Re: [PATCH] ext4: fix JBD2 credit overflow with large folios

[Sasha Levin <sashal@kernel.org>]

On Mon, Jun 30, 2025 at 09:58:52PM +0800, Zhang Yi wrote:
>On 2025/6/30 21:13, Sasha Levin wrote:
>> When large folios are enabled, the blocks-per-folio calculation in
>> ext4_da_writepages_trans_blocks() can overflow the journal transaction
>> limits, causing the writeback path to fail with errors like:
>>
>>   JBD2: kworker/u8:0 wants too many credits credits:416 rsv_credits:21 max:334
>>
>> This occurs with small block sizes (1KB) and large folios (32MB), where
>> the calculation results in 32768 blocks per folio. The transaction credit
>> calculation then requests more credits than the journal can handle, leading
>> to the following warning and writeback failure:
>>
>>   WARNING: CPU: 1 PID: 43 at fs/jbd2/transaction.c:334 start_this_handle+0x4c0/0x4e0
>>   EXT4-fs (loop0): ext4_do_writepages: jbd2_start: 9223372036854775807 pages, ino 14; err -28
>>
>> Call trace leading to the issue:
>>   ext4_do_writepages()
>>     ext4_da_writepages_trans_blocks()
>>       bpp = ext4_journal_blocks_per_folio() // Returns 32768 for 32MB folio with 1KB blocks
>>       ext4_meta_trans_blocks(inode, MAX_WRITEPAGES_EXTENT_LEN + bpp - 1, bpp)
>>         // With bpp=32768, lblocks=34815, pextents=32768
>>         // Returns credits=415, but with overhead becomes 416 > max 334
>>     ext4_journal_start_with_reserve()
>>       jbd2_journal_start_reserved()
>>         start_this_handle()
>>           // Fails with warning when credits:416 > max:334
>>
>> The issue was introduced by commit d6bf294773a47 ("ext4/jbd2: convert
>> jbd2_journal_blocks_per_page() to support large folio"), which added
>> support for large folios but didn't account for the journal credit limits.
>>
>> Fix this by capping the blocks-per-folio value at 8192 in the writeback
>> path. This is the value we'd get with 32MB folios and 4KB blocks, or 8MB
>> folios with 1KB blocks, which is reasonable and safe for typical journal
>> configurations.
>>
>> Fixes: d6bf294773a4 ("ext4/jbd2: convert jbd2_journal_blocks_per_page() to support large folio")
>> Cc: stable@vger.kernel.org
>> Signed-off-by: Sasha Levin <sashal@kernel.org>
>
>Hi, Sasha!
>
>Thank you for the fix. However, simply limiting the credits is not enough,
>as this may result in a scenario where there are not enough credits
>available to map a large, non-contiguous folio. I've been working on this
>issue[1] and I'll release v3 tomorrow if my tests looks fine.
>
>[1] https://lore.kernel.org/linux-ext4/20250611111625.1668035-1-yi.zhang@huaweicloud.com/

Ah perfect, I haven't seen your work, thank you for that.

Please ignore my patch.

-- 
Thanks,
Sasha