[PATCH RFC] NFS: Add some knobs for disabling delegations in sysfs

[PATCH RFC] NFS: Add some knobs for disabling delegations in sysfs

[Scott Mayhew]

There's occasionally a need to disable delegations, whether it be due to
known bugs or simply to give support staff some breathing room to
troubleshoot issues.  Currently the only real method for disabling
delegations in Linux NFS is via /proc/sys/fs/leases-enable, which has
some major drawbacks in that 1) it's only applicable to knfsd, and 2) it
affects all clients using that server.

Technically it's not really possible to disable delegations from the
client side since it's ultimately up to the server whether grants a
delegation or not, but we can achieve a similar affect in NFSv4.1+ by
manipulating the OPEN4_SHARE_ACCESS_WANT* flags.

Rather than proliferating a bunch of new mount options, add some sysfs
knobs to allow some of the nfs_server->caps flags related to delegations
to be adjusted.

Signed-off-by: Scott Mayhew <smayhew@redhat.com>
---
 fs/nfs/nfs4proc.c         |  8 ++++++
 fs/nfs/sysfs.c            | 55 +++++++++++++++++++++++++++++++++++++++
 include/linux/nfs_fs_sb.h |  1 +
 3 files changed, 64 insertions(+)

diff --git a/fs/nfs/nfs4proc.c b/fs/nfs/nfs4proc.c
index 93c6ce04332b..42810260f7d8 100644
--- a/fs/nfs/nfs4proc.c
+++ b/fs/nfs/nfs4proc.c
@@ -1396,6 +1396,11 @@ nfs4_map_atomic_open_share(struct nfs_server *server,
 		res |= NFS4_SHARE_WANT_NO_DELEG;
 		goto out;
 	}
+	/* open delegation disabled in sysfs */
+	if (!(server->caps & NFS_CAP_DELEGATION)) {
+		res |= NFS4_SHARE_WANT_NO_DELEG;
+		goto out;
+	}
 	/* res |= NFS4_SHARE_WANT_NO_PREFERENCE; */
 	if (server->caps & NFS_CAP_DELEGTIME)
 		res |= NFS4_SHARE_WANT_DELEG_TIMESTAMPS;
@@ -10796,6 +10801,7 @@ static const struct nfs4_minor_version_ops nfs_v4_0_minor_ops = {
 	.minor_version = 0,
 	.init_caps = NFS_CAP_READDIRPLUS
 		| NFS_CAP_ATOMIC_OPEN
+		| NFS_CAP_DELEGATION
 		| NFS_CAP_POSIX_LOCK,
 	.init_client = nfs40_init_client,
 	.shutdown_client = nfs40_shutdown_client,
@@ -10822,6 +10828,7 @@ static const struct nfs4_minor_version_ops nfs_v4_1_minor_ops = {
 	.minor_version = 1,
 	.init_caps = NFS_CAP_READDIRPLUS
 		| NFS_CAP_ATOMIC_OPEN
+		| NFS_CAP_DELEGATION
 		| NFS_CAP_POSIX_LOCK
 		| NFS_CAP_STATEID_NFSV41
 		| NFS_CAP_ATOMIC_OPEN_V1
@@ -10848,6 +10855,7 @@ static const struct nfs4_minor_version_ops nfs_v4_2_minor_ops = {
 	.minor_version = 2,
 	.init_caps = NFS_CAP_READDIRPLUS
 		| NFS_CAP_ATOMIC_OPEN
+		| NFS_CAP_DELEGATION
 		| NFS_CAP_POSIX_LOCK
 		| NFS_CAP_STATEID_NFSV41
 		| NFS_CAP_ATOMIC_OPEN_V1
diff --git a/fs/nfs/sysfs.c b/fs/nfs/sysfs.c
index ea6e6168092b..cce2b7f594d5 100644
--- a/fs/nfs/sysfs.c
+++ b/fs/nfs/sysfs.c
@@ -323,6 +323,43 @@ implid_name_show(struct kobject *kobj, struct kobj_attribute *attr,
 
 static struct kobj_attribute nfs_sysfs_attr_implid_name = __ATTR_RO(implid_name);
 
