From: "J. Bruce Fields" <bfields@fieldses.org>
Subject: Re: [PATCH] fs: ext4: inode->i_generation not assigned 0.
Date: Thu, 29 Jun 2017 14:50:22 -0400
Message-ID: <20170629185022.GB4178@fieldses.org>
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Cc: William Koh <kkc6196@fb.com>, Andreas Dilger <adilger@dilger.ca>,
        Theodore Ts'o <tytso@mit.edu>,
        linux-ext4 <linux-ext4@vger.kernel.org>,
        lkml <linux-kernel@vger.kernel.org>,
        Kernel Team <Kernel-team@fb.com>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Trond Myklebust <trond.myklebust@primarydata.com>,
        xfs <linux-xfs@vger.kernel.org>
To: "Darrick J. Wong" <darrick.wong@oracle.com>
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On Thu, Jun 29, 2017 at 02:30:53PM -0400, J. Bruce Fields wrote:
> On Thu, Jun 29, 2017 at 10:25:28AM -0700, Darrick J. Wong wrote:
> > Was there ever a version of NFS (or more generally callers of the
> > exportfs code) that couldn't deal with i_generation in the file handle,
> > and therefore we invented this generation hack to work around the loss
> > of the generation information?
> > 
> > There's a comment in xfs_fs_encode_fh about not supporting 64bit inodes
> > with subtree_check (which seems to require one ino/gen pair for the file
> > and a second pair for the file's parent) on NFSv2 because v2 doesn't
> > provide enough space for all the file handle information, but that's the
> > furthest I got with lazy-mining the git history. :)
> 
> There's a comment in fs/ext4/super.c:ext4_nfs_get_inode
> 
> 	* Currently we don't know the generation for parent directory, so
> 	* a generation of 0 means "accept any"
> 
> But I don't see that used.
> 
> It was used once upon a time; I see it actually used in old 2.5 code in
> nfsd_get_dentry.  Hm.

Oh, maybe it's here in fs/libfs.c:generic_fh_to_parent:

	switch (fh_type) {
	case FILEID_INO32_GEN_PARENT:
		inode = get_inode(sb, fid->i32.parent_ino,
        			  (fh_len > 3 ? fid->i32.parent_gen : 0));
		break;
	}

I'm not sure under what conditions that filehandle encoding is used.

--b.