linux-kernel - Re: [PATCH v2 1/3] ext4: Add alignment check for DAX mount

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Date:	Wed, 04 May 2016 09:42:23 -0600
From:	Toshi Kani <toshi.kani@....com>
To:	Ross Zwisler <ross.zwisler@...ux.intel.com>,
	Jan Kara <jack@...e.cz>, Christoph Hellwig <hch@...radead.org>
Cc:	"tytso@....edu" <tytso@....edu>,
	"linux-nvdimm@...1.01.org" <linux-nvdimm@...1.01.org>,
	"david@...morbit.com" <david@...morbit.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Parrish, Micah (HP Servers Linux R&D)" <micah.parrish@....com>,
	"adilger.kernel@...ger.ca" <adilger.kernel@...ger.ca>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH v2 1/3] ext4: Add alignment check for DAX mount

On Tue, 2016-05-03 at 15:41 +0000, Kani, Toshimitsu wrote:
> On Tue, 2016-05-03 at 08:43 -0600, Ross Zwisler wrote:
> > On Tue, May 03, 2016 at 11:00:21AM +0200, Jan Kara wrote:
> > > On Tue 03-05-16 01:44:10, Christoph Hellwig wrote:
> > > > 
> > > > Please come up with a version that doesn't require tons of
> > > > boilerplate code in every file system.
> > >
> > > Well, I was thinking about some helper as well but we could save ~4
> > > lines with that and that didn't seem significant to me. Most of the
> > > lines is actually reporting appropriate mount error in dmesg and that
> > > is fs-dependent so it needs to stay in the filesystem... So what do
> > > you have in mind?
> >
> > I guess if you wanted to reduce the code needed in each filesystem, you
> > could avoid having different error messages for each of the failure
> > conditions, and just print the error value.  All the error cases caught
> > by the current code are unique, so we aren't losing any
> > information.  The resulting patch for ext4 would look like this:
>
> I'd prefer to keep the "error: device does not support dax" and "error:
> unaligned partition for dax" messages since they clarify the problem as a
> user error. The former case is especially common with BTT. The "error:
> dax access failed (%d)" message is helpful when we need to look into the
> case.
> 
> If XFS is OK to use the messages similar to ext2/4, then we can do: 
> 
>  * Add a new helper function, say bdev_check_dax_mount(), which logs
> common error messages with pr_err(), such as:
>        "VFS (pmem0): error: device does not support dax"
>  * When bdev_check_dax_mount() returns with a negative value:
>    - XFS logs an additional message below via xfs_alert() and proceeds
> without dax option.
>        "XFS (pmem0): Turning DAX off."
>    - ext2/4 fails the mount.

Since v2 got multiple reviews, and Jan has added 2/3 into his tree
(thanks!), I will send "[PATCH v2-UPDATE 3/3]" which addresses Ross's
comments on v2 patch 3/3.  This keeps the v2 series.

Christoph, let me know if you'd still like to see v3 with the above changes
to share the same error messages among ext2/4 and xfs.

Thanks,
-Toshi