[PATCH v3] efivarfs: add nostatfs mount option to skip QueryVariableInfo()

[PATCH v3] efivarfs: add nostatfs mount option to skip QueryVariableInfo()

[Prashant Singh]

QueryVariableInfo() is an EFI runtime service that, on some firmware,
takes tens of milliseconds and runs with preemption disabled, stalling
the CPU that services it. efivarfs_statfs() calls it (rate-limited since
commit b2326338dc68 ("efivarfs: Rate limit statfs() handler")) to report
the variable-store used/available capacity, so any statfs(2) -- e.g.
every "df" -- can inject that stall into unrelated latency-sensitive
workloads on the same CPU.

Add a negatable "nostatfs" mount option: with nostatfs, statfs(2) skips
QueryVariableInfo() and reports zero used/available; with statfs it
reports the capacity as before. It defaults to nostatfs on
CONFIG_PREEMPT_RT so real-time kernels do not take the stall out of the
box, but statfs can be passed there to force reporting back on -- e.g.
for tools such as fwupd that need the efivars free space to update Secure
Boot key databases.

The option can also be toggled on a live mount via remount, so reporting
can be enabled only for the duration of a firmware update without
unmounting the boot-time efivarfs mount:

  mount -o remount,statfs   /sys/firmware/efi/efivars   # reporting on
  mount -o remount,nostatfs /sys/firmware/efi/efivars   # reporting off

Tested on an Intel Xeon E5-2628L v4, 6.12 kernel, via
"strace -T -e trace=statfs df" on the efivarfs mount.

Cost of the firmware call (CONFIG_PREEMPT_RT not set, so reporting is
enabled by default). Default mount calls QueryVariableInfo() and returns
the real store capacity, ~66-129 ms per call:

  statfs("/sys/firmware/efi/efivars", {f_type=EFIVARFS_MAGIC,
      f_bsize=1, f_blocks=90024, f_bfree=33387, f_bavail=28267, ...})
      = 0 <0.129124>

With -o nostatfs the firmware call is skipped, capacity reported as
zero, ~11 us per call:

  statfs("/sys/firmware/efi/efivars", {f_type=EFIVARFS_MAGIC,
      f_bsize=1, f_blocks=0, f_bfree=0, f_bavail=0, ...}) = 0 <0.000011>

PREEMPT_RT default and live remount toggle (CONFIG_PREEMPT_RT=y). The
boot-time mount defaults to nostatfs (mount shows nostatfs); no firmware
call, capacity zero:

  statfs(... f_blocks=0, f_bfree=0, f_bavail=0, ...) = 0 <0.000018>

Enabling reporting in place with "mount -o remount,statfs" (mount now
shows statfs) takes the ~70 ms firmware call and returns the real
capacity, without unmounting:

  statfs(... f_blocks=90024, f_bfree=30315, f_bavail=25195, ...)
      = 0 <0.070293>

Disabling it again with "mount -o remount,nostatfs" returns to the fast,
zero-capacity path:

  statfs(... f_blocks=0, f_bfree=0, f_bavail=0, ...) = 0 <0.000014>

An unrelated remount that does not respecify the option preserves it: a
"mount -o remount,rw" after "remount,statfs" leaves the mount showing
statfs.

Suggested-by: Ard Biesheuvel <ardb@kernel.org>
Suggested-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Prashant Singh <singhpra@juniper.net>
---
Changes since v2:
- Make the flag negatable (fsparam_flag_no): "statfs" forces reporting
  back on, "nostatfs" skips QueryVariableInfo(). Default stays nostatfs
  on PREEMPT_RT. This lets fwupd get the efivars free space it needs for
  Secure Boot key (KEK/DB/DBX) updates on an RT kernel by mounting with
  -o statfs (Luis Claudio R. Goncalves, Steve McIntyre).
- Support toggling the option on a live mount via remount, so reporting
  can be enabled just for a firmware update without unmounting efivarfs
  (Sebastian Andrzej Siewior). Options not respecified on remount are
  preserved.
- Read/write nostatfs with READ_ONCE()/WRITE_ONCE(): statfs() reads it
  without s_umount while remount can update it.
- Add PREEMPT_RT + remount test data to the commit message.

