linux-kernel - Re: [PATCH 12/41] union-mount: Allow removal of a directory

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Message-Id: <200911300613.nAU6Da0Z022008@agora.fsl.cs.sunysb.edu>
Date:	Mon, 30 Nov 2009 01:13:36 -0500
From:	Erez Zadok <ezk@...sunysb.edu>
To:	Valerie Aurora <vaurora@...hat.com>
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
Subject: Re: [PATCH 12/41] union-mount: Allow removal of a directory 

In message <1256152779-10054-13-git-send-email-vaurora@...hat.com>, Valerie Aurora writes:
> From: Jan Blunck <jblunck@...e.de>
> 
> do_whiteout() allows removal of a directory when it has whiteouts but
> is logically empty.
> 
> XXX - This patch abuses readdir() to check if the union directory is
> logically empty - that is, all the entries are whiteouts (or "." or
> "..").  Currently, we have no clean VFS interface to ask the lower
> file system if a directory is empty.
> 
> Fixes:
>  - Add ->is_directory_empty() op
>  - Add is_directory_empty flag to dentry (ugly dcache populate)
>  - Ask underlying fs to remove it and look for an error return
>  - (your idea here)

Yeah, this is a difficult issue.  I think the best way would be to

1. add an OPTIONAL ->is_directory_empty() inode op.

2. have the VFS use some default/generic behavior ala filldir_is_empty()
   below if inode->i_op->is_directory_empty is NULL.  I assume this behavior
   will only need to be checked for file systems that support whiteouts in
   the first place.

This'll provide some working behavior for all whiteout-supporting file
systems, but allow anyone who wants to develop a more efficient method to
provide one.

> Signed-off-by: Jan Blunck <jblunck@...e.de>
> Signed-off-by: Valerie Aurora <vaurora@...hat.com>
> ---
>  fs/namei.c |   85 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 files changed, 85 insertions(+), 0 deletions(-)
> 
> diff --git a/fs/namei.c b/fs/namei.c
> index 5da1635..9a62c75 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -2284,6 +2284,91 @@ int vfs_whiteout(struct inode *dir, struct dentry *dentry, int isdir)
>  }
>  
>  /*
> + * This is abusing readdir to check if a union directory is logically empty.
> + * Al Viro barfed when he saw this, but Val said: "Well, at this point I'm
> + * aiming for working, pretty can come later"
> + */
> +static int filldir_is_empty(void *__buf, const char *name, int namlen,
> +			    loff_t offset, u64 ino, unsigned int d_type)
> +{

Why not make filldir_is_empty() return a bool?  That explains more clearly
the function's return code.

> +static int directory_is_empty(struct dentry *dentry, struct vfsmount *mnt)
> +{

This can also return a bool.

> +static int do_whiteout(struct nameidata *nd, struct path *path, int isdir)
> +{

'isdir' can be bool.

> +	struct path safe = { .dentry = dget(nd->path.dentry),
> +			     .mnt = mntget(nd->path.mnt) };
> +	struct dentry *dentry = path->dentry;
> +	int err;

You might want to move the initialization of 'struct path safe' down below,
and add a BUG_ON(!nd) before that.  I think during the development phases of
UM, it's a good idea to have a few more debugging BUG_ON's.

Erez.
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