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

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Date:	Mon, 02 May 2016 09:23:58 -0600
From:	Toshi Kani <toshi.kani@....com>
To:	Boaz Harrosh <boaz@...xistor.com>,
	Christoph Hellwig <hch@...radead.org>
Cc:	dan.j.williams@...el.com, david@...morbit.com, jack@...e.cz,
	tytso@....edu, adilger.kernel@...ger.ca,
	ross.zwisler@...ux.intel.com, linux-nvdimm@...1.01.org,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] ext4: Add alignment check for DAX mount

On Mon, 2016-05-02 at 12:49 +0300, Boaz Harrosh wrote:
> On 05/01/2016 08:31 PM, Christoph Hellwig wrote:
> > 
> > On Fri, Apr 29, 2016 at 02:39:33PM -0600, Toshi Kani wrote:
> > > 
> > > 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)) {
> > > +			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");
> >
> > Factor your new checks and the ->direct_access into a new helper. It
> > should take the block device as file systems might have multiple
> > underlying devices.
>
> This already exists as part of bdev_direct_access()
> 
> All the code needs to do is call bdev_direct_access() on sector ZERO and
> see if it is successful. If the alignment is broken it will fail.
> 
> Infact the FS does not even need the second if (!...bd_disk->fops-
> >direct_access) check either because it is also done inside
> bdev_direct_access(). The only check needed is calling
> bdev_direct_access() and the generic layer does all the
> checks needed already.

Sounds good. I will call bdev_direct_access() for this check. I assume we
do not need to check the alignment of 'bd_part->nr_sects' since the rest of
unaligned sectors will be ignored with block size 4KB. 

Thanks!
-Toshi