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Date:   Wed, 5 May 2021 14:27:11 -0700
From:   "Darrick J. Wong" <djwong@kernel.org>
To:     Leah Rumancik <leah.rumancik@gmail.com>
Cc:     linux-ext4@vger.kernel.org, tytso@mit.edu
Subject: Re: [PATCH v3 2/3] ext4: add ioctl EXT4_IOC_CHECKPOINT
Message-ID: <20210505212711.GA8532@magnolia>
References: <20210504163550.1486337-1-leah.rumancik@gmail.com>
 <20210504163550.1486337-2-leah.rumancik@gmail.com>
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On Tue, May 04, 2021 at 04:35:49PM +0000, Leah Rumancik wrote:
> ioctl EXT4_IOC_CHECKPOINT checkpoints and flushes the journal. This
> includes forcing all the transactions to the log, checkpointing the
> transactions, and flushing the log to disk. This ioctl takes u64 "flags"
> as an argument. With the EXT4_IOC_CHECKPOINT_FLAG_DISCARD flag set, the
> journal blocks are also discarded.
> 
> Systems that wish to achieve content deletion SLO can set up a daemon
> that calls this ioctl at a regular interval such that it matches with the
> SLO requirement. Thus, with this patch, the ext4_dir_entry2 wipeout
> patch[1], and the Ext4 "-o discard" mount option set, Ext4 can now
> guarantee that all data will be erased

Er... what specifically does "data" mean?  File data, or just the dirent
blocks?

I think this is only true if discard_zeroes_data == 1, right?  The last
I looked, ext4 was calling REQ_OP_DISCARD, not REQ_OP_WRITE_ZEROES.

Also, there are some SSDs that "implement" discard as nop, which means
that the old contents can still be read by re-reading the LBAs.  What
about those?

(Also wondering if this is where FS_SECRM_FL files should get their
freed file blocks erased with REQ_OP_SECURE_ERASE...)

Like Dave says, the commit message needs to be a lot more precise about
what data are being targeted, and what the user can expect afterwards.

Something like (setting aside my questions about discard for a moment):

"...and with the ext4 '-o discard' mount option set, ext4 can now
guarantee that all file contents, file metadata, and directory names
will not be accessible through the filesystem or raw block device reads
after a file deletion."

> and discarded on deletion.  Note
> that this ioctl won't write zeros if the device doesn't support discards.

AFAICT the patch doesn't call blkdev_issue_zeroout, so this statement is
always true.

> The __jbd2_journal_issue_discard function could also be used to discard the
> journal (if discard is supported) during journal load after recovery. This
> would provide a potential solution to a journal replay bug reported earlier
> this year[2] for block devices that support discard. After a successful
> journal recovery, e2fsck can call this ioctl to discard the journal as
> well.
> 
> [1] https://lore.kernel.org/linux-ext4/YIHknqxngB1sUdie@mit.edu/
> [2] https://lore.kernel.org/linux-ext4/YDZoaacIYStFQT8g@mit.edu/
> 
> Signed-off-by: Leah Rumancik <leah.rumancik@gmail.com>
> ---
>  fs/ext4/ext4.h    |  4 +++
>  fs/ext4/ioctl.c   | 38 +++++++++++++++++++++++
>  fs/jbd2/journal.c | 79 +++++++++++++++++++++++++++++++++++++++++++++++
>  3 files changed, 121 insertions(+)
> 
> diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
> index 18f021c988a1..2fe8565706fc 100644
> --- a/fs/ext4/ext4.h
> +++ b/fs/ext4/ext4.h
> @@ -715,6 +715,7 @@ enum {
>  #define EXT4_IOC_CLEAR_ES_CACHE		_IO('f', 40)
>  #define EXT4_IOC_GETSTATE		_IOW('f', 41, __u32)
>  #define EXT4_IOC_GET_ES_CACHE		_IOWR('f', 42, struct fiemap)
> +#define EXT4_IOC_CHECKPOINT		_IOW('f', 43, __u64)
>  
>  #define EXT4_IOC_SHUTDOWN _IOR ('X', 125, __u32)
>  
> @@ -736,6 +737,9 @@ enum {
>  #define EXT4_STATE_FLAG_NEWENTRY	0x00000004
>  #define EXT4_STATE_FLAG_DA_ALLOC_CLOSE	0x00000008
>  
> +/* flag to enable discarding journal blocks through ioctl EXT4_IOC_CHECKPOINT */
> +#define EXT4_IOC_CHECKPOINT_FLAG_DISCARD	1
> +
>  #if defined(__KERNEL__) && defined(CONFIG_COMPAT)
>  /*
>   * ioctl commands in 32 bit emulation
> diff --git a/fs/ext4/ioctl.c b/fs/ext4/ioctl.c
> index ef809feb7e77..839ffd067357 100644
> --- a/fs/ext4/ioctl.c
> +++ b/fs/ext4/ioctl.c
> @@ -794,6 +794,40 @@ static int ext4_ioctl_get_es_cache(struct file *filp, unsigned long arg)
>  	return error;
>  }
>  
> +static int ext4_ioctl_checkpoint(struct file *filp, unsigned long arg)
> +{
> +	int err = 0;
> +	unsigned long long flags = 0;
> +	struct super_block *sb = file_inode(filp)->i_sb;
> +
> +	if (!capable(CAP_SYS_ADMIN))
> +		return -EPERM;
> +
> +	/* file argument is not the mount point */
> +	if (file_dentry(filp) != sb->s_root)
> +		return -EINVAL;
> +
> +	/* filesystem is not backed by block device */
> +	if (sb->s_bdev == NULL)
> +		return -EINVAL;
> +
> +	if (copy_from_user(&flags, (__u64 __user *)arg,
> +				sizeof(__u64)))
> +		return -EFAULT;
> +
> +	/* flags can only be 0 or EXT4_IOC_CHECKPOINT_FLAG_DISCARD */
> +	if (flags & ~EXT4_IOC_CHECKPOINT_FLAG_DISCARD)
> +		return -EINVAL;
> +
> +	if (EXT4_SB(sb)->s_journal) {
> +		jbd2_journal_lock_updates(EXT4_SB(sb)->s_journal);
> +		err = jbd2_journal_flush(EXT4_SB(sb)->s_journal,
> +			flags & EXT4_IOC_CHECKPOINT_FLAG_DISCARD);
> +		jbd2_journal_unlock_updates(EXT4_SB(sb)->s_journal);
> +	}
> +	return err;
> +}
> +
>  static long __ext4_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
>  {
>  	struct inode *inode = file_inode(filp);
> @@ -1205,6 +1239,9 @@ static long __ext4_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
>  		return fsverity_ioctl_read_metadata(filp,
>  						    (const void __user *)arg);
>  
> +	case EXT4_IOC_CHECKPOINT:
> +		return ext4_ioctl_checkpoint(filp, arg);
> +
>  	default:
>  		return -ENOTTY;
>  	}
> @@ -1285,6 +1322,7 @@ long ext4_compat_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
>  	case EXT4_IOC_CLEAR_ES_CACHE:
>  	case EXT4_IOC_GETSTATE:
>  	case EXT4_IOC_GET_ES_CACHE:
> +	case EXT4_IOC_CHECKPOINT:
>  		break;
>  	default:
>  		return -ENOIOCTLCMD;
> diff --git a/fs/jbd2/journal.c b/fs/jbd2/journal.c
> index 4b7953934c82..ce33e4817aab 100644
> --- a/fs/jbd2/journal.c
> +++ b/fs/jbd2/journal.c
> @@ -1686,6 +1686,80 @@ static void jbd2_mark_journal_empty(journal_t *journal, int write_op)
>  	write_unlock(&journal->j_state_lock);
>  }
>  
> +/* discard journal blocks excluding journal superblock */
> +static int __jbd2_journal_issue_discard(journal_t *journal)
> +{
> +	int err = 0;
> +	unsigned long block, log_offset; /* logical */
> +	unsigned long long phys_block, block_start, block_stop; /* physical */
> +	loff_t byte_start, byte_stop, byte_count;
> +	struct request_queue *q = bdev_get_queue(journal->j_dev);
> +
> +	if (!q)
> +		return -ENXIO;
> +
> +	if (!blk_queue_discard(q))
> +		return -EOPNOTSUPP;
> +
> +	/* lookup block mapping and issue discard for each contiguous region */
> +	log_offset = be32_to_cpu(journal->j_superblock->s_first);
> +
> +	err = jbd2_journal_bmap(journal, log_offset, &block_start);
> +	if (err) {
> +		printk(KERN_ERR "JBD2: bad block at offset %lu", log_offset);
> +		return err;
> +	}
> +
> +	/*
> +	 * use block_start - 1 to meet check for contiguous with previous region:
> +	 * phys_block == block_stop + 1
> +	 */
> +	block_stop = block_start - 1;
> +
> +	for (block = log_offset; block < journal->j_total_len; block++) {
> +		err = jbd2_journal_bmap(journal, block, &phys_block);
> +		if (err) {
> +			printk(KERN_ERR "JBD2: bad block at offset %lu", block);
> +			return err;
> +		}
> +
> +		if (block == journal->j_total_len - 1)
> +			block_stop = phys_block;
> +		else if (phys_block == block_stop + 1) {
> +			block_stop++;
> +			continue;
> +		}
> +
> +		/*
> +		 * not contiguous with prior physical block or this is last
> +		 * block of journal, take care of the region
> +		 */
> +		byte_start = block_start * journal->j_blocksize;
> +		byte_stop = block_stop * journal->j_blocksize;
> +		byte_count = (block_stop - block_start + 1) *
> +			journal->j_blocksize;
> +
> +		truncate_inode_pages_range(journal->j_dev->bd_inode->i_mapping,
> +			byte_start, byte_stop);
> +
> +		err = blkdev_issue_discard(journal->j_dev,
> +			byte_start >> SECTOR_SHIFT,
> +			byte_count >> SECTOR_SHIFT,
> +			GFP_NOFS, 0);

