linux-kernel - Re: [PATCH 10/41] whiteout: Add vfs_whiteout() and whiteout inode operation

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Message-ID: <20100121003550.GA26459@shell>
Date:	Wed, 20 Jan 2010 19:35:50 -0500
From:	Valerie Aurora <vaurora@...hat.com>
To:	Erez Zadok <ezk@...sunysb.edu>
Cc:	Jan Blunck <jblunck@...e.de>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Christoph Hellwig <hch@...radead.org>,
	Andy Whitcroft <apw@...onical.com>,
	Scott James Remnant <scott@...onical.com>,
	Sandu Popa Marius <sandupopamarius@...il.com>,
	Jan Rekorajski <baggins@...h.mimuw.edu.pl>,
	"J. R. Okajima" <hooanon05@...oo.co.jp>,
	Arnd Bergmann <arnd@...db.de>,
	Vladimir Dronnikov <dronnikov@...il.com>,
	Felix Fietkau <nbd@...nwrt.org>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, David Woodhouse <dwmw2@...radead.org>
Subject: Re: [PATCH 10/41] whiteout: Add vfs_whiteout() and whiteout inode operation

On Sun, Nov 29, 2009 at 10:04:16PM -0500, Erez Zadok wrote:
> In message <1256152779-10054-11-git-send-email-vaurora@...hat.com>, Valerie Aurora writes:
> > From: Jan Blunck <jblunck@...e.de>
> > 
> > Simply white-out a given directory entry. This functionality is usually used
> > in the sense of unlink. Therefore the given dentry can still be in-use and
> > contains an in-use inode. The filesystems inode operation has to do what
> > unlink or rmdir would in that case. Since the dentry still might be in-use
> > we have to provide a fresh unhashed dentry that is used as the whiteout
> > dentry instead. The given dentry is dropped and the whiteout dentry is
> > rehashed instead.
> > 
> > Signed-off-by: Jan Blunck <jblunck@...e.de>
> > Signed-off-by: David Woodhouse <dwmw2@...radead.org>
> > Signed-off-by: Valerie Aurora <vaurora@...hat.com>
> > ---
> >  fs/dcache.c            |    4 +-
> >  fs/namei.c             |  104 ++++++++++++++++++++++++++++++++++++++++++++++++
> >  include/linux/dcache.h |    6 +++
> >  include/linux/fs.h     |    3 +
> >  4 files changed, 116 insertions(+), 1 deletions(-)
> > 
> > diff --git a/fs/dcache.c b/fs/dcache.c
> > index 3415e9e..0fcae4b 100644
> > --- a/fs/dcache.c
> > +++ b/fs/dcache.c
> > @@ -1076,8 +1076,10 @@ struct dentry *d_alloc_name(struct dentry *parent, const char *name)
> >  /* the caller must hold dcache_lock */
> >  static void __d_instantiate(struct dentry *dentry, struct inode *inode)
> >  {
> > -	if (inode)
> > +	if (inode) {
> > +		dentry->d_flags &= ~DCACHE_WHITEOUT;
> >  		list_add(&dentry->d_alias, &inode->i_dentry);
> > +	}
> >  	dentry->d_inode = inode;
> >  	fsnotify_d_instantiate(dentry, inode);
> >  }
> > diff --git a/fs/namei.c b/fs/namei.c
> > index 46cf1cb..d2fc8c9 100644
> > --- a/fs/namei.c
> > +++ b/fs/namei.c
> > @@ -2169,6 +2169,110 @@ SYSCALL_DEFINE2(mkdir, const char __user *, pathname, int, mode)
> >  	return sys_mkdirat(AT_FDCWD, pathname, mode);
> >  }
> >  
> > +
> > +/* Checks on the victim for whiteout */
> > +static inline int may_whiteout(struct inode *dir, struct dentry *victim,
> > +			       int isdir)
> 
> Why not make 'isdir' a boolean?
> 
> I'd prefer to see more documentation above this function: explain what each
> arg does, return value, etc.

Indeed!  I added descriptions of the new whiteout and fallthru VFS ops
to Documentation/filesystems/vfs.txt, where it belongs.

> > +/**
> > + * vfs_whiteout: creates a white-out for the given directory entry
> > + * @dir: parent inode
> > + * @dentry: directory entry to white-out
> 
> Nit: is it 'white-out' or 'whiteout'?  Whatever you choose is fine, but
> please use consistent hypenation/spelling everywhere (code, comments, and
> documentation).

Done, thanks.

> > + *
> > + * Simply white-out a given directory entry. This functionality is usually used
> > + * in the sense of unlink. Therefore the given dentry can still be in-use and
> > + * contains an in-use inode. The filesystem has to do what unlink or rmdir
> 
> Nit: other than the line of comment just above, the other two instances of
> "in-use" in this comment (and the patch header) should be changed to "in
> use" (no hyphen).
> 
> > + * would in that case. Since the dentry still might be in-use we have to
> > + * provide a fresh unhashed dentry that whiteout can fill the new inode into.
> > + * In that case the given dentry is dropped and the fresh dentry containing the
> > + * whiteout is rehashed instead. If the given dentry is unused, the whiteout
> > + * inode is instantiated into it instead.
> > + *
> > + * After this returns with success, don't make any assumptions about the inode.
> 
> What kinds of assumptions one should not make?  Perhaps it'd be better to
> document what you can/should assume, instead of what you shouldn't (or
> both?)
> 
> > + * Just dput() it dentry.
> 
> The last line is awkward: do you mean "its dentry"?

All rewritten to address the above comments.

Thanks,

-VAL
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