linux-kernel - Re: [PATCH] fs: ext4: inode->i

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Date:   Mon, 3 Jul 2017 21:04:46 -0700
From:   "Darrick J. Wong" <darrick.wong@...cle.com>
To:     "J. Bruce Fields" <bfields@...ldses.org>
Cc:     William Koh <kkc6196@...com>, Andreas Dilger <adilger@...ger.ca>,
        "Theodore Ts'o" <tytso@....edu>,
        linux-ext4 <linux-ext4@...r.kernel.org>,
        lkml <linux-kernel@...r.kernel.org>,
        Kernel Team <Kernel-team@...com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Trond Myklebust <trond.myklebust@...marydata.com>,
        xfs <linux-xfs@...r.kernel.org>
Subject: Re: [PATCH] fs: ext4: inode->i_generation not assigned 0.

On Thu, Jun 29, 2017 at 02:50:22PM -0400, J. Bruce Fields wrote:
> 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.

The best guess I can come up with is the old nfs_fhbase_old style handles,
which (afaict) do not carry parent i_generation?

--D

> 
> --b.