+#define DEFINE_NFS_SYSFS_DELEG_KNOB(name, capname)			\
+	static ssize_t							\
+	name##_show(struct kobject *kobj, struct kobj_attribute *attr,	\
+					char *buf)			\
+	{								\
+		struct nfs_server *server = container_of(kobj, struct nfs_server, kobj); \
+		bool val = server->caps & NFS_CAP_##capname;		\
+									\
+		return sysfs_emit(buf, "%d\n", val);			\
+	}								\
+									\
+	static ssize_t							\
+	name##_store(struct kobject *kobj, struct kobj_attribute *attr,	\
+					const char *buf, size_t count)	\
+	{								\
+		struct nfs_server *server = container_of(kobj, struct nfs_server, kobj); \
+		bool val;						\
+		int ret;						\
+									\
+		ret = kstrtobool(buf, &val);				\
+		if (ret < 0)						\
+			return ret;					\
+									\
+		if (val == true)					\
+			server->caps |= NFS_CAP_##capname;		\
+		else							\
+			server->caps &= ~NFS_CAP_##capname;		\
+									\
+		return count;						\
+	}								\
+									\
+	static struct kobj_attribute nfs_sysfs_attr_##name = __ATTR_RW(name); \
+
+DEFINE_NFS_SYSFS_DELEG_KNOB(open_deleg, DELEGATION)
+DEFINE_NFS_SYSFS_DELEG_KNOB(open_xor_deleg, OPEN_XOR)
+DEFINE_NFS_SYSFS_DELEG_KNOB(timestamp_deleg, DELEGTIME)
+
 #endif /* IS_ENABLED(CONFIG_NFS_V4_1) */
 
 #define RPC_CLIENT_NAME_SIZE 64
@@ -381,6 +418,24 @@ static void nfs_sysfs_add_nfsv41_server(struct nfs_server *server)
 	if (ret < 0)
 		pr_warn("NFS: sysfs_create_file_ns for server-%d failed (%d)\n",
 			server->s_sysfs_id, ret);
+
+	ret = sysfs_create_file_ns(&server->kobj, &nfs_sysfs_attr_open_deleg.attr,
+				   nfs_netns_server_namespace(&server->kobj));
+	if (ret < 0)
+		pr_warn("NFS: sysfs_create_file_ns for server-%d failed (%d)\n",
+			server->s_sysfs_id, ret);
+
+	ret = sysfs_create_file_ns(&server->kobj, &nfs_sysfs_attr_open_xor_deleg.attr,
+				   nfs_netns_server_namespace(&server->kobj));
+	if (ret < 0)
+		pr_warn("NFS: sysfs_create_file_ns for server-%d failed (%d)\n",
+			server->s_sysfs_id, ret);
+
+	ret = sysfs_create_file_ns(&server->kobj, &nfs_sysfs_attr_timestamp_deleg.attr,
+				   nfs_netns_server_namespace(&server->kobj));
+	if (ret < 0)
+		pr_warn("NFS: sysfs_create_file_ns for server-%d failed (%d)\n",
+			server->s_sysfs_id, ret);
 }
 #else /* CONFIG_NFS_V4_1 */
 static inline void nfs_sysfs_add_nfsv41_server(struct nfs_server *server)
diff --git a/include/linux/nfs_fs_sb.h b/include/linux/nfs_fs_sb.h
index d30c0245031c..dcbb4ce6b3fd 100644
--- a/include/linux/nfs_fs_sb.h
+++ b/include/linux/nfs_fs_sb.h
@@ -305,6 +305,7 @@ struct nfs_server {
 #define NFS_CAP_REBOOT_LAYOUTRETURN	(1U << 8)
 #define NFS_CAP_OFFLOAD_STATUS	(1U << 9)
 #define NFS_CAP_ZERO_RANGE	(1U << 10)
+#define NFS_CAP_DELEGATION	(1U << 11)
 #define NFS_CAP_OPEN_XOR	(1U << 12)
 #define NFS_CAP_DELEGTIME	(1U << 13)
 #define NFS_CAP_POSIX_LOCK	(1U << 14)
-- 
2.51.0

