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Re: [RFC PATCH 2/2] xfs_export_operations.commit_metadata

To: Ben Myers <bpm@xxxxxxx>
Subject: Re: [RFC PATCH 2/2] xfs_export_operations.commit_metadata
From: Christoph Hellwig <hch@xxxxxxxxxxxxx>
Date: Wed, 10 Feb 2010 04:07:50 -0500
Cc: linux-nfs@xxxxxxxxxxxxxxx, xfs@xxxxxxxxxxx
In-reply-to: <20100210003337.6021.10942.stgit@case>
References: <20100210003220.6021.74943.stgit@case> <20100210003337.6021.10942.stgit@case>
User-agent: Mutt/1.5.19 (2009-01-05)
On Tue, Feb 09, 2010 at 06:33:37PM -0600, Ben Myers wrote:
> Here is the commit_metadata export_operation for xfs.  We take two dentries 
> and
> force the log up to the larger lsn.  It looks to me that in nfsd the child is
> always modified after the parent so generally we expect the child's lsn to be
> larger.  If that's not the case we'll just force the entire thing.
> The basic form of this is based upon one of Christoph's suggestions.  I'm an
> xfs newbie so I'm not very comfortable with it yet.  My understanding is that 
> I
> need to verify that all of the necessary changes make it into the transations
> we're forcing into the log here.  I am still looking into that and hopefully
> the XFS gurus can continue to provide guidance.

Ccing the xfs list would help with that :)  Anyway, I think it looks
pretty good, but there's quite a few smaller nitpicks:

> +STATIC int
> +xfs_fs_nfs_commit_metadata(
> +     struct dentry           *parent,
> +     struct dentry           *child)
> +{
> +     struct xfs_inode        *p_xip = NULL, *c_xip = NULL;

Normal xfs naming would be dp for the parent, and ip for the child,
it would be good to stick to that.

> +     struct xfs_mount        *i_mount = NULL;        

Normal name all over xfs would be mp.

> +     } else if (parent && child) {
> +             p_xip = XFS_I(parent->d_inode);
> +             c_xip = XFS_I(child->d_inode);
> +             xfs_ilock(p_xip, XFS_ILOCK_SHARED);
> +             xfs_ilock(c_xip, XFS_ILOCK_SHARED);

If we need to lock both parent and child we need to use
xfs_lock_two_inodes to make sure the lock order is correct.

> +             if (xfs_ipincount(c_xip)) {
> +                     /*
> +                      * AFAICS the child is always modified after the parent
> +                      * in nfsd so should always have a larger lsn.
> +                      */
> +                     if (c_xip->i_itemp->ili_last_lsn > force_lsn) {
> +                             force_lsn = c_xip->i_itemp->ili_last_lsn;
> +                     } else {
> +                             force_lsn = 0; /* whole thing */
> +                     }

I wouldn't rely on that and always take the larger one.

Now with the simplification of always having a non-zero first argument
suggested in the previous mail this might be simplified down to:

        struct dentry           *parent,
        struct dentry           *child)
        struct xfs_inode        *dp = XFS_I(parent->d_inode);
        struct xfs_inode        *ip = NULL;
        struct xfs_mount        *mp = dp->i_mount;
        xfs_lsn_t               force_lsn = 0;
        int                     error = 0;

        if (child) {
                ip = XFS_I(child->d_inode);
                xfs_lock_two_inodes(dp, ip, XFS_ILOCK_SHARED);
        } else {
                xfs_ilock(dp, XFS_ILOCK_SHARED);

        if (xfs_ipincount(dp))
                force_lsn = dp->i_itemp->ili_last_lsn;
        if (ip && xfs_ipincount(ip))
                force_lsn = max(force_lsn, ip->i_itemp->ili_last_lsn);

        error = _xfs_log_force_lsn(mp, force_lsn, NULL);

        if (ip)
                xfs_iunlock(ip, XFS_ILOCK_SHARED);
        xfs_iunlock(dp, XFS_ILOCK_SHARED);

        return error;

Note that _xfs_log_force_lsn is new in the XFS tree, mainline still
has _xfs_log_force with an lsn argument.

Also note that ->commit_metadata probably should take just two inodes
instead of two dentires given the level it operates on, but it shouldn't
matter too much.

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