Dumb style nit: I think kernel style rules say to indent second lines
more than one tab.

(Dumb in the sense of "ha look at the xfs code!" :P)

> +
> +		if (unlikely(err != 0)) {
> +			printk(KERN_ERR "JBD2: unable to discard "
> +				"journal at physical blocks %llu - %llu",
> +				block_start, block_stop);
> +			return err;
> +		}
> +
> +		block_start = phys_block;
> +		block_stop = phys_block;
> +	}
> +
> +	return blkdev_issue_flush(journal->j_dev);
> +}
>  
>  /**
>   * jbd2_journal_update_sb_errno() - Update error in the journal.
> @@ -2246,6 +2320,7 @@ EXPORT_SYMBOL(jbd2_journal_clear_features);
>  /**
>   * jbd2_journal_flush() - Flush journal
>   * @journal: Journal to act on.
> + * @discard: discard the journal blocks
>   *
>   * Flush all data for a given journal to disk and empty the journal.
>   * Filesystems can use this when remounting readonly to ensure that
> @@ -2305,6 +2380,10 @@ int jbd2_journal_flush(journal_t *journal, bool discard)
>  	 * commits of data to the journal will restore the current
>  	 * s_start value. */
>  	jbd2_mark_journal_empty(journal, REQ_SYNC | REQ_FUA);
> +
> +	if (discard)
> +		err = __jbd2_journal_issue_discard(journal);
> +
>  	mutex_unlock(&journal->j_checkpoint_mutex);
>  	write_lock(&journal->j_state_lock);
>  	J_ASSERT(!journal->j_running_transaction);
> -- 
> 2.31.1.527.g47e6f16901-goog
>