Re: [PATCH RFC] NFS: Add some knobs for disabling delegations in sysfs

[Trond Myklebust]

Hi Scott,

On Mon, 2025-11-24 at 19:15 -0500, Scott Mayhew wrote:
> There's occasionally a need to disable delegations, whether it be due
> to
> known bugs or simply to give support staff some breathing room to
> troubleshoot issues.  Currently the only real method for disabling
> delegations in Linux NFS is via /proc/sys/fs/leases-enable, which has
> some major drawbacks in that 1) it's only applicable to knfsd, and 2)
> it
> affects all clients using that server.
> 
> Technically it's not really possible to disable delegations from the
> client side since it's ultimately up to the server whether grants a
> delegation or not, but we can achieve a similar affect in NFSv4.1+ by
> manipulating the OPEN4_SHARE_ACCESS_WANT* flags.
> 
> Rather than proliferating a bunch of new mount options, add some
> sysfs
> knobs to allow some of the nfs_server->caps flags related to
> delegations
> to be adjusted.
> 

Shouldn't we rather be allowing the application to select whether it
wants to request a delegation or not?

IOW: while there may or may not be a place for a 'big hammer' solution
like you propose, should we not rather first try to enable a solution
in which someone could add a O_DELEGATION or O_NODELEGATION flag to
open() in order to specify what they want.

That might also allow someone to add an LD_PRELOAD library to add or
remove these flags from an existing application's open() calls.

It might also be useful for the directory delegation functionality that
Anna and Jeff have been working on...

Just some food for thought while you're digesting on Thursday :-)...

-- 
Trond Myklebust
Linux NFS client maintainer, Hammerspace
trondmy@kernel.org, trond.myklebust@hammerspace.com

Re: [PATCH RFC] NFS: Add some knobs for disabling delegations in sysfs

[Scott Mayhew]

On Mon, 24 Nov 2025, Trond Myklebust wrote:

> Hi Scott,
> 
> On Mon, 2025-11-24 at 19:15 -0500, Scott Mayhew wrote:
> > There's occasionally a need to disable delegations, whether it be due
> > to
> > known bugs or simply to give support staff some breathing room to
> > troubleshoot issues.  Currently the only real method for disabling
> > delegations in Linux NFS is via /proc/sys/fs/leases-enable, which has
> > some major drawbacks in that 1) it's only applicable to knfsd, and 2)
> > it
> > affects all clients using that server.
> > 
> > Technically it's not really possible to disable delegations from the
> > client side since it's ultimately up to the server whether grants a
> > delegation or not, but we can achieve a similar affect in NFSv4.1+ by
> > manipulating the OPEN4_SHARE_ACCESS_WANT* flags.
> > 
> > Rather than proliferating a bunch of new mount options, add some
> > sysfs
> > knobs to allow some of the nfs_server->caps flags related to
> > delegations
> > to be adjusted.
> > 
> 
> Shouldn't we rather be allowing the application to select whether it
> wants to request a delegation or not?
> 
> IOW: while there may or may not be a place for a 'big hammer' solution
> like you propose, should we not rather first try to enable a solution
> in which someone could add a O_DELEGATION or O_NODELEGATION flag to
> open() in order to specify what they want.
> 
> That might also allow someone to add an LD_PRELOAD library to add or
> remove these flags from an existing application's open() calls.