Changes since v1:
- Replace the sysctl with a mount option named "nostatfs", per review
  (Ard Biesheuvel).

v1: https://lore.kernel.org/all/20260917071600.5587-1-singhpra@juniper.net/
v2: https://lore.kernel.org/all/20260919044124.8268-1-singhpra@juniper.net/

 Documentation/filesystems/efivarfs.rst | 16 +++++++++
 fs/efivarfs/internal.h                 |  1 +
 fs/efivarfs/super.c                    | 46 +++++++++++++++++++++++---
 3 files changed, 58 insertions(+), 5 deletions(-)

diff --git a/Documentation/filesystems/efivarfs.rst b/Documentation/filesystems/efivarfs.rst
index f646c3f0980f..cd81ee84115b 100644
--- a/Documentation/filesystems/efivarfs.rst
+++ b/Documentation/filesystems/efivarfs.rst
@@ -37,6 +37,22 @@ accidentally.
           |4_bytes_of_attributes + efivar_data|
           +-----------------------------------+
 
+Mount options
+=============
+
+statfs / nostatfs
+      Control whether ``statfs(2)`` reports the variable-store used/available
+      capacity. Obtaining it requires the ``QueryVariableInfo()`` EFI runtime
+      service, which on some firmware takes tens of milliseconds and runs with
+      preemption disabled, stalling the calling CPU. With ``nostatfs`` the call
+      is skipped and ``statfs(2)`` reports zero. The default is to report,
+      except on ``CONFIG_PREEMPT_RT`` where it defaults to ``nostatfs``; pass
+      ``statfs`` there to force reporting back on (e.g. for tools such as fwupd
+      that need the free space for firmware updates). The option can also be
+      flipped on a live mount with ``mount -o remount,statfs`` /
+      ``mount -o remount,nostatfs``, so reporting can be enabled only for the
+      duration of a firmware update without unmounting efivarfs.
+
 *See also:*
 
 - Documentation/admin-guide/acpi/ssdt-overlays.rst
diff --git a/fs/efivarfs/internal.h b/fs/efivarfs/internal.h
index f913b6824289..0cb053884b4b 100644
--- a/fs/efivarfs/internal.h
+++ b/fs/efivarfs/internal.h
@@ -11,6 +11,7 @@
 struct efivarfs_mount_opts {
 	kuid_t uid;
 	kgid_t gid;
+	bool nostatfs;		/* skip QueryVariableInfo() in statfs() */
 };
 
 struct efivarfs_fs_info {
diff --git a/fs/efivarfs/super.c b/fs/efivarfs/super.c
index 8d33f11db2a1..d2e1f31a72d1 100644
--- a/fs/efivarfs/super.c
+++ b/fs/efivarfs/super.c
@@ -74,6 +74,15 @@ static int efivarfs_show_options(struct seq_file *m, struct dentry *root)
 	if (!gid_eq(opts->gid, GLOBAL_ROOT_GID))
 		seq_printf(m, ",gid=%u",
 				from_kgid_munged(&init_user_ns, opts->gid));
+	/*
+	 * Always show nostatfs (it makes statfs() report zero capacity, which
+	 * is otherwise surprising); only show statfs where it is not the
+	 * default, i.e. on PREEMPT_RT.
+	 */
+	if (opts->nostatfs)
+		seq_puts(m, ",nostatfs");
+	else if (IS_ENABLED(CONFIG_PREEMPT_RT))
+		seq_puts(m, ",statfs");
 	return 0;
 }
 
@@ -82,13 +91,19 @@ static int efivarfs_statfs(struct dentry *dentry, struct kstatfs *buf)
 	const u32 attr = EFI_VARIABLE_NON_VOLATILE |
 			 EFI_VARIABLE_BOOTSERVICE_ACCESS |
 			 EFI_VARIABLE_RUNTIME_ACCESS;
+	struct efivarfs_fs_info *sfi = dentry->d_sb->s_fs_info;
 	u64 storage_space, remaining_space, max_variable_size;
 	u64 id = huge_encode_dev(dentry->d_sb->s_dev);
 	efi_status_t status;
 
-	/* Some UEFI firmware does not implement QueryVariableInfo() */
+	/*
+	 * Some UEFI firmware does not implement QueryVariableInfo(); the
+	 * nostatfs mount option also disables this (preempt-disabled,
+	 * potentially slow) call entirely, reporting zero used/available.
+	 */
 	storage_space = remaining_space = 0;
-	if (efi_rt_services_supported(EFI_RT_SUPPORTED_QUERY_VARIABLE_INFO)) {
+	if (!READ_ONCE(sfi->mount_opts.nostatfs) &&
+	    efi_rt_services_supported(EFI_RT_SUPPORTED_QUERY_VARIABLE_INFO)) {
 		static DEFINE_RATELIMIT_STATE(_rs, 2 * HZ, 5);
 		static u64 storage, remaining;
 		static DEFINE_SPINLOCK(lock);
@@ -323,12 +338,13 @@ static int efivarfs_callback(efi_char16_t *name16, efi_guid_t vendor,
 }
 
 enum {
-	Opt_uid, Opt_gid,
+	Opt_uid, Opt_gid, Opt_statfs,
 };
 
 static const struct fs_parameter_spec efivarfs_parameters[] = {
 	fsparam_uid("uid", Opt_uid),
 	fsparam_gid("gid", Opt_gid),
+	fsparam_flag_no("statfs", Opt_statfs),
 	{},
 };
 
