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

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Message-ID: <Z0a1x7yksOE4Jsha@li-bb2b2a4c-3307-11b2-a85c-8fa5c3a69313.ibm.com>
Date: Wed, 27 Nov 2024 11:31:43 +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, 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:

> > @@ -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?

> 
> 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>

Thanks Baokun!

Regards,
ojaswin
>