Sure, allowing the application to specify whether it wants a delegation
or not would be better... but with open() being governed by POSIX and
with delegations being an NFS-specific feature, I figured adding open
flags related to delegations wasn't feasible.  I discussed it with Olga,
and she had the same opinion.  Are we mistaken here?

> 
> It might also be useful for the directory delegation functionality that
> Anna and Jeff have been working on...
> 
> Just some food for thought while you're digesting on Thursday :-)...
> 
> -- 
> Trond Myklebust
> Linux NFS client maintainer, Hammerspace
> trondmy@kernel.org, trond.myklebust@hammerspace.com
> 

Re: [PATCH RFC] NFS: Add some knobs for disabling delegations in sysfs

[Trond Myklebust]

On Mon, 2025-12-01 at 06:34 -0500, Scott Mayhew wrote:
> On Mon, 24 Nov 2025, Trond Myklebust wrote:
> 
> > Hi Scott,
> > 
> > On Mon, 2025-11-24 at 19:15 -0500, Scott Mayhew wrote:
> > > There's occasionally a need to disable delegations, whether it be
> > > due
> > > to
> > > known bugs or simply to give support staff some breathing room to
> > > troubleshoot issues.  Currently the only real method for
> > > disabling
> > > delegations in Linux NFS is via /proc/sys/fs/leases-enable, which
> > > has
> > > some major drawbacks in that 1) it's only applicable to knfsd,
> > > and 2)
> > > it
> > > affects all clients using that server.
> > > 
> > > Technically it's not really possible to disable delegations from
> > > the
> > > client side since it's ultimately up to the server whether grants
> > > a
> > > delegation or not, but we can achieve a similar affect in
> > > NFSv4.1+ by
> > > manipulating the OPEN4_SHARE_ACCESS_WANT* flags.
> > > 
> > > Rather than proliferating a bunch of new mount options, add some
> > > sysfs
> > > knobs to allow some of the nfs_server->caps flags related to
> > > delegations
> > > to be adjusted.
> > > 
> > 
> > Shouldn't we rather be allowing the application to select whether
> > it
> > wants to request a delegation or not?
> > 
> > IOW: while there may or may not be a place for a 'big hammer'
> > solution
> > like you propose, should we not rather first try to enable a
> > solution
> > in which someone could add a O_DELEGATION or O_NODELEGATION flag to
> > open() in order to specify what they want.
> > 
> > That might also allow someone to add an LD_PRELOAD library to add
> > or
> > remove these flags from an existing application's open() calls.
> 
> Sure, allowing the application to specify whether it wants a
> delegation
> or not would be better... but with open() being governed by POSIX and
> with delegations being an NFS-specific feature, I figured adding open
> flags related to delegations wasn't feasible.  I discussed it with
> Olga,
> and she had the same opinion.  Are we mistaken here?
> 

O_DIRECT, O_NOATIME, O_PATH and O_TMPFILE are all open modes that are
unsupported by POSIX.
Furthermore, the openat2() system call is designed to be extensible.

> > 
> > It might also be useful for the directory delegation functionality
> > that
> > Anna and Jeff have been working on...
> > 
> > Just some food for thought while you're digesting on Thursday :-
> > )...
> > 

-- 
Trond Myklebust
Linux NFS client maintainer, Hammerspace
trondmy@kernel.org, trond.myklebust@hammerspace.com

Re: [PATCH RFC] NFS: Add some knobs for disabling delegations in sysfs

[Olga Kornievskaia]