@@ -350,6 +366,9 @@ static int efivarfs_parse_param(struct fs_context *fc, struct fs_parameter *para
 	case Opt_gid:
 		opts->gid = result.gid;
 		break;
+	case Opt_statfs:
+		opts->nostatfs = result.negated;
+		break;
 	default:
 		return -EINVAL;
 	}
@@ -402,11 +421,17 @@ static int efivarfs_get_tree(struct fs_context *fc)
 
 static int efivarfs_reconfigure(struct fs_context *fc)
 {
+	struct efivarfs_fs_info *sfi = fc->root->d_sb->s_fs_info;
+	struct efivarfs_fs_info *new_sfi = fc->s_fs_info;
+
 	if (!efivar_supports_writes() && !(fc->sb_flags & SB_RDONLY)) {
 		pr_err("Firmware does not support SetVariableRT. Can not remount with rw\n");
 		return -EINVAL;
 	}
 
+	/* statfs() reads nostatfs locklessly (no s_umount); pair with WRITE_ONCE. */
+	WRITE_ONCE(sfi->mount_opts.nostatfs, new_sfi->mount_opts.nostatfs);
+
 	return 0;
 }
 
@@ -524,8 +549,19 @@ static int efivarfs_init_fs_context(struct fs_context *fc)
 	if (!sfi)
 		return -ENOMEM;
 
-	sfi->mount_opts.uid = GLOBAL_ROOT_UID;
-	sfi->mount_opts.gid = GLOBAL_ROOT_GID;
+	if (fc->purpose == FS_CONTEXT_FOR_RECONFIGURE) {
+		/* Preserve options not respecified on remount. */
+		struct efivarfs_fs_info *old = fc->root->d_sb->s_fs_info;
+
+		sfi->mount_opts.uid = old->mount_opts.uid;
+		sfi->mount_opts.gid = old->mount_opts.gid;
+		sfi->mount_opts.nostatfs = READ_ONCE(old->mount_opts.nostatfs);
+	} else {
+		sfi->mount_opts.uid = GLOBAL_ROOT_UID;
+		sfi->mount_opts.gid = GLOBAL_ROOT_GID;
+		/* QueryVariableInfo() stalls the CPU; default nostatfs on PREEMPT_RT. */
+		sfi->mount_opts.nostatfs = IS_ENABLED(CONFIG_PREEMPT_RT);
+	}
 
 	fc->s_fs_info = sfi;
 	fc->ops = &efivarfs_context_ops;

base-commit: 9b87fdc9af2fbfcdb5c24a64139685ef80f6573f
-- 
2.34.1

Re: [PATCH v3] efivarfs: add nostatfs mount option to skip QueryVariableInfo()

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Low] efivarfs: data race in efivarfs_show_options() on opts->nostatfs
--

commit 33ecb04473d9adc8fe8bcd4b68acea9d071545b7
Author: Prashant Singh <singhpra@juniper.net>

efivarfs: add nostatfs mount option to skip QueryVariableInfo()

QueryVariableInfo() is an EFI runtime service that, on some firmware,
takes tens of milliseconds and runs with preemption disabled, stalling
the CPU that services it. efivarfs_statfs() calls it (rate-limited since
commit b2326338dc68 ("efivarfs: Rate limit statfs() handler")) to report
the variable-store used/available capacity, so any statfs(2) -- e.g.
every "df" -- can inject that stall into unrelated latency-sensitive
workloads on the same CPU.

Add a negatable "nostatfs" mount option: with nostatfs, statfs(2) skips
QueryVariableInfo() and reports zero used/available; with statfs it
reports the capacity as before.

> diff --git a/fs/efivarfs/super.c b/fs/efivarfs/super.c
> index 8d33f11db2a1b..d2e1f31a72d1e 100644
> --- a/fs/efivarfs/super.c
> +++ b/fs/efivarfs/super.c
> @@ -74,6 +74,15 @@ static int efivarfs_show_options(struct seq_file *m, struct dentry *root)
>  	if (!gid_eq(opts->gid, GLOBAL_ROOT_GID))
