Re: [PATCH 1/3] ext4: Add alignment check for DAX mount

[Boaz Harrosh]

On 04/29/2016 11:39 PM, Toshi Kani wrote:
> When a partition is not aligned by 4KB, mount -o dax succeeds,
> but any read/write access to the filesystem fails, except for
> metadata update.
> 
> Add alignment check to ext4_fill_super() when -o dax is specified.
> 
> Reported-by: Micah Parrish <micah.parrish@xxxxxxx>
> Signed-off-by: Toshi Kani <toshi.kani@xxxxxxx>
> Cc: "Theodore Ts'o" <tytso@xxxxxxx>
> Cc: Andreas Dilger <adilger.kernel@xxxxxxxxx>
> Cc: Jan Kara <jack@xxxxxxx>
> Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
> Cc: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx>
> ---
>  fs/ext4/super.c |    6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/fs/ext4/super.c b/fs/ext4/super.c
> index 304c712..90a8670 100644
> --- a/fs/ext4/super.c
> +++ b/fs/ext4/super.c
> @@ -3421,6 +3421,12 @@ static int ext4_fill_super(struct super_block *sb, void *data, int silent)
>  					"error: unsupported blocksize for dax");
>  			goto failed_mount;
>  		}
> +		if (sb->s_bdev->bd_part->start_sect % (PAGE_SIZE / 512) ||
> +		    sb->s_bdev->bd_part->nr_sects % (PAGE_SIZE / 512)) {

Can you please not do this like this? For me is a layering violation only
the device should know what are its limits.

I would prefer if you just try to bdev_direct_access() the 0 sector and if
it fails then fail here. This way you let the device decide what it needs
and if/how to support unaligned partitions.
(For example it could by shrinking its size)

Thanks
Boaz

> +			ext4_msg(sb, KERN_ERR,
> +					"error: unaligned partition for dax");
> +			goto failed_mount;
> +		}
>  		if (!sb->s_bdev->bd_disk->fops->direct_access) {
>  			ext4_msg(sb, KERN_ERR,
>  					"error: device does not support dax");
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