On Mon, Nov 24, 2025 at 8:01 PM Trond Myklebust <trondmy@kernel.org> wrote:
>
> Hi Scott,
>
> On Mon, 2025-11-24 at 19:15 -0500, Scott Mayhew wrote:
> > There's occasionally a need to disable delegations, whether it be due
> > to
> > known bugs or simply to give support staff some breathing room to
> > troubleshoot issues.  Currently the only real method for disabling
> > delegations in Linux NFS is via /proc/sys/fs/leases-enable, which has
> > some major drawbacks in that 1) it's only applicable to knfsd, and 2)
> > it
> > affects all clients using that server.
> >
> > Technically it's not really possible to disable delegations from the
> > client side since it's ultimately up to the server whether grants a
> > delegation or not, but we can achieve a similar affect in NFSv4.1+ by
> > manipulating the OPEN4_SHARE_ACCESS_WANT* flags.
> >
> > Rather than proliferating a bunch of new mount options, add some
> > sysfs
> > knobs to allow some of the nfs_server->caps flags related to
> > delegations
> > to be adjusted.
> >
>
> Shouldn't we rather be allowing the application to select whether it
> wants to request a delegation or not?
>
> IOW: while there may or may not be a place for a 'big hammer' solution
> like you propose, should we not rather first try to enable a solution
> in which someone could add a O_DELEGATION or O_NODELEGATION flag to
> open() in order to specify what they want.

Hi Trond,

Shouldn't open flags be something that's a generic concept for all
filesystems? O_DELEGATION seems to be NFS specific (or at most network
filesystem specific).

> That might also allow someone to add an LD_PRELOAD library to add or
> remove these flags from an existing application's open() calls.
>
> It might also be useful for the directory delegation functionality that
> Anna and Jeff have been working on...

Being able to control directory delegations feature (might be) useful
too? We used to be able to toggle features via mount options, wouldn't
it be nice if we could again.... Toggling features via sysfs is useful
for when mount is already in place though.

>
> Just some food for thought while you're digesting on Thursday :-)...
>
> --
> Trond Myklebust
> Linux NFS client maintainer, Hammerspace
> trondmy@kernel.org, trond.myklebust@hammerspace.com
>

Re: [PATCH RFC] NFS: Add some knobs for disabling delegations in sysfs

[Trond Myklebust]

On Tue, 2026-01-20 at 15:46 -0500, Olga Kornievskaia wrote:
> On Mon, Nov 24, 2025 at 8:01 PM Trond Myklebust <trondmy@kernel.org>
> wrote:
> > 
> > Hi Scott,
> > 
> > On Mon, 2025-11-24 at 19:15 -0500, Scott Mayhew wrote:
> > > There's occasionally a need to disable delegations, whether it be
> > > due
> > > to
> > > known bugs or simply to give support staff some breathing room to
> > > troubleshoot issues.  Currently the only real method for
> > > disabling
> > > delegations in Linux NFS is via /proc/sys/fs/leases-enable, which
> > > has
> > > some major drawbacks in that 1) it's only applicable to knfsd,
> > > and 2)
> > > it
> > > affects all clients using that server.
> > > 
> > > Technically it's not really possible to disable delegations from
> > > the
> > > client side since it's ultimately up to the server whether grants
> > > a
> > > delegation or not, but we can achieve a similar affect in
> > > NFSv4.1+ by
> > > manipulating the OPEN4_SHARE_ACCESS_WANT* flags.
> > > 
> > > Rather than proliferating a bunch of new mount options, add some
> > > sysfs
> > > knobs to allow some of the nfs_server->caps flags related to
> > > delegations
> > > to be adjusted.
> > > 
> > 
> > Shouldn't we rather be allowing the application to select whether
> > it
> > wants to request a delegation or not?
> > 
> > IOW: while there may or may not be a place for a 'big hammer'
> > solution
> > like you propose, should we not rather first try to enable a
> > solution
> > in which someone could add a O_DELEGATION or O_NODELEGATION flag to
> > open() in order to specify what they want.
> 
> Hi Trond,
> 
> Shouldn't open flags be something that's a generic concept for all
> filesystems? O_DELEGATION seems to be NFS specific (or at most
> network
> filesystem specific).
> 
> > That might also allow someone to add an LD_PRELOAD library to add
> > or
> > remove these flags from an existing application's open() calls.
> > 
> > It might also be useful for the directory delegation functionality
> > that
> > Anna and Jeff have been working on...
> 
> Being able to control directory delegations feature (might be) useful
> too? We used to be able to toggle features via mount options,
> wouldn't
> it be nice if we could again.... Toggling features via sysfs is
> useful
> for when mount is already in place though.
> > 

Isn't Jeff Layton in the process of exporting the NFSv4 delegation
functionality to userspace through the lease mechanisms?

