Re: [PATCH v2 01/20] dax: Modify bdev_dax_pgoff() to handle NULL bdev

From: Jan Kara <jack@suse.cz>
To: Vivek Goyal <vgoyal@redhat.com>
Cc: linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
	virtio-fs@redhat.com, miklos@szeredi.hu, stefanha@redhat.com,
	dgilbert@redhat.com, Christoph Hellwig <hch@infradead.org>,
	Dan Williams <dan.j.williams@intel.com>,
	linux-nvdimm@lists.01.org
Subject: Re: [PATCH v2 01/20] dax: Modify bdev_dax_pgoff() to handle NULL bdev
Date: Mon, 17 Aug 2020 18:57:31 +0200	[thread overview]
Message-ID: <20200817165731.GB22500@quack2.suse.cz> (raw)
In-Reply-To: <20200807195526.426056-2-vgoyal@redhat.com>

On Fri 07-08-20 15:55:07, Vivek Goyal wrote:
> virtiofs does not have a block device but it has dax device.
> Modify bdev_dax_pgoff() to be able to handle that.
> 
> If there is no bdev, that means dax offset is 0. (It can't be a partition
> block device starting at an offset in dax device).
> 
> This is little hackish. There have been discussions about getting rid
> of dax not supporting partitions.
> 
> https://lore.kernel.org/linux-fsdevel/20200107125159.GA15745@infradead.org/
> 
> IMHO, this path can easily break exisitng users. For example
> ioctl(BLKPG_ADD_PARTITION) will start breaking on block devices
> supporting DAX. Also, I personally find it very useful to be able to
> partition dax devices and still be able to use DAX.
> 
> Alternatively, I tried to store offset into dax device information in iomap
> interface, but that got NACKed.
> 
> https://lore.kernel.org/linux-fsdevel/20200217133117.GB20444@infradead.org/
> 
> I can't think of a good path to solve this issue properly. So to make
> progress, it seems this patch is least bad option for now and I hope
> we can take it.
> 
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
> Cc: Christoph Hellwig <hch@infradead.org>
> Cc: Dan Williams <dan.j.williams@intel.com>
> Cc: linux-nvdimm@lists.01.org

This patch looks OK to me. You can add:

Reviewed-by: Jan Kara <jack@suse.cz>

								Honza

> ---
>  drivers/dax/super.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/dax/super.c b/drivers/dax/super.c
> index 8e32345be0f7..c4bec437e88b 100644
> --- a/drivers/dax/super.c
> +++ b/drivers/dax/super.c
> @@ -46,7 +46,8 @@ EXPORT_SYMBOL_GPL(dax_read_unlock);
>  int bdev_dax_pgoff(struct block_device *bdev, sector_t sector, size_t size,
>  		pgoff_t *pgoff)
>  {
> -	phys_addr_t phys_off = (get_start_sect(bdev) + sector) * 512;
> +	sector_t start_sect = bdev ? get_start_sect(bdev) : 0;
> +	phys_addr_t phys_off = (start_sect + sector) * 512;
>  
>  	if (pgoff)
>  		*pgoff = PHYS_PFN(phys_off);
> -- 
> 2.25.4
> 
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR