Re: [PATCH v3 05/10] ext4: restart handle if credits are insufficient during allocating blocks

[Zhang Yi]

On 2025/7/2 22:18, Jan Kara wrote:
> On Tue 01-07-25 21:06:30, 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_block_write_begin(). 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>
> 
> Very nice. Feel free to add:
> 
> Reviewed-by: Jan Kara <jack@xxxxxxx>
> 
> One small comment below:
> 
>> +/*
>> + * Make sure that the current journal transaction has enough credits to map
>> + * one extent. Return -EAGAIN if it cannot extend the current running
>> + * transaction.
>> + */
>> +static inline int ext4_journal_ensure_extent_credits(handle_t *handle,
>> +						     struct inode *inode)
>> +{
>> +	int credits;
>> +	int ret;
>> +
>> +	if (!handle)
> 
> Shouldn't this rather be ext4_handle_valid(handle) to catch nojournal mode
> properly?
> 
__ext4_journal_ensure_credits() already calls ext4_handle_valid() to handle
nojournal mode, and the '!handle' check here is to handle the case where
ext4_block_write_begin() passes in a NULL 'handle'.

Thanks,
Yi.