Re: [PATCH v14 15/25] nfs_common: introduce nfs_localio_ctx struct and interfaces

[Mike Snitzer <snitzer@kernel.org>]

On Fri, Aug 30, 2024 at 01:01:55AM -0400, Mike Snitzer wrote:
> On Fri, Aug 30, 2024 at 02:36:13PM +1000, NeilBrown wrote:
> > On Fri, 30 Aug 2024, Jeff Layton wrote:
> > > 
> > > Have a pointer to a struct nfsd_localio_ops or something in the
> > > nfs_common module. That's initially set to NULL. Then, have a static
> > > structure of that type in nfsd.ko, and have its __init routine set the
> > > pointer in nfs_common to point to the right structure. The __exit
> > > routine will later set it to NULL.
> > > 
> > > > I really don't want all calls in NFS client (or nfs_common) to have to
> > > > first check if nfs_common's 'nfs_to' ops structure is NULL or not.
> > > 
> > > Neil seems to think that's not necessary:
> > > 
> > > "If nfs/localio holds an auth_domain, then it implicitly holds a
> > > reference to the nfsd module and the functions cannot disappear."
> > 
> > On reflection that isn't quite right, but it is the sort of approach
> > that I think we need to take.
> > There are several things that the NFS client needs to hold one to.
> > 
> > 1/ It needs a reference to the nfsd module (or symbols in the module).
> >    I think this can be held long term but we need a clear mechanism for
> >    it to be dropped.
> > 2/ It needs a reference to the nfsd_serv which it gets through the
> >    'struct net' pointer.  I've posted patches to handle that better.
> > 3/ It needs a reference to an auth_domain.  This can safely be a long
> >    term reference.  It can already be invalidated and the code to free
> >    it is in sunrpc which nfs already pins.  Any delay in freeing it only
> >    wastes memory (not much), it doesn't impact anything else.
> > 4/ It needs a reference to the nfsd_file and/or file.  This is currently
> >    done only while the ref to the nfsd_serv is held, so I think there is
> >    no problem there.
> > 
> > So possibly we could take a reference to the nfsd module whenever we
> > store a net in nfs_uuid. and drop the ref whenever we clear that.
> > 
> > That means we cannot call nfsd_open_local_fh() without first getting a
> > ref on the nfsd_serv which my latest code doesn't do.  That is easily
> > fixed.  I'll send a patch for consideration...
> 
> I already implemented 2 different versions today, meant for v15.
> 
> First is a relaxed version of the v14 code (less code, only using
> symbol_request on nfsd_open_local_fh.
> 
> Second is much more relaxed, because it leverages your original
> assumption that the auth_domain ref sufficient.

Corresponding code needed in fs/nfsd/localio.c:

static const struct nfsd_localio_operations nfsd_localio_ops = {
        .nfsd_open_local_fh = nfsd_open_local_fh,
        .nfsd_file_get = nfsd_file_get,
        .nfsd_file_put = nfsd_file_put,
        .nfsd_file_file = nfsd_file_file,
        .nfsd_serv_put = nfsd_serv_put,
};

void nfsd_localio_ops_init(void)
{
        memcpy(&nfs_to, &nfsd_localio_ops, sizeof(nfsd_localio_ops));
}

From: Mike Snitzer <snitzer@kernel.org>
Date: Wed, 28 Aug 2024 17:04:44 -0500
Subject: [PATCH v15.option2] nfs_common: introduce nfs_localio_ctx struct and interfaces

Introduce struct nfs_localio_ctx (which has nfsd_file and nfsd_net
members) and the interfaces nfs_localio_ctx_alloc() and
nfs_localio_ctx_free().  The next commit will introduce
nfsd_open_local_fh() which returns a nfs_localio_ctx structure.

Also, expose localio's required NFSD symbols to NFS client:
- Make nfsd_open_local_fh() symbol and other required NFSD symbols
  available to NFS in a global 'nfs_to' nfsd_localio_operations
  struct. The next commit will also introduce nfsd_localio_ops_init()
  that init_nfsd() will call to initialize 'nfs_to'.

- Introduce nfsd_file_file() wrapper that provides access to
  nfsd_file's backing file.  Keeps nfsd_file structure opaque to NFS
  client (as suggested by Jeff Layton).

- The addition of nfsd_file_get, nfsd_file_put and nfsd_file_file
  symbols prepares for the NFS client to use nfsd_file for localio.

