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

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Message-ID: <Z1/u2jIpSjAxOB6r@li-bb2b2a4c-3307-11b2-a85c-8fa5c3a69313.ibm.com>
Date: Mon, 16 Dec 2024 14:41:54 +0530
From: Ojaswin Mujoo <ojaswin@...ux.ibm.com>
To: Baokun Li <libaokun1@...wei.com>
Cc: linux-ext4@...r.kernel.org, Jan Kara <jack@...e.com>,
        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 Tue, Dec 03, 2024 at 04:29:31PM +0800, Baokun Li wrote:
> On 2024/11/27 14:01, Ojaswin Mujoo wrote:
> > On Tue, Nov 26, 2024 at 10:49:14PM +0800, Baokun Li wrote:
> > > 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().
> > Oh that's strange, shouldn't it match the complete line like:
> A rough look at the `git am` source code looks like it only focuses on
> line numbers between two ‘@@’.
> 
> am_run
>  run_apply
>   apply_all_patches
>    apply_patch
>     parse_chunk
>      find_header
>       parse_fragment_header
>     check_patch_list
>      check_patch
>       apply_data
>        load_preimage
>        apply_fragments
>         apply_one_fragment
>          find_pos
>           match_fragment
> > > > @@ -6903,6 +6916,10 @@ static int ext4_release_dquot(struct dquot *dquot)
> > That should only have one occurence around line 6903? Or does it try to
> > fuzzy match which ends up matching ext4_write_dquot etc?
> In find_pos(), start from line 6903, compare the hash value of each line
> of code line by line in forward direction, if it can't match, then match
> the hash value of each line of code line by line in reverse direction from
> line 6903. Fuzzy matching is used if some whitespace characters should be
> ignored.

Ahh okay thanks for looking into this Baokun, pretty interesting since
I've never looked at git code :)

Regards,
ojaswin
> 
> 
> Regards,
> Baokun
>