linux-ext4 - Re: [RFC PATCH v3] ext4: convert symlink external data block mapping to bdev

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Date:   Wed, 20 Apr 2022 14:32:07 +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 v3] ext4: convert symlink external data block mapping
 to bdev

On Mon 18-04-22 14:37:35, 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>

The patch looks good now except for one problem:

> +static const char *ext4_get_link(struct dentry *dentry, struct inode *inode,
> +				 struct delayed_call *callback)
> +{
> +	struct buffer_head *bh;
> +
> +	if (!dentry) {
> +		bh = ext4_getblk(NULL, inode, 0, 0);

This is problematic because in RCU walk mode we must not sleep and
ext4_getblk() may sleep in ext4_map_blocks(). So we'd need some trick to
avoid that.

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