Suggested-by: Trond Myklebust <trond.myklebust@hammerspace.com> # nfs_to
Suggested-by: NeilBrown <neilb@suse.de> # nfsd_localio_operations
Suggested-by: Jeff Layton <jlayton@kernel.org> # nfsd_file_file
Signed-off-by: Mike Snitzer <snitzer@kernel.org>
---
 fs/nfs_common/nfslocalio.c | 62 ++++++++++++++++++++++++++++++++++++++
 fs/nfsd/filecache.c        | 16 ++++++++++
 fs/nfsd/filecache.h        |  1 +
 fs/nfsd/nfssvc.c           |  2 ++
 include/linux/nfslocalio.h | 43 ++++++++++++++++++++++++++
 5 files changed, 124 insertions(+)

diff --git a/fs/nfs_common/nfslocalio.c b/fs/nfs_common/nfslocalio.c
index 0b2e17c2068f..175064e37a75 100644
--- a/fs/nfs_common/nfslocalio.c
+++ b/fs/nfs_common/nfslocalio.c
@@ -72,3 +72,65 @@ bool nfs_uuid_is_local(const uuid_t *uuid, struct net *net, struct auth_domain *
 	return is_local;
 }
 EXPORT_SYMBOL_GPL(nfs_uuid_is_local);
+
+/*
+ * The NFS LOCALIO code needs to call into NFSD using various symbols,
+ * but cannot be statically linked, because that will make the NFS
+ * module always depend on the NFSD module.
+ *
+ * 'nfs_to' provides NFS access to NFSD functions needed for LOCALIO,
+ * its lifetime is tightly coupled to the NFSD module and will always
+ * be available to NFS LOCALIO because any successful client<->server
+ * LOCALIO handshake results in a reference taken on an auth_domain,
+ * so NFS implicitly holds a reference to the NFSD module and its
+ * functions in the 'nfs_to' nfsd_localio_operations cannot disappear.
+ *
+ * If the last NFS client using LOCALIO disconnects (and its reference
+ * on NFSD dropped) then NFSD could be unloaded, resulting in 'nfs_to'
+ * functions being invalid pointers. But if NFSD isn't loaded then NFS
+ * will not be able to handshake with NFSD and will have no cause to
+ * try to call 'nfs_to' function pointers. If/when NFSD is reloaded it
+ * will reinitialize the 'nfs_to' function pointers and make LOCALIO
+ * possible.
+ */
+struct nfsd_localio_operations nfs_to;
+EXPORT_SYMBOL_GPL(nfs_to);
+
+/*
+ * nfs_localio_ctx cache and alloc/free interfaces.
+ */
+static struct kmem_cache *nfs_localio_ctx_cache;
+
+struct nfs_localio_ctx *nfs_localio_ctx_alloc(void)
+{
+	return kmem_cache_alloc(nfs_localio_ctx_cache,
+				GFP_KERNEL | __GFP_ZERO);
+}
+EXPORT_SYMBOL_GPL(nfs_localio_ctx_alloc);
+
+void nfs_localio_ctx_free(struct nfs_localio_ctx *localio)
+{
+	if (localio->nf)
+		nfs_to.nfsd_file_put(localio->nf);
+	if (localio->nn)
+		nfs_to.nfsd_serv_put(localio->nn);
+	kmem_cache_free(nfs_localio_ctx_cache, localio);
+}
+EXPORT_SYMBOL_GPL(nfs_localio_ctx_free);
+
+static int __init nfslocalio_init(void)
+{
+	nfs_localio_ctx_cache = KMEM_CACHE(nfs_localio_ctx, 0);
+	if (!nfs_localio_ctx_cache)
+		return -ENOMEM;
+
+	return 0;
+}
+
+static void __exit nfslocalio_exit(void)
+{
+	kmem_cache_destroy(nfs_localio_ctx_cache);
+}
+
+module_init(nfslocalio_init);
+module_exit(nfslocalio_exit);
diff --git a/fs/nfsd/filecache.c b/fs/nfsd/filecache.c
index 2dc72de31f61..1a26f5812578 100644
--- a/fs/nfsd/filecache.c
+++ b/fs/nfsd/filecache.c
@@ -39,6 +39,7 @@
 #include <linux/fsnotify.h>
 #include <linux/seq_file.h>
 #include <linux/rhashtable.h>
+#include <linux/nfslocalio.h>
 
 #include "vfs.h"
 #include "nfsd.h"
@@ -345,6 +346,7 @@ nfsd_file_get(struct nfsd_file *nf)
 		return nf;
 	return NULL;
 }
+EXPORT_SYMBOL_GPL(nfsd_file_get);
 
