Re: [RFC PATCH] ext4: convert symlink external data block mapping to bdev

[Zhang Yi <yi.zhang@huawei.com>]

On 2022/4/7 1:17, Jan Kara wrote:
> On Wed 06-04-22 16:45:03, Zhang Yi wrote:
>> Symlink's external data block is one kind of metadata block, and now
>> that almost all ext4 metadata block's page cache (e.g. directory blocks,
>> quota blocks...) belongs to bdev backing inode except the symlink. It
>> is essentially worked in data=journal mode like other regular file's
>> data block because probably in order to make it simple for generic VFS
>> code handling symlinks or some other historical reasons, but the logic
>> of creating external data block in ext4_symlink() is complicated. and it
>> also make things confused if user do not want to let the filesystem
>> worked in data=journal mode. This patch convert the final exceptional
>> case and make things clean, move the mapping of the symlink's external
>> data block to bdev like any other metadata block does.
>>
>> Signed-off-by: Zhang Yi <yi.zhang@huawei.com>
>> ---
>> This RFC patch follow the talking of whether if we could unify the
>> journal mode of ext4 metadata blocks[1], it stop using the data=journal
>> mode for the final exception case of symlink's external data block. Any
>> comments are welcome, thanks.
>>
>> [1]. https://lore.kernel.org/linux-ext4/20220321151141.hypnhr6o4vng2sa6@quack3.lan/T/#m84b942a6bb838ba60ae8afd906ebbb987a577488
>>
>>  fs/ext4/inode.c   |   9 +---
>>  fs/ext4/namei.c   | 123 +++++++++++++++++++++-------------------------
>>  fs/ext4/symlink.c |  44 ++++++++++++++---
>>  3 files changed, 93 insertions(+), 83 deletions(-)
> 
> Hum, we don't save on code but I'd say the result is somewhat more
> standard. So I guess this makes some sense. Let's see what Ted thinks...
> 
> Otherwise I've found just one small bug below.
> 
>> @@ -3270,26 +3296,8 @@ static int ext4_symlink(struct user_namespace *mnt_userns, struct inode *dir,
>>  	if (err)
>>  		return err;
>>  
>> -	if ((disk_link.len > EXT4_N_BLOCKS * 4)) {
>> -		/*
>> -		 * For non-fast symlinks, we just allocate inode and put it on
>> -		 * orphan list in the first transaction => we need bitmap,
>> -		 * group descriptor, sb, inode block, quota blocks, and
>> -		 * possibly selinux xattr blocks.
>> -		 */
>> -		credits = 4 + EXT4_MAXQUOTAS_INIT_BLOCKS(dir->i_sb) +
>> -			  EXT4_XATTR_TRANS_BLOCKS;
>> -	} else {
>> -		/*
>> -		 * Fast symlink. We have to add entry to directory
>> -		 * (EXT4_DATA_TRANS_BLOCKS + EXT4_INDEX_EXTRA_TRANS_BLOCKS),
>> -		 * allocate new inode (bitmap, group descriptor, inode block,
>> -		 * quota blocks, sb is already counted in previous macros).
>> -		 */
>> -		credits = EXT4_DATA_TRANS_BLOCKS(dir->i_sb) +
>> -			  EXT4_INDEX_EXTRA_TRANS_BLOCKS + 3;
>> -	}
>> -
>> +	credits = EXT4_DATA_TRANS_BLOCKS(dir->i_sb) +
>> +		  EXT4_INDEX_EXTRA_TRANS_BLOCKS + 3;
> 
> This does not seem like enough credits - we may need to allocate inode, add
> entry to directory, allocate & initialize symlink block. So I think you
> need to add 4 for block allocation + init in case of non-fast symlink. And
> please keep the comment explaining what is actually counted in the number
> of credits...
> 

Thanks for pointing this out, and ext4_mkdir() seems has the same problem too
because we also need to allocate one more block to store '.' and '..' entries
for a new created empty directory.

BTW, look the credits calculation in depth, the definition of
EXT4_DATA_TRANS_BLOCKS is weird, the '-2' subtraction looks wrong.

> #define EXT4_DATA_TRANS_BLOCKS(sb)	(EXT4_SINGLEDATA_TRANS_BLOCKS(sb) + \
> 					 EXT4_XATTR_TRANS_BLOCKS - 2 + \
> 					 EXT4_MAXQUOTAS_TRANS_BLOCKS(sb))

I see the history log, before commit[1], the '-2' subtract the 2 more duplicate
counted super block in '3 * EXT3_SINGLEDATA_TRANS_BLOCKS', but after this commit,
it seems buggy because we have only count the super block once. It's a long time
ago, I'm not sure am I missing something?

[1]. https://git.kernel.org/pub/scm/linux/kernel/git/history/history.git/commit/?id=2df2c24aa6d2cd56777570d96494b921568b4405

Thanks,
Yi.