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

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Message-ID: <f73b6973-3f7c-4e0e-9908-3a3f151715b5@huaweicloud.com>
Date: Fri, 4 Jul 2025 09:40:23 +0800
From: Zhang Yi <yi.zhang@...weicloud.com>
To: Jan Kara <jack@...e.cz>
Cc: linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
 linux-kernel@...r.kernel.org, tytso@....edu, adilger.kernel@...ger.ca,
 ojaswin@...ux.ibm.com, sashal@...nel.org, yi.zhang@...wei.com,
 libaokun1@...wei.com, yukuai3@...wei.com, yangerkun@...wei.com
Subject: Re: [PATCH v3 05/10] ext4: restart handle if credits are insufficient
 during allocating blocks

On 2025/7/4 0:27, Jan Kara wrote:
> On Thu 03-07-25 10:13:07, Zhang Yi wrote:
>> 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@...wei.com>
>>>>
>>>> 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@...e.cz>
>>>> Signed-off-by: Zhang Yi <yi.zhang@...wei.com>
>>>
>>> Very nice. Feel free to add:
>>>
>>> Reviewed-by: Jan Kara <jack@...e.cz>
>>>
>>> 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'.
> 
> Ah, right. But then you don't need the test at all, do you? Anyway,
> whatever you decide to do with this (or nothing) is fine by me.
> 

Yeah, remove this test is fine with me. I added this one is because the
comments in ext4_handle_valid() said "Do not use this for NULL handles."
I think it is best to follow this rule. :)

Best regards,
Yi.