linux-kernel - Re: [PATCH 13/41] whiteout: tmpfs whiteout support

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Date:	Wed, 20 Jan 2010 21:02:57 -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 13/41] whiteout: tmpfs whiteout support

On Mon, Nov 30, 2009 at 01:26:53AM -0500, Erez Zadok wrote:
> In message <1256152779-10054-14-git-send-email-vaurora@...hat.com>, Valerie Aurora writes:
> > From: Jan Blunck <jblunck@...e.de>
> > 
> > Add support for whiteout dentries to tmpfs.
> 
> Shouldn't you CC Hugh Dickins here?  He's probably best positioned to review
> the changes in mm/shmem.c.

Thanks, I added him and linux-mm.

> > XXX - Not sure this is the right patch to put the code for supporting
> > whiteouts in d_genocide().
> > 
> > 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 |    3 +-
> >  mm/shmem.c  |  149 +++++++++++++++++++++++++++++++++++++++++++++++++++++------
> >  2 files changed, 137 insertions(+), 15 deletions(-)
> > 
> > diff --git a/fs/dcache.c b/fs/dcache.c
> > index 0fcae4b..1fae1df 100644
> > --- a/fs/dcache.c
> > +++ b/fs/dcache.c
> > @@ -2280,7 +2280,8 @@ resume:
> >  		struct list_head *tmp = next;
> >  		struct dentry *dentry = list_entry(tmp, struct dentry, d_u.d_child);
> >  		next = tmp->next;
> > -		if (d_unhashed(dentry)||!dentry->d_inode)
> > +		if (d_unhashed(dentry)||(!dentry->d_inode &&
> > +					 !d_is_whiteout(dentry)))
> 
> I think this d_genocide patch should go elsewhere.  What does it have to do
> with tmpfs?

Without this patch, you can't unmount a tmpfs file system with
whiteouts.  d_genocide() is called by kill_litter_super() to evict all
the dcache entries used by tmpfs.

> Also, is your logic above correct?  If I understood d_genocide correctly,
> then the code you changed attempts to skip over dentries for which
> d_genocide has no work to do, like unhashed and negative dentries.  So I
> assume it should also skip over whiteout dentries.  Your condition is
> 
> if (d_unhashed(dentry) || (!dentry->d_inode && !d_is_whiteout(dentry)))
> 
> but perhaps it needs to be
> 
> if (d_unhashed(dentry) || !dentry->d_inode || d_is_whiteout(dentry))
> 
> No?
> 
> Either way, you may want to document any complex conditional that may be
> confusing to parse.

This is a good thing to document.  What we're dealing with here is
dropping the ref count on persistent dentries.  How about this comment?

  /*
   * Skip unhashed and negative dentries, but process
   * positive dentries and whiteouts.  A whiteout looks
   * kind of like a negative dentry for purposes of
   * lookup, but it has an extra pinning ref count
   * because it can't be evicted like a negative dentry
   * can.  What we care about here is ref counts - and
   * we need to drop the ref count on a whiteout before
   * we can evict it.
   */
  if (d_unhashed(dentry)||(!dentry->d_inode &&
                           !d_is_whiteout(dentry)))
          continue;

-VAL
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