Re: [PATCH v2 3/6] ext4: restart handle if credits are insufficient during allocating blocks

[Jan Kara]

On Wed 11-06-25 19:16:22, Zhang Yi wrote:
> From: Zhang Yi <yi.zhang@xxxxxxxxxx>
> 
> After large folios are supported on ext4, writing back a sufficiently
> large and discontinuous folio may consume a significant number of
> journal credits, placing considerable strain on the journal. For
> example, in a 20GB filesystem with 1K block size and 1MB journal size,
> writing back a 2MB folio could require thousands of credits in the
> worst-case scenario (when each block is discontinuous and distributed
> across different block groups), potentially exceeding the journal size.
> This issue can also occur in ext4_write_begin() and ext4_page_mkwrite()
> when delalloc is not enabled.
> 
> Fix this by ensuring that there are sufficient journal credits before
> allocating an extent in mpage_map_one_extent() and _ext4_get_block(). If
> there are not enough credits, return -EAGAIN, exit the current mapping
> loop, restart a new handle and a new transaction, and allocating blocks
> on this folio again in the next iteration.
> 
> Suggested-by: Jan Kara <jack@xxxxxxx>
> Signed-off-by: Zhang Yi <yi.zhang@xxxxxxxxxx>

...

>  static int _ext4_get_block(struct inode *inode, sector_t iblock,
>  			   struct buffer_head *bh, int flags)
>  {
>  	struct ext4_map_blocks map;
> +	handle_t *handle = ext4_journal_current_handle();
>  	int ret = 0;
>  
>  	if (ext4_has_inline_data(inode))
>  		return -ERANGE;
>  
> +	/* Make sure transaction has enough credits for this extent */
> +	if (flags & EXT4_GET_BLOCKS_CREATE) {
> +		ret = ext4_journal_ensure_extent_credits(handle, inode);
> +		if (ret)
> +			return ret;
> +	}
> +
>  	map.m_lblk = iblock;
>  	map.m_len = bh->b_size >> inode->i_blkbits;
>  
> -	ret = ext4_map_blocks(ext4_journal_current_handle(), inode, &map,
> -			      flags);
> +	ret = ext4_map_blocks(handle, inode, &map, flags);

Good spotting with ext4_page_mkwrite() and ext4_write_begin() also needing
this treatment! But rather then hiding the transaction extension in
_ext4_get_block() I'd do this in ext4_block_write_begin() where it is much
more obvious (and also it is much more obvious who needs to be prepared for
handling EAGAIN error). Otherwise the patch looks good!

								Honza

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