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From: Ira Weiny <ira.weiny@intel.com>
To: "Theodore Y. Ts'o" <tytso@mit.edu>
Cc: Jan Kara <jack@suse.cz>,
	linux-kernel@vger.kernel.org,
	"Darrick J. Wong" <darrick.wong@oracle.com>,
	Dan Williams <dan.j.williams@intel.com>,
	Dave Chinner <david@fromorbit.com>,
	Christoph Hellwig <hch@lst.de>, Jeff Moyer <jmoyer@redhat.com>,
	linux-ext4@vger.kernel.org, linux-xfs@vger.kernel.org,
	linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH RFC 2/8] fs/ext4: Disallow verity if inode is DAX
Date: Wed, 15 Apr 2020 12:21:10 -0700	[thread overview]
Message-ID: <20200415192110.GB2305801@iweiny-DESK2.sc.intel.com> (raw)
In-Reply-To: <20200415155525.GI90651@mit.edu>

On Wed, Apr 15, 2020 at 11:55:25AM -0400, Theodore Y. Ts'o wrote:
> On Wed, Apr 15, 2020 at 02:00:02PM +0200, Jan Kara wrote:
> > On Mon 13-04-20 21:00:24, ira.weiny@intel.com wrote:
> > > From: Ira Weiny <ira.weiny@intel.com>
> > > 
> > > Verity and DAX are incompatible.  Changing the DAX mode due to a verity
> > > flag change is wrong without a corresponding address_space_operations
> > > update.
> > > 
> > > Make the 2 options mutually exclusive by returning an error if DAX was
> > > set first.
> > > 
> > > (Setting DAX is already disabled if Verity is set first.)
> > > 
> > > Signed-off-by: Ira Weiny <ira.weiny@intel.com>
> > > ---
> > >  fs/ext4/verity.c | 3 +++
> > >  1 file changed, 3 insertions(+)
> > > 
> > > diff --git a/fs/ext4/verity.c b/fs/ext4/verity.c
> > > index dc5ec724d889..ce3f9a198d3b 100644
> > > --- a/fs/ext4/verity.c
> > > +++ b/fs/ext4/verity.c
> > > @@ -113,6 +113,9 @@ static int ext4_begin_enable_verity(struct file *filp)
> > >  	handle_t *handle;
> > >  	int err;
> > >  
> > > +	if (WARN_ON_ONCE(IS_DAX(inode)))
> > > +		return -EINVAL;
> > > +
> > 
> > Hum, one question, is there a reason for WARN_ON_ONCE()? If I understand
> > correctly, user could normally trigger this, couldn't he?
> 
> Tes, the WARN_ON_ONCE isn't appropriate here.  We should also disallow
> setting the DAX flag if the inode has the verity flag set already.

This is taken care of and is part of ext4_enable_dax() after this series.

> 
> And if we need to decide what to if the file system is mounted with
> "-o dax=always" and the verity file system feature is enabled.  We
> could either (a) reject the mount with if the mount option is given
> and the file system can have verity files, or (b) make "-o dax=always"
> mean "-o dax=mostly_always" and treat verity files as not using dax
> even when dax=always is selected.

The later is implemented in this series...  Not the most explicit thing.  :-(

> 
> Also, in theory, we *could* support dax and verity files, but
> verifying the crypto checksums of all of the pages when the file is
> first accessed, and then marking that in a flag in the in-inode flag.
> Or we could have a per-page flag stored somewhere that indicates that
> the page has been verified, so that we can on-demand verify the
> integrity of the page.  Given that verity files are read-only, the
> main reason why someone might want to use dax && verity would be to
> reduce page cache overhead of system files; if executing out of dax
> pages doesn't have significant performance impacts, this might be
> something which might be a nice-to-have.  I don't think we need to
> worry about this for now; if there are use cases where mobile devices
> want to use dax && verity, we can let them figure out how to make it
> work.  I'm just pointing out that it's not really a completely insane
> combination.

Fair enough.  The main issue I need to correct here is to keep the 2 mutually
exclusive.  Which AFAICT is not true today.  This makes it so even without the
per-file enablement.

Ira

> 
> Cheers,
> 
> 						- Ted

  reply	other threads:[~2020-04-15 19:21 UTC|newest]

Thread overview: 42+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-04-14  4:00 [PATCH RFC 0/8] Enable ext4 support for per-file/directory DAX operations ira.weiny
2020-04-14  4:00 ` [PATCH RFC 1/8] fs/ext4: Narrow scope of DAX check in setflags ira.weiny
2020-04-15 11:45   ` Jan Kara
2020-04-14  4:00 ` [PATCH RFC 2/8] fs/ext4: Disallow verity if inode is DAX ira.weiny
2020-04-15 11:58   ` Jan Kara
2020-04-15 12:00   ` Jan Kara
2020-04-15 15:55     ` Theodore Y. Ts'o
2020-04-15 19:21       ` Ira Weiny [this message]
2020-04-15 19:14     ` Ira Weiny
2020-04-16  1:29       ` Eric Biggers
2020-04-16  3:48         ` Ira Weiny
2020-04-15 20:34     ` Ira Weiny
2020-04-14  4:00 ` [PATCH RFC 3/8] fs/ext4: Disallow encryption " ira.weiny
2020-04-15 12:02   ` Jan Kara
2020-04-15 20:35     ` Ira Weiny
2020-04-15 16:03   ` Theodore Y. Ts'o
2020-04-15 17:27     ` Dan Williams
2020-04-15 19:54     ` Ira Weiny
2020-04-21 18:41       ` Ira Weiny
2020-04-21 18:56         ` Darrick J. Wong
2020-04-14  4:00 ` [PATCH RFC 4/8] fs/ext4: Introduce DAX inode flag ira.weiny
2020-04-15 12:08   ` Jan Kara
2020-04-15 20:39     ` Ira Weiny
2020-04-16 10:32       ` Jan Kara
2020-04-16 18:01       ` Theodore Y. Ts'o
2020-04-16 16:25   ` Darrick J. Wong
2020-04-16 22:33     ` Ira Weiny
2020-04-16 22:49       ` Darrick J. Wong
2020-04-17  0:37         ` Ira Weiny
2020-04-17  1:57           ` Darrick J. Wong
2020-04-17  2:20             ` Ira Weiny
2020-04-17  6:43               ` Andreas Dilger
2020-04-17 17:19                 ` Ira Weiny
2020-04-14  4:00 ` [PATCH RFC 5/8] fs/ext4: Make DAX mount option a tri-state ira.weiny
2020-04-15 12:51   ` Jan Kara
2020-04-14  4:00 ` [PATCH RFC 6/8] fs/ext4: Update ext4_should_use_dax() ira.weiny
2020-04-15 13:58   ` Jan Kara
2020-04-17 17:16     ` Ira Weiny
2020-04-14  4:00 ` [PATCH RFC 7/8] fs/ext4: Only change S_DAX on inode load ira.weiny
2020-04-15 14:03   ` Jan Kara
2020-04-17 17:18     ` Ira Weiny
2020-04-14  4:00 ` [PATCH RFC 8/8] Documentation/dax: Update DAX enablement for ext4 ira.weiny

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