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Date:   Wed, 13 May 2020 14:41:55 -0700
From:   Ira Weiny <ira.weiny@...el.com>
To:     Jan Kara <jack@...e.cz>
Cc:     linux-ext4@...r.kernel.org,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        "Theodore Y. Ts'o" <tytso@....edu>,
        Al Viro <viro@...iv.linux.org.uk>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Chinner <david@...morbit.com>,
        Christoph Hellwig <hch@....de>, Jeff Moyer <jmoyer@...hat.com>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 8/9] fs/ext4: Introduce DAX inode flag

On Wed, May 13, 2020 at 04:47:06PM +0200, Jan Kara wrote:
> On Tue 12-05-20 22:43:23, ira.weiny@...el.com wrote:
> > From: Ira Weiny <ira.weiny@...el.com>
> > 
> > Add a flag to preserve FS_XFLAG_DAX in the ext4 inode.
> > 
> > Set the flag to be user visible and changeable.  Set the flag to be
> > inherited.  Allow applications to change the flag at any time.
> > 
> > Finally, on regular files, flag the inode to not be cached to facilitate
> > changing S_DAX on the next creation of the inode.
> > 
> > Signed-off-by: Ira Weiny <ira.weiny@...el.com>
> > 
> > ---
> > Change from RFC:
> > 	use new d_mark_dontcache()
> > 	Allow caching if ALWAYS/NEVER is set
> > 	Rebased to latest Linus master
> > 	Change flag to unused 0x01000000
> > 	update ext4_should_enable_dax()
> > ---
> >  fs/ext4/ext4.h  | 13 +++++++++----
> >  fs/ext4/inode.c |  4 +++-
> >  fs/ext4/ioctl.c | 25 ++++++++++++++++++++++++-
> >  3 files changed, 36 insertions(+), 6 deletions(-)
> > 
> > diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
> > index 01d1de838896..715f8f2029b2 100644
> > --- a/fs/ext4/ext4.h
> > +++ b/fs/ext4/ext4.h
> > @@ -415,13 +415,16 @@ struct flex_groups {
> >  #define EXT4_VERITY_FL			0x00100000 /* Verity protected inode */
> >  #define EXT4_EA_INODE_FL	        0x00200000 /* Inode used for large EA */
> >  /* 0x00400000 was formerly EXT4_EOFBLOCKS_FL */
> > +
> > +#define EXT4_DAX_FL			0x01000000 /* Inode is DAX */
> > +
> >  #define EXT4_INLINE_DATA_FL		0x10000000 /* Inode has inline data. */
> >  #define EXT4_PROJINHERIT_FL		0x20000000 /* Create with parents projid */
> >  #define EXT4_CASEFOLD_FL		0x40000000 /* Casefolded file */
> >  #define EXT4_RESERVED_FL		0x80000000 /* reserved for ext4 lib */
> >  
> > -#define EXT4_FL_USER_VISIBLE		0x705BDFFF /* User visible flags */
> > -#define EXT4_FL_USER_MODIFIABLE		0x604BC0FF /* User modifiable flags */
> > +#define EXT4_FL_USER_VISIBLE		0x715BDFFF /* User visible flags */
> > +#define EXT4_FL_USER_MODIFIABLE		0x614BC0FF /* User modifiable flags */
> 
> Hum, I think this was already mentioned but there are also definitions in
> include/uapi/linux/fs.h which should be kept in sync... Also if DAX flag
> gets modified through FS_IOC_SETFLAGS, we should call ext4_doncache() as
> well, shouldn't we?

Ah yea it was mentioned.  Sorry.

> 
> > @@ -802,6 +807,21 @@ static int ext4_ioctl_get_es_cache(struct file *filp, unsigned long arg)
> >  	return error;
> >  }
> >  
> > +static void ext4_dax_dontcache(struct inode *inode, unsigned int flags)
> > +{
> > +	struct ext4_inode_info *ei = EXT4_I(inode);
> > +
> > +	if (S_ISDIR(inode->i_mode))
> > +		return;
> > +
> > +	if (test_opt2(inode->i_sb, DAX_NEVER) ||
> > +	    test_opt(inode->i_sb, DAX_ALWAYS))
> > +		return;
> > +
> > +	if (((ei->i_flags ^ flags) & EXT4_DAX_FL) == EXT4_DAX_FL)
> > +		d_mark_dontcache(inode);
> > +}
> > +
> >  long ext4_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
> >  {
> >  	struct inode *inode = file_inode(filp);
> > @@ -1267,6 +1287,9 @@ long ext4_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
> >  			return err;
> >  
> >  		inode_lock(inode);
> > +
> > +		ext4_dax_dontcache(inode, flags);
> > +
> 
> I don't think we should set dontcache flag when setting of DAX flag fails -
> it could event be a security issue).

good point.

>
> So I think you'll have to check
> whether DAX flag is being changed,

ext4_dax_dontcache() does check if the flag is being changed.

> call vfs_ioc_fssetxattr_check(), and
> only if it succeeded and DAX flags was changing call ext4_dax_dontcache().

Yes I think it would be better to ensure all of the ioctl succeeds prior to
setting the don't cache.  The logic is easier to follow.

Ira

> 
> 								Honza
> -- 
> Jan Kara <jack@...e.com>
> SUSE Labs, CR

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