linux-ext4 - Re: [PATCH v6 03/20] ext4, jbd2: add fast commit initialization routines

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Message-Id: <C29FFBAA-43A0-4307-B0A3-ED91A572308F@dilger.ca>
Date:   Fri, 10 Apr 2020 06:12:09 -0600
From:   Andreas Dilger <adilger@...ger.ca>
To:     Harshad Shirwadkar <harshadshirwadkar@...il.com>
Cc:     linux-ext4@...r.kernel.org, tytso@....edu
Subject: Re: [PATCH v6 03/20] ext4, jbd2: add fast commit initialization
 routines

On Apr 8, 2020, at 3:55 PM, Harshad Shirwadkar <harshadshirwadkar@...il.com> wrote:
> 
> From: Harshad Shirwadkar <harshadshirwadkar@...il.com>
> 
> Define feature flags for fast commits and add routines to allow ext4 to
> initialize fast commits. Note that we allow 128 blocks to be used for
> fast commits. As of now, that's the default constant value.
> 
> Signed-off-by: Harshad Shirwadkar <harshadshirwadkar@...il.com>

> +static inline int ext4_should_fast_commit(struct super_block *sb)
> +{
> +	if (!ext4_has_feature_fast_commit(sb))
> +		return 0;
> +	if (!test_opt2(sb, JOURNAL_FAST_COMMIT))
> +		return 0;
> +	if (test_opt(sb, QUOTA))
> +		return 0;
> +	return 1;
> +}

This function seems more complex than it should be.  In this patch the
ext4_should_fast_commit() function is only called once during mount, but
in later patches it looks like it is called many times per file/inode.

Why not just check JOURNAL_FAST_COMMIT, and clear it at mount/remount
time if the other conditions prevent fast commits being used at all?
It seems that JOURNAL_FAST_COMMIT is only set if the FAST_COMMIT feature
is already in the superblock, so always doing both checks seems redundant.

Also, maybe I missed the discussion, but why does having quotas enabled
on the filesystem disable fast commits entirely?  I see in patch 11/20
that EXT4_FC_REASON_QUOTA is a reason not to do fast commit on the quota
inodes themselves, which seems like a reasonable limitation if needed,
but the above check disables FC for any filesystem with quota, and I
can't find anywhere that this line is later removed in this series.

Cheers, Andreas

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