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Date:   Thu, 13 May 2021 14:09:26 -0400
From:   "Theodore Ts'o" <tytso@mit.edu>
To:     Leah Rumancik <leah.rumancik@gmail.com>
Cc:     linux-ext4@vger.kernel.org
Subject: Re: [PATCH v4 1/3] ext4: add discard/zeroout flags to journal flush
Message-ID: <YJ1rVr7Sf7Az+MQm@mit.edu>
References: <20210511180428.3358267-1-leah.rumancik@gmail.com>
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In-Reply-To: <20210511180428.3358267-1-leah.rumancik@gmail.com>
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On Tue, May 11, 2021 at 06:04:26PM +0000, Leah Rumancik wrote:
> @@ -3223,7 +3223,7 @@ static sector_t ext4_bmap(struct address_space *mapping, sector_t block)
>  		ext4_clear_inode_state(inode, EXT4_STATE_JDATA);
>  		journal = EXT4_JOURNAL(inode);
>  		jbd2_journal_lock_updates(journal);
> -		err = jbd2_journal_flush(journal);
> +		err = jbd2_journal_flush(journal, 0);

In the ocfs2 changes, I noticed you are using "false", instead of 0,
in the second argument to jbd2_journal_flush.

When I looked more closely, the function signature of
jbd2_journal_flush is also using an unsigned long long for flags,
which struck me as strange:

> +extern int	 jbd2_journal_flush(journal_t *journal, unsigned long long flags);

I then noticed that later in the patch series, the ioctl argument is
taking an unsigned long long and we're passing that straight through
to jbd2_journal_flush().

First of all, unsigned long long is not very efficient on many
platforms (especially 32-bit platforms), but also on platforms where
int is 32 bits.  If we don't expect us to need more than 32 flag bits,
I'd suggest explicit ly using __u32 in ioctl interface.  (__u32 is
fine; it's the use of the base int type which can get us into trouble,
since int can be either 32 or 64 bits depending on the architecture).

Secondly, I'd suggest using a different set of flags for
jbd2_journal_flush(), which is an internal kernel interface, and the
EXT4_IOC_CHECKPOINT interface.  We might in the future want to add
some internal flags to jbd2_journal_flush that we do *not* want to
expose via EXT4_IOC_CHECKPOINT, and so it's best that we keep those
two interfaces separate.

> diff --git a/fs/jbd2/journal.c b/fs/jbd2/journal.c
> index 2dc944442802..f86929dbca3c 100644
> --- a/fs/jbd2/journal.c
> +++ b/fs/jbd2/journal.c
> @@ -1686,6 +1686,106 @@ static void jbd2_mark_journal_empty(journal_t *journal, int write_op)
>  	write_unlock(&journal->j_state_lock);
>  }
>  
> +#define JBD2_ERASE_FLAG_DISCARD	1
> +#define JBD2_ERASE_FLAG_ZEROOUT	2

I'd suggest defining these in include/linux/jbd2.h, and giving them
names like: JBD2_JOURNAL_FLUSH_DISCARD and JBD2_JOURNAL_FLUSH_ERASE...
(and making the flags parameter an unsigned int).

> +	/* flags must be set to either discard or zeroout */
> +	if ((flags & JBD2_ERASE_FLAG_DISCARD & JBD2_ERASE_FLAG_ZEROOUT) || !flags)
> +		return -EINVAL;

The expression (flags & JBD2_ERASE_FLAG_DISCARD & JBD2_ERASE_FLAG_ZEROOUT)
is always going to evaluate to zero, since (1 & 2) is 0.

What you probably want is something like:

#define JBD2_JOURNAL_FLUSH_DISCARD	0x0001
#define JBD2_JOURNAL_FLUSH_ZEROOUT	0x0002
#define JBD2_JOURNAL_FLUSH_VALID	0x0003

     if ((flags & ~JBD2_JOURNAL_FLUSH_VALID) ||
         ((flags & JBD2_JOURNAL_FLUSH_DISCARD) &&
	  (flags & JBD2_JOURNAL_FLUSH_ZEROOUT)))
	return -EINVAL;

> +
> +	err = jbd2_journal_bmap(journal, log_offset, &block_start);
> +	if (err) {
> +		printk(KERN_ERR "JBD2: bad block at offset %lu", log_offset);
> +		return err;
> +	}

We could get rid of this, and instead make sure block_start is initialized
to ~((unsigned long long) 0).  Then in the loop we can do...

> +
> +	/*
> +	 * 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_start == ~((unsigned long long) 0)) {
			block_start = phys_block;
			block_Stop = block_start - 1;
		}

> +
> +		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);
> +
> +		if (flags & JBD2_ERASE_FLAG_DISCARD) {
> +			err = blkdev_issue_discard(journal->j_dev,
> +					byte_start >> SECTOR_SHIFT,
> +					byte_count >> SECTOR_SHIFT,
> +					GFP_NOFS, 0);
> +		} else if (flags & JBD2_ERASE_FLAG_ZEROOUT) {
> +			err = blkdev_issue_zeroout(journal->j_dev,
> +					byte_start >> SECTOR_SHIFT,
> +					byte_count >> SECTOR_SHIFT,
> +					GFP_NOFS, 0);
> +		}
> +
> +		if (unlikely(err != 0)) {
> +			printk(KERN_ERR "JBD2: (error %d) unable to wipe journal at physical blocks %llu - %llu",
> +					err, block_start, block_stop);
> +			return err;
> +		}
> +
> +		block_start = phys_block;
> +		block_stop = phys_block;

Is this right?  When we initialized the loop, above, block_stop was
set to block_start-1 (where block_start == phys_block).  So I think it
might be more correct to replace the above two lines with:

		block_start = ~((unsigned long long) 0);

... and then let block_start and block_stop be initialized in a single
place.  Do you agree?  Does this make sense to you?

	       	       	    	      	    - Ted