linux-ext4 - Re: [PATCH V3 4/8] fs/ext4: Update ext4_should_use

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Date:   Wed, 20 May 2020 15:37:28 +0200
From:   Jan Kara <jack@...e.cz>
To:     ira.weiny@...el.com
Cc:     linux-ext4@...r.kernel.org,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        "Theodore Y. Ts'o" <tytso@....edu>, Jan Kara <jack@...e.cz>,
        Eric Biggers <ebiggers@...nel.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Chinner <david@...morbit.com>,
        Christoph Hellwig <hch@....de>, Jeff Moyer <jmoyer@...hat.com>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH V3 4/8] fs/ext4: Update ext4_should_use_dax()

On Tue 19-05-20 22:57:49, ira.weiny@...el.com wrote:
> From: Ira Weiny <ira.weiny@...el.com>
> 
> S_DAX should only be enabled when the underlying block device supports
> dax.
> 
> Change ext4_should_use_dax() to check for device support prior to the
> over riding mount option.
> 
> While we are at it change the function to ext4_should_enable_dax() as
> this better reflects the ask as well as matches xfs.
> 
> Reviewed-by: Jan Kara <jack@...e.cz>
> Signed-off-by: Ira Weiny <ira.weiny@...el.com>

...

> @@ -4412,7 +4410,13 @@ static bool ext4_should_use_dax(struct inode *inode)
>  		return false;
>  	if (ext4_test_inode_flag(inode, EXT4_INODE_VERITY))
>  		return false;
> -	return true;
> +	if (!bdev_dax_supported(inode->i_sb->s_bdev,
> +				inode->i_sb->s_blocksize))
> +		return false;
> +	if (test_opt(inode->i_sb, DAX_ALWAYS))
> +		return true;
> +
> +	return false;
>  }

Now that I think about it - shouldn't we rather cache the result of
bdev_dax_supported() in sb on mount and then just check the flag here?
Because bdev_dax_supported() isn't exactly cheap (it does a lot of checks
and mappings, tries to read from the pmem, ...).

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