Re: Adventures in NFS re-exporting

[Jeff Layton <jlayton@kernel.org>]

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

-- 
Jeff Layton <jlayton@kernel.org>