-- 
Trond Myklebust
Linux NFS client maintainer, Hammerspace
trondmy@kernel.org, trond.myklebust@hammerspace.com

Re: [PATCH RFC] NFS: Add some knobs for disabling delegations in sysfs

[Jeff Layton]

On Tue, 2026-01-20 at 18:22 -0500, Trond Myklebust wrote:
> On Tue, 2026-01-20 at 15:46 -0500, Olga Kornievskaia wrote:
> > On Mon, Nov 24, 2025 at 8:01 PM Trond Myklebust <trondmy@kernel.org>
> > wrote:
> > > 
> > > Hi Scott,
> > > 
> > > On Mon, 2025-11-24 at 19:15 -0500, Scott Mayhew wrote:
> > > > There's occasionally a need to disable delegations, whether it be
> > > > due
> > > > to
> > > > known bugs or simply to give support staff some breathing room to
> > > > troubleshoot issues.  Currently the only real method for
> > > > disabling
> > > > delegations in Linux NFS is via /proc/sys/fs/leases-enable, which
> > > > has
> > > > some major drawbacks in that 1) it's only applicable to knfsd,
> > > > and 2)
> > > > it
> > > > affects all clients using that server.
> > > > 
> > > > Technically it's not really possible to disable delegations from
> > > > the
> > > > client side since it's ultimately up to the server whether grants
> > > > a
> > > > delegation or not, but we can achieve a similar affect in
> > > > NFSv4.1+ by
> > > > manipulating the OPEN4_SHARE_ACCESS_WANT* flags.
> > > > 
> > > > Rather than proliferating a bunch of new mount options, add some
> > > > sysfs
> > > > knobs to allow some of the nfs_server->caps flags related to
> > > > delegations
> > > > to be adjusted.
> > > > 
> > > 
> > > Shouldn't we rather be allowing the application to select whether
> > > it
> > > wants to request a delegation or not?
> > > 
> > > IOW: while there may or may not be a place for a 'big hammer'
> > > solution
> > > like you propose, should we not rather first try to enable a
> > > solution
> > > in which someone could add a O_DELEGATION or O_NODELEGATION flag to
> > > open() in order to specify what they want.
> > 
> > Hi Trond,
> > 
> > Shouldn't open flags be something that's a generic concept for all
> > filesystems? O_DELEGATION seems to be NFS specific (or at most
> > network
> > filesystem specific).
> > 
> > > That might also allow someone to add an LD_PRELOAD library to add
> > > or
> > > remove these flags from an existing application's open() calls.
> > > 
> > > It might also be useful for the directory delegation functionality
> > > that
> > > Anna and Jeff have been working on...
> > 
> > Being able to control directory delegations feature (might be) useful
> > too? We used to be able to toggle features via mount options,
> > wouldn't
> > it be nice if we could again.... Toggling features via sysfs is
> > useful
> > for when mount is already in place though.
> > > 
> 
> Isn't Jeff Layton in the process of exporting the NFSv4 delegation
> functionality to userspace through the lease mechanisms?

It's not quite the same thing, as they aren't acquired at open time.
Similar to leases, you have to call fcntl(..., F_SETDELEG, ...) to get
one. It doesn't seem like it really meshes with the idea of an
O_NODELEGATION flag.

Being able to disable delegations on a per-mount basis seems like it
could be useful though.

-- 
Jeff Layton <jlayton@kernel.org>