spec=32
           mount  3519 [000]  2544.958860:    probe:__ext4_remount_L49: (ffffffff81479da2) journal_ioprio=16384 spec=32
qemu-> ionice -p 3506
best-effort: prio 0

"remount with no mount options"
=================================
qemu-> sudo perf record -e probe:* -aR mount -o remount /dev/loop2 /mnt
qemu-> ionice -p 3506
best-effort: prio 0
qemu-> sudo perf script
           mount  3530 [000]  2575.964048:    probe:__ext4_remount_L49: (ffffffff81479da2) journal_ioprio=16384 spec=0


>
> Signed-off-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>

We should add fixes tag too. Can you please confirm if that would be this patch?
"ext4: Completely separate options parsing and sb setup".

With that feel free to add below -

Reviewed-by: Ritesh Harjani <riteshh@linux.ibm.com>
Tested-by: Ritesh Harjani <riteshh@linux.ibm.com>


-ritesh

> ---
>  fs/ext4/super.c | 15 ++++++++++-----
>  1 file changed, 10 insertions(+), 5 deletions(-)
>
> diff --git a/fs/ext4/super.c b/fs/ext4/super.c
> index c5a9ffbf7f4f..bfd767c51203 100644
> --- a/fs/ext4/super.c
> +++ b/fs/ext4/super.c
> @@ -4427,7 +4427,8 @@ static int __ext4_fill_super(struct fs_context *fc, struct super_block *sb)
>  	int silent = fc->sb_flags & SB_SILENT;
>
>  	/* Set defaults for the variables that will be set during parsing */
> -	ctx->journal_ioprio = DEFAULT_JOURNAL_IOPRIO;
> +	if (!(ctx->spec & EXT4_SPEC_JOURNAL_IOPRIO))
> +		ctx->journal_ioprio = DEFAULT_JOURNAL_IOPRIO;
>
>  	sbi->s_inode_readahead_blks = EXT4_DEF_INODE_READAHEAD_BLKS;
>  	sbi->s_sectors_written_start =
> @@ -6289,7 +6290,6 @@ static int __ext4_remount(struct fs_context *fc, struct super_block *sb)
>  	char *to_free[EXT4_MAXQUOTAS];
>  #endif
>
> -	ctx->journal_ioprio = DEFAULT_JOURNAL_IOPRIO;
>
>  	/* Store the original options */
>  	old_sb_flags = sb->s_flags;
> @@ -6315,9 +6315,14 @@ static int __ext4_remount(struct fs_context *fc, struct super_block *sb)
>  		} else
>  			old_opts.s_qf_names[i] = NULL;
>  #endif
> -	if (sbi->s_journal && sbi->s_journal->j_task->io_context)
> -		ctx->journal_ioprio =
> -			sbi->s_journal->j_task->io_context->ioprio;
> +	if (!(ctx->spec & EXT4_SPEC_JOURNAL_IOPRIO)) {
> +		if (sbi->s_journal && sbi->s_journal->j_task->io_context)
> +			ctx->journal_ioprio =
> +				sbi->s_journal->j_task->io_context->ioprio;
> +		else
> +			ctx->journal_ioprio = DEFAULT_JOURNAL_IOPRIO;
> +
> +	}
>
>  	ext4_apply_options(fc, sb);
>
> --
> 2.27.0
>