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

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Date:   Wed, 6 Apr 2022 19:17:15 +0200
From:   Jan Kara <jack@...e.cz>
To:     Zhang Yi <yi.zhang@...wei.com>
Cc:     linux-ext4@...r.kernel.org, tytso@....edu,
        adilger.kernel@...ger.ca, jack@...e.cz, yukuai3@...wei.com,
        yebin10@...wei.com
Subject: Re: [RFC PATCH] ext4: convert symlink external data block mapping to
 bdev

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@...wei.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...

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR