Re: [PATCH 3/3] ext4: fix a potential assertion failure due to improperly dirtied buffer

[Jan Kara]

On Thu 29-08-24 16:54:07, zhangshida wrote:
> From: Shida Zhang <zhangshida@xxxxxxxxxx>
> 
> On an old kernel version(4.19, ext3, data=journal, pagesize=64k),
> an assertion failure will occasionally be triggered by the line below:
> -----------
> jbd2_journal_commit_transaction
> {
> ...
> J_ASSERT_BH(bh, !buffer_dirty(bh));
> /*
> * The buffer on BJ_Forget list and not jbddirty means
> ...
> }
> -----------
> 
> The same condition may also be applied to the lattest kernel version.
> 
> When blocksize < pagesize and we truncate a file, there can be buffers in
> the mapping tail page beyond i_size. These buffers will be filed to
> transaction's BJ_Forget list by ext4_journalled_invalidatepage() during
> truncation. When the transaction doing truncate starts committing, we can
> grow the file again. This calls __block_write_begin() which allocates new
> blocks under these buffers in the tail page we go through the branch:
> 
>                         if (buffer_new(bh)) {
>                                 clean_bdev_bh_alias(bh);
>                                 if (folio_test_uptodate(folio)) {
>                                         clear_buffer_new(bh);
>                                         set_buffer_uptodate(bh);
>                                         mark_buffer_dirty(bh);
>                                         continue;
>                                 }
>                                 ...
>                         }
> 
> Hence buffers on BJ_Forget list of the committing transaction get marked
> dirty and this triggers the jbd2 assertion.
> 
> Teach ext4_block_write_begin() to properly handle files with data
> journalling by avoiding dirtying them directly. Instead of
> folio_zero_new_buffers() we use ext4_journalled_zero_new_buffers() which
> takes care of handling journalling. We also don't need to mark new uptodate
> buffers as dirty in ext4_block_write_begin(). That will be either done
> either by block_commit_write() in case of success or by
> folio_zero_new_buffers() in case of failure.
> 
> Reported-by: Baolin Liu <liubaolin@xxxxxxxxxx>
> Suggested-by: Jan Kara <jack@xxxxxxx>
> Signed-off-by: Shida Zhang <zhangshida@xxxxxxxxxx>

One small comment below but regardless whether you decide to address it or
not, feel free to add:

Reviewed-by: Jan Kara <jack@xxxxxxx>

> @@ -1083,11 +1090,22 @@ int ext4_block_write_begin(struct folio *folio, loff_t pos, unsigned len,
>  			err = get_block(inode, block, bh, 1);
>  			if (err)
>  				break;
> +			/*
> +			 * We may be zeroing partial buffers or all new
> +			 * buffers in case of failure. Prepare JBD2 for
> +			 * that.
> +			 */
> +			if (should_journal_data)
> +				do_journal_get_write_access(handle, inode, bh);

Thanks for adding comments! I also mentioned this hunk can be moved inside
the if (buffer_new(bh)) check below to make it more obvious that this is
indeed about handling of newly allocated buffers. But this is just a nit
and the comment explains is well enough so I don't insist.

>  			if (buffer_new(bh)) {
>  				if (folio_test_uptodate(folio)) {
> -					clear_buffer_new(bh);
> +					/*
> +					 * Unlike __block_write_begin() we leave
> +					 * dirtying of new uptodate buffers to
> +					 * ->write_end() time or
> +					 * folio_zero_new_buffers().
> +					 */
>  					set_buffer_uptodate(bh);
> -					mark_buffer_dirty(bh);
>  					continue;
>  				}
>  				if (block_end > to || block_start < from)

Thanks!

								Honza

-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR