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Date:   Mon, 16 Nov 2020 10:56:19 -0500
From:   bfields <bfields@fieldses.org>
To:     Jeff Layton <jlayton@kernel.org>
Cc:     Daire Byrne <daire@dneg.com>,
        Trond Myklebust <trondmy@hammerspace.com>,
        linux-cachefs <linux-cachefs@redhat.com>,
        linux-nfs <linux-nfs@vger.kernel.org>
Subject: Re: Adventures in NFS re-exporting
Message-ID: <20201116155619.GF898@fieldses.org>
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On Mon, Nov 16, 2020 at 10:29:29AM -0500, Jeff Layton wrote:
> On Fri, 2020-11-13 at 17:26 -0500, bfields wrote:
> > On Fri, Nov 13, 2020 at 09:50:50AM -0500, bfields wrote:
> > > On Thu, Nov 12, 2020 at 11:05:57PM +0000, Daire Byrne wrote:
> > > > So, I can't lay claim to identifying the exact optimisation/hack that
> > > > improves the retention of the re-export server's client cache when
> > > > re-exporting an NFSv3 server (which is then read by many clients). We
> > > > were working with an engineer at the time who showed an interest in
> > > > our use case and after we supplied a reproducer he suggested modifying
> > > > the nfs/inode.c
> > > > 
> > > > -		if (!inode_eq_iversion_raw(inode, fattr->change_attr)) {
> > > > +		if (inode_peek_iversion_raw(inode) < fattr->change_attr)
> > > > {
> > > > 
> > > > His reasoning at the time was:
> > > > 
> > > > "Fixes inode invalidation caused by read access. The least important
> > > > bit is ORed with 1 and causes the inode version to differ from the one
> > > > seen on the NFS share. This in turn causes unnecessary re-download
> > > > impacting the performance significantly. This fix makes it only
> > > > re-fetch file content if inode version seen on the server is newer
> > > > than the one on the client."
> > > > 
> > > > But I've always been puzzled by why this only seems to be the case
> > > > when using knfsd to re-export the (NFSv3) client mount. Using multiple
> > > > processes on a standard client mount never causes any similar
> > > > re-validations. And this happens with a completely read-only share
> > > > which is why I started to think it has something to do with atimes as
> > > > that could perhaps still cause a "write" modification even when
> > > > read-only?
> > > 
> > > Ah-hah!  So, it's inode_query_iversion() that's modifying a nfs inode's
> > > i_version.  That's a special thing that only nfsd would do.
> > > 
> > > I think that's totally fixable, we'll just have to think a little about
> > > how....
> > 
> > I wonder if something like this helps?--b.
> > 
> > commit 0add88a9ccc5
> > Author: J. Bruce Fields <bfields@redhat.com>
> > Date:   Fri Nov 13 17:03:04 2020 -0500
> > 
> >     nfs: don't mangle i_version on NFS
> >     
> > 
> >     The i_version on NFS has pretty much opaque to the client, so we don't
> >     want to give the low bit any special interpretation.
> >     
> > 
> >     Define a new FS_PRIVATE_I_VERSION flag for filesystems that manage the
> >     i_version on their own.
> >     
> > 
> >     Signed-off-by: J. Bruce Fields <bfields@redhat.com>
> > 
> > diff --git a/fs/nfs/fs_context.c b/fs/nfs/fs_context.c
> > index 29ec8b09a52d..9b8dd5b713a7 100644
> > --- a/fs/nfs/fs_context.c
> > +++ b/fs/nfs/fs_context.c
> > @@ -1488,7 +1488,8 @@ struct file_system_type nfs_fs_type = {
> >  	.init_fs_context	= nfs_init_fs_context,
> >  	.parameters		= nfs_fs_parameters,
> >  	.kill_sb		= nfs_kill_super,
> > -	.fs_flags		= FS_RENAME_DOES_D_MOVE|FS_BINARY_MOUNTDATA,
> > +	.fs_flags		= FS_RENAME_DOES_D_MOVE|FS_BINARY_MOUNTDATA|
> > +				  FS_PRIVATE_I_VERSION,
> >  };
> >  MODULE_ALIAS_FS("nfs");
> >  EXPORT_SYMBOL_GPL(nfs_fs_type);
> > @@ -1500,7 +1501,8 @@ struct file_system_type nfs4_fs_type = {
> >  	.init_fs_context	= nfs_init_fs_context,
> >  	.parameters		= nfs_fs_parameters,
> >  	.kill_sb		= nfs_kill_super,
> > -	.fs_flags		= FS_RENAME_DOES_D_MOVE|FS_BINARY_MOUNTDATA,
> > +	.fs_flags		= FS_RENAME_DOES_D_MOVE|FS_BINARY_MOUNTDATA|
> > +				  FS_PRIVATE_I_VERSION,
> >  };
> >  MODULE_ALIAS_FS("nfs4");
> >  MODULE_ALIAS("nfs4");
> > diff --git a/include/linux/fs.h b/include/linux/fs.h
> > index 21cc971fd960..c5bb4268228b 100644
> > --- a/include/linux/fs.h
> > +++ b/include/linux/fs.h
> > @@ -2217,6 +2217,7 @@ struct file_system_type {
> >  #define FS_HAS_SUBTYPE		4
> >  #define FS_USERNS_MOUNT		8	/* Can be mounted by userns root */
> >  #define FS_DISALLOW_NOTIFY_PERM	16	/* Disable fanotify permission events */
> > +#define FS_PRIVATE_I_VERSION	32	/* i_version managed by filesystem */
> >  #define FS_THP_SUPPORT		8192	/* Remove once all fs converted */
> >  #define FS_RENAME_DOES_D_MOVE	32768	/* FS will handle d_move() during rename() internally. */
> >  	int (*init_fs_context)(struct fs_context *);
> > diff --git a/include/linux/iversion.h b/include/linux/iversion.h
> > index 2917ef990d43..52c790a847de 100644
> > --- a/include/linux/iversion.h
> > +++ b/include/linux/iversion.h
> > @@ -307,6 +307,8 @@ inode_query_iversion(struct inode *inode)
> >  	u64 cur, old, new;
> >  
> > 
> >  	cur = inode_peek_iversion_raw(inode);
> > +	if (inode->i_sb->s_type->fs_flags & FS_PRIVATE_I_VERSION)
> > +		return cur;
> >  	for (;;) {
> >  		/* If flag is already set, then no need to swap */
> >  		if (cur & I_VERSION_QUERIED) {
> 
> 
> It's probably more correct to just check the already-existing
> SB_I_VERSION flag here

So the check would be

	if (!IS_I_VERSION(inode))
		return cur;

?

> (though in hindsight a fstype flag might have made more sense).

I_VERSION support can vary by superblock (for example, xfs supports it
or not depending on on-disk format version).

--b.