Re: [RFC PATCH v4 4/4] ext4: data=journal: write-protect pages on j_submit_inode_data_buffers()

[Jan Kara <jack@suse.cz>]

On Mon 28-09-20 16:41:03, Mauricio Faria de Oliveira wrote:
> This implements journal callbacks j_submit|finish_inode_data_buffers()
> with different behavior for data=journal: to write-protect pages under
> commit, preventing changes to buffers writeably mapped to userspace.
> 
> If a buffer's content changes between commit's checksum calculation
> and write-out to disk, it can cause journal recovery/mount failures
> upon a kernel crash or power loss.
> 
>     [   27.334874] EXT4-fs: Warning: mounting with data=journal disables delayed allocation, dioread_nolock, and O_DIRECT support!
>     [   27.339492] JBD2: Invalid checksum recovering data block 8705 in log
>     [   27.342716] JBD2: recovery failed
>     [   27.343316] EXT4-fs (loop0): error loading journal
>     mount: /ext4: can't read superblock on /dev/loop0.
> 
> In j_submit_inode_data_buffers() we write-protect the inode's pages
> with write_cache_pages() and redirty w/ writepage callback if needed.
> 
> In j_finish_inode_data_buffers() there is nothing do to.
> 
> And in order to use the callbacks, inodes are added to the inode list
> in transaction in __ext4_journalled_writepage() and ext4_page_mkwrite().
> 
> In ext4_page_mkwrite() we must make sure that the buffers are attached
> to the transaction as jbddirty with write_end_fn(), as already done in
> __ext4_journalled_writepage().
> 
> Signed-off-by: Mauricio Faria de Oliveira <mfo@canonical.com>
> Reported-by: Dann Frazier <dann.frazier@canonical.com>
> Reported-by: kernel test robot <lkp@intel.com> # wbc.nr_to_write
> Suggested-by: Jan Kara <jack@suse.cz>

The patch looks good to me. Just one nit below. After fixing that feel free
to add:

Reviewed-by: Jan Kara <jack@suse.cz>

> + * However, we have to redirty a page in these cases:
> + * 1) some buffer is dirty (needs checkpointing)
> + * 2) some buffer is not part of the committing transaction
> + * 3) some buffer already has b_next_transaction set
> + */

Maybe I'd move this comment inside ext4_journalled_writepage_callback()
just before the if () to make it clear what it speaks about. I'd also
somewhat expand it like:

/*
 * However, we have to redirty a page in these cases:
 * 1) If buffer is dirty, it means the page was dirty because it contains a
 * buffer that needs checkpointing. So dirty bit needs to be preserved so
 * that checkpointing writes the buffer properly.
 * 2) If buffer is not part of the committing transaction (we may have just
 * accidentally come across this buffer because inode range tracking is not
 * exact) or if the currently running transaction already contains this
 * buffer as well, dirty bit needs to be preserved so that the buffer gets
 * properly writeprotected on running transaction's commit.
 */

> +
> +static int ext4_journalled_writepage_callback(struct page *page,
> +					      struct writeback_control *wbc,
> +					      void *data)
> +{
> +	transaction_t *transaction = (transaction_t *) data;
> +	struct buffer_head *bh, *head;
> +	struct journal_head *jh;
> +
> +	bh = head = page_buffers(page);
> +	do {
> +		jh = bh2jh(bh);
> +		if (buffer_dirty(bh) ||
> +			(jh && (jh->b_transaction != transaction ||
> +				jh->b_next_transaction))) {

Also we usually indent the condition like:
		if (buffer_dirty(bh) ||
		    (jh && (jh->b_transaction != transaction ||
			    jh->b_next_transaction))) {

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR