Re: [PATCH v13 27/28] ovl: Verify a data dentry has been found for metacopy inode

[Vivek Goyal <vgoyal@redhat.com>]

On Fri, Mar 30, 2018 at 01:53:24PM +0300, Amir Goldstein wrote:
> On Thu, Mar 29, 2018 at 10:38 PM, Vivek Goyal <vgoyal@redhat.com> wrote:
> > If we find a upper metacopy inode, make sure we also found associated data
> > dentry in lower. Otherwise copy up operation later will fail.
> >
> > There are two cases where this can happen. First case is that somehow
> > data file was removed from lower layer. Other case is that REDIRECT
> > xattr was removed due to copy up of file on another cpu (when inode is
> > shared between two dentries) and hence ovl_lookup() could not find the
> > correct dentry.
> >
> 
> Remind me again why we remove REDIRECT xattr?
> Is it a must for functionality or just for being boy scouts?

Cleaning REDIRECT is not must for the functionality. It is just more
about cleanup as that redirect is not needed anymore.

> I would prefer to avoid having to deal with races of this sort.
> You can cleanup REDIRECT for non-dir that is not metacopy
> on lookup when finding a I_NEW inode.

Hmm.., ok, that sounds reasonable too. Can look into it.

> 
> 
> > First case is an error while second case is not error. If file has been
> > copied up, then it does not matter if data dentry was found or not.
> >
> > Redirect removal is protected using ovl_inode->lock and ovl_lookup() does
> > not have access to that lock. So to differentiate between these two
> > cases, take appropriate inode lock in ovl_get_inode() and make sure a
> > data dentry has been found for metacopy inode. Otherwise lookup failed
> > and its an error.
> >
> > For example, say two files are hardlinked, foo.txt and bar.txt. Say foo.txt
> > is renamed to foo-renamed.txt gets copied up metadata only. This will also
> > put a redirect "/foo.txt" on hardlnk  inode. Now assume foo-renamed.txt
> > is opened for write and is undergoing data copy up on one cpu and bar.txt
> > is under going ovl_lookup() on other cpu. Data copy up path will remove
> > REDIRECT and METACOPY xattr. It is possible that METACOPY xattr is
> > visible to ovl_lookup() but by the REDIRECT xattr was gone by the time.
> > That means no data dentry will be found but at the same time now inode
> > is not metacopy inode. So data dentry is not required. So this is not
> > error case. But if inode was still metacopy but data dentry was not found
> > this is error case. (May be due to underlying layer changed). Fix it by
> > returning -ESTALE.
> >
> > If inode was found in cache, then take ovl_inode->lock before checking
> > status of inode. If inode has been allocated, then it is returned with
> > inode lock anyway and other threads will block on that lock, so no need
> > to take ovl_inode->lock.
> >
> > Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
> > ---
> >  fs/overlayfs/export.c    |  3 ++-
> >  fs/overlayfs/inode.c     | 49 +++++++++++++++++++++++++++++++++++++++++++++++-
> >  fs/overlayfs/namei.c     | 14 ++++----------
> >  fs/overlayfs/overlayfs.h |  3 ++-
> >  4 files changed, 56 insertions(+), 13 deletions(-)
> >
> > diff --git a/fs/overlayfs/export.c b/fs/overlayfs/export.c
> > index 1c233096e59c..e8575d4d2c77 100644
> > --- a/fs/overlayfs/export.c
> > +++ b/fs/overlayfs/export.c
> > @@ -305,7 +305,8 @@ static struct dentry *ovl_obtain_alias(struct super_block *sb,
> >         if (d_is_dir(upper ?: lower))
> >                 return ERR_PTR(-EIO);
> >
> > -       inode = ovl_get_inode(sb, dget(upper), lower, index, !!lower, NULL);
> > +       inode = ovl_get_inode(sb, dget(upper), lower, index, !!lower, NULL,
> > +                             false);
> >         if (IS_ERR(inode)) {
> >                 dput(upper);
> >                 return ERR_CAST(inode);
> > diff --git a/fs/overlayfs/inode.c b/fs/overlayfs/inode.c
> > index 6a0c85699024..7e30f4a7cdd9 100644
> > --- a/fs/overlayfs/inode.c
> > +++ b/fs/overlayfs/inode.c
> > @@ -685,9 +685,42 @@ static bool ovl_hash_bylower(struct super_block *sb, struct dentry *upper,
> >         return true;
> >  }
> >
> > +static bool ovl_verify_metacopy_data(struct super_block *sb,
> > +                                    struct inode *inode, bool metacopydata)
> > +{
> > +       struct ovl_fs *ofs = sb->s_fs_info;
> > +       struct ovl_inode *oi = OVL_I(inode);
> > +       bool metacopy = false;
> > +
> > +       /* A metacopy data dentry was found. So no need to do further checks */
> > +       if (metacopydata)
> > +               return true;
> > +
> > +       /*
> > +        * Metacopy feature not enabled. No metadata data copy up should take
> > +        * place. So no further checks needed.
> > +        */
> > +       if (!ofs->config.metacopy)
> > +               return true;
> > +
> > +       if (!S_ISREG(inode->i_mode))
> > +               return true;
> > +
> > +       /*
> > +        * Metacopy feature is enabled and we have not found metacopy data
> > +        * dentry. Make sure this inode is not metacopy inode.
> > +        */
> > +       mutex_lock(&oi->lock);
> > +       metacopy = !ovl_test_flag(OVL_UPPERDATA, inode);
> > +       mutex_unlock(&oi->lock);
> > +
> 
> FYI, the reason you need mutex_lock_interruptible is so if other CPU
> is busy with large copy up, the process that does lookup() can
> be interrupted by user.
> This is not likley to be a problem here, because race condition suggests that
> copy up of data is complete by now, but nevertheless.

Ok, good to know.

Vivek