 /**
  * nfsd_file_put - put the reference to a nfsd_file
@@ -389,6 +391,20 @@ nfsd_file_put(struct nfsd_file *nf)
 	if (refcount_dec_and_test(&nf->nf_ref))
 		nfsd_file_free(nf);
 }
+EXPORT_SYMBOL_GPL(nfsd_file_put);
+
+/**
+ * nfsd_file_file - get the backing file of an nfsd_file
+ * @nf: nfsd_file of which to access the backing file.
+ *
+ * Return backing file for @nf.
+ */
+struct file *
+nfsd_file_file(struct nfsd_file *nf)
+{
+	return nf->nf_file;
+}
+EXPORT_SYMBOL_GPL(nfsd_file_file);
 
 static void
 nfsd_file_dispose_list(struct list_head *dispose)
diff --git a/fs/nfsd/filecache.h b/fs/nfsd/filecache.h
index 26ada78b8c1e..6fbbb2e32e95 100644
--- a/fs/nfsd/filecache.h
+++ b/fs/nfsd/filecache.h
@@ -56,6 +56,7 @@ int nfsd_file_cache_start_net(struct net *net);
 void nfsd_file_cache_shutdown_net(struct net *net);
 void nfsd_file_put(struct nfsd_file *nf);
 struct nfsd_file *nfsd_file_get(struct nfsd_file *nf);
+struct file *nfsd_file_file(struct nfsd_file *nf);
 void nfsd_file_close_inode_sync(struct inode *inode);
 void nfsd_file_net_dispose(struct nfsd_net *nn);
 bool nfsd_file_is_cached(struct inode *inode);
diff --git a/fs/nfsd/nfssvc.c b/fs/nfsd/nfssvc.c
index c639fbe4d8c2..7b9119b8dd1b 100644
--- a/fs/nfsd/nfssvc.c
+++ b/fs/nfsd/nfssvc.c
@@ -19,6 +19,7 @@
 #include <linux/sunrpc/svc_xprt.h>
 #include <linux/lockd/bind.h>
 #include <linux/nfsacl.h>
+#include <linux/nfslocalio.h>
 #include <linux/seq_file.h>
 #include <linux/inetdevice.h>
 #include <net/addrconf.h>
@@ -201,6 +202,7 @@ void nfsd_serv_put(struct nfsd_net *nn)
 {
 	percpu_ref_put(&nn->nfsd_serv_ref);
 }
+EXPORT_SYMBOL_GPL(nfsd_serv_put);
 
 static void nfsd_serv_done(struct percpu_ref *ref)
 {
diff --git a/include/linux/nfslocalio.h b/include/linux/nfslocalio.h
index 9735ae8d3e5e..fdb1f278afb6 100644
--- a/include/linux/nfslocalio.h
+++ b/include/linux/nfslocalio.h
@@ -7,6 +7,7 @@
 
 #include <linux/list.h>
 #include <linux/uuid.h>
+#include <linux/sunrpc/clnt.h>
 #include <linux/sunrpc/svcauth.h>
 #include <linux/nfs.h>
 #include <net/net_namespace.h>
@@ -28,4 +29,46 @@ void nfs_uuid_begin(nfs_uuid_t *);
 void nfs_uuid_end(nfs_uuid_t *);
 bool nfs_uuid_is_local(const uuid_t *, struct net *, struct auth_domain *);
 
+struct nfsd_file;
+struct nfsd_net;
+
+struct nfs_localio_ctx {
+	struct nfsd_file *nf;
+	struct nfsd_net *nn;
+};
+
+/* localio needs to map filehandle -> struct nfsd_file */
+typedef struct nfs_localio_ctx *
+(*nfs_to_nfsd_open_local_fh_t)(struct net *, struct auth_domain *,
+			       struct rpc_clnt *, const struct cred *,
+			       const struct nfs_fh *, const fmode_t);
+
+extern struct nfs_localio_ctx *
+nfsd_open_local_fh(struct net *, struct auth_domain *,
+		   struct rpc_clnt *, const struct cred *,
+		   const struct nfs_fh *, const fmode_t);
+
+/* localio needs to acquire an nfsd_file */
+typedef struct nfsd_file * (*nfs_to_nfsd_file_get_t)(struct nfsd_file *);
+/* localio needs to release an nfsd_file */
+typedef void (*nfs_to_nfsd_file_put_t)(struct nfsd_file *);
+/* localio needs to access the nf->nf_file */
+typedef struct file * (*nfs_to_nfsd_file_file_t)(struct nfsd_file *);
+/* localio needs to release nn->nfsd_serv */
+typedef void (*nfs_to_nfsd_serv_put_t)(struct nfsd_net *);
+
+struct nfsd_localio_operations {
+	nfs_to_nfsd_open_local_fh_t	nfsd_open_local_fh;
+	nfs_to_nfsd_file_get_t		nfsd_file_get;
+	nfs_to_nfsd_file_put_t		nfsd_file_put;
+	nfs_to_nfsd_file_file_t		nfsd_file_file;
+	nfs_to_nfsd_serv_put_t		nfsd_serv_put;
+} ____cacheline_aligned;
+
+extern void nfsd_localio_ops_init(void);
+extern struct nfsd_localio_operations nfs_to;
+
+struct nfs_localio_ctx *nfs_localio_ctx_alloc(void);
+void nfs_localio_ctx_free(struct nfs_localio_ctx *);
+
 #endif  /* __LINUX_NFSLOCALIO_H */
-- 
2.44.0