Re: [PATCH v4 11/11] ext4: limit the maximum folio order

[Joseph Qi <jiangqi903@gmail.com>]

On 2025/7/7 22:08, Zhang Yi wrote:
> From: Zhang Yi <yi.zhang@huawei.com>
> 
> In environments with a page size of 64KB, the maximum size of a folio
> can reach up to 128MB. Consequently, during the write-back of folios,
> the 'rsv_blocks' will be overestimated to 1,577, which can make
> pressure on the journal space where the journal is small. This can
> easily exceed the limit of a single transaction. Besides, an excessively
> large folio is meaningless and will instead increase the overhead of
> traversing the bhs within the folio. Therefore, limit the maximum order
> of a folio to 2048 filesystem blocks.
> 
> Reported-by: Naresh Kamboju <naresh.kamboju@linaro.org>
> Reported-by: Joseph Qi <jiangqi903@gmail.com>
> Closes: https://lore.kernel.org/linux-ext4/CA+G9fYsyYQ3ZL4xaSg1-Tt5Evto7Zd+hgNWZEa9cQLbahA1+xg@mail.gmail.com/
> Signed-off-by: Zhang Yi <yi.zhang@huawei.com>

Confirmed that this can fix the following jbd2 warning in start_this_handle():
"JBD2: kworker/u32:0 wants too many credits credits:32 rsv_credits:1577 max:2695"

Tested-by: Joseph Qi <joseph.qi@linux.alibaba.com>

> ---
>  fs/ext4/ext4.h   |  2 +-
>  fs/ext4/ialloc.c |  3 +--
>  fs/ext4/inode.c  | 22 +++++++++++++++++++---
>  3 files changed, 21 insertions(+), 6 deletions(-)
> 
> diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
> index f705046ba6c6..9ac0a7d4fa0c 100644
> --- a/fs/ext4/ext4.h
> +++ b/fs/ext4/ext4.h
> @@ -3020,7 +3020,7 @@ int ext4_walk_page_buffers(handle_t *handle,
>  				     struct buffer_head *bh));
>  int do_journal_get_write_access(handle_t *handle, struct inode *inode,
>  				struct buffer_head *bh);
> -bool ext4_should_enable_large_folio(struct inode *inode);
> +void ext4_set_inode_mapping_order(struct inode *inode);
>  #define FALL_BACK_TO_NONDELALLOC 1
>  #define CONVERT_INLINE_DATA	 2
>  
> diff --git a/fs/ext4/ialloc.c b/fs/ext4/ialloc.c
> index 79aa3df8d019..df4051613b29 100644
> --- a/fs/ext4/ialloc.c
> +++ b/fs/ext4/ialloc.c
> @@ -1335,8 +1335,7 @@ struct inode *__ext4_new_inode(struct mnt_idmap *idmap,
>  		}
>  	}
>  
> -	if (ext4_should_enable_large_folio(inode))
> -		mapping_set_large_folios(inode->i_mapping);
> +	ext4_set_inode_mapping_order(inode);
>  
>  	ext4_update_inode_fsync_trans(handle, inode, 1);
>  
> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> index 4b679cb6c8bd..1bce9ebaedb7 100644
> --- a/fs/ext4/inode.c
> +++ b/fs/ext4/inode.c
> @@ -5181,7 +5181,7 @@ static int check_igot_inode(struct inode *inode, ext4_iget_flags flags,
>  	return -EFSCORRUPTED;
>  }
>  
> -bool ext4_should_enable_large_folio(struct inode *inode)
> +static bool ext4_should_enable_large_folio(struct inode *inode)
>  {
>  	struct super_block *sb = inode->i_sb;
>  
> @@ -5198,6 +5198,22 @@ bool ext4_should_enable_large_folio(struct inode *inode)
>  	return true;
>  }
>  
> +/*
> + * Limit the maximum folio order to 2048 blocks to prevent overestimation
> + * of reserve handle credits during the folio writeback in environments
> + * where the PAGE_SIZE exceeds 4KB.
> + */
> +#define EXT4_MAX_PAGECACHE_ORDER(i)		\
> +		min(MAX_PAGECACHE_ORDER, (11 + (i)->i_blkbits - PAGE_SHIFT))
> +void ext4_set_inode_mapping_order(struct inode *inode)
> +{
> +	if (!ext4_should_enable_large_folio(inode))
> +		return;
> +
> +	mapping_set_folio_order_range(inode->i_mapping, 0,
> +				      EXT4_MAX_PAGECACHE_ORDER(inode));
> +}
> +
>  struct inode *__ext4_iget(struct super_block *sb, unsigned long ino,
>  			  ext4_iget_flags flags, const char *function,
>  			  unsigned int line)
> @@ -5515,8 +5531,8 @@ struct inode *__ext4_iget(struct super_block *sb, unsigned long ino,
>  		ret = -EFSCORRUPTED;
>  		goto bad_inode;
>  	}
> -	if (ext4_should_enable_large_folio(inode))
> -		mapping_set_large_folios(inode->i_mapping);
> +
> +	ext4_set_inode_mapping_order(inode);
>  
>  	ret = check_igot_inode(inode, flags, function, line);
>  	/*