linux-kernel - Re: [PATCH v2 2/2] ext4: protect ext4_release

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Message-ID: <cc2fcc33-9024-4ce8-bd52-cdcd23f6b455@huawei.com>
Date: Tue, 26 Nov 2024 22:49:14 +0800
From: Baokun Li <libaokun1@...wei.com>
To: Ojaswin Mujoo <ojaswin@...ux.ibm.com>, <linux-ext4@...r.kernel.org>, Jan
 Kara <jack@...e.com>
CC: Ritesh Harjani <ritesh.list@...il.com>, <linux-kernel@...r.kernel.org>,
	<linux-fsdevel@...r.kernel.org>, Jan Kara <jack@...e.cz>, Yang Erkun
	<yangerkun@...wei.com>
Subject: Re: [PATCH v2 2/2] ext4: protect ext4_release_dquot against freezing

On 2024/11/21 20:38, Ojaswin Mujoo wrote:
> Protect ext4_release_dquot against freezing so that we
> don't try to start a transaction when FS is frozen, leading
> to warnings.
>
> Further, avoid taking the freeze protection if a transaction
> is already running so that we don't need end up in a deadlock
> as described in
>
>    46e294efc355 ext4: fix deadlock with fs freezing and EA inodes
>
> Suggested-by: Jan Kara <jack@...e.cz>
> Signed-off-by: Ojaswin Mujoo <ojaswin@...ux.ibm.com>
> ---
>   fs/ext4/super.c | 17 +++++++++++++++++
>   1 file changed, 17 insertions(+)
>
> diff --git a/fs/ext4/super.c b/fs/ext4/super.c
> index 16a4ce704460..f7437a592359 100644
> --- a/fs/ext4/super.c
> +++ b/fs/ext4/super.c
> @@ -6887,12 +6887,25 @@ static int ext4_release_dquot(struct dquot *dquot)
>   {
>   	int ret, err;
>   	handle_t *handle;
> +	bool freeze_protected = false;
> +
> +	/*
> +	 * Trying to sb_start_intwrite() in a running transaction
> +	 * can result in a deadlock. Further, running transactions
> +	 * are already protected from freezing.
> +	 */
> +	if (!ext4_journal_current_handle()) {
> +		sb_start_intwrite(dquot->dq_sb);
> +		freeze_protected = true;
> +	}
>   
>   	handle = ext4_journal_start(dquot_to_inode(dquot), EXT4_HT_QUOTA,
>   				    EXT4_QUOTA_DEL_BLOCKS(dquot->dq_sb));
>   	if (IS_ERR(handle)) {
>   		/* Release dquot anyway to avoid endless cycle in dqput() */
>   		dquot_release(dquot);
> +		if (freeze_protected)
> +			sb_end_intwrite(dquot->dq_sb);
>   		return PTR_ERR(handle);
>   	}
>   	ret = dquot_release(dquot);
> @@ -6903,6 +6916,10 @@ static int ext4_release_dquot(struct dquot *dquot)
The `git am` command looks for the following context code from line 6903
to apply the changes. But there are many functions in fs/ext4/super.c that
have similar code, such as ext4_write_dquot() and ext4_acquire_dquot().

So when the code before ext4_release_dquot() is added, the first matching
context found could be in ext4_write_dquot() or ext4_acquire_dquot().
>   	err = ext4_journal_stop(handle);
>   	if (!ret)
>   		ret = err;
> +
> +	if (freeze_protected)
> +		sb_end_intwrite(dquot->dq_sb);
> +
>   	return ret;
>   }
>   

Thus this is actually a bug in `git am`, which can be avoided by increasing
the number of context lines with `git format-patch -U8 -1`.

Otherwise it looks good. Feel free to add:

Reviewed-by: Baokun Li <libaokun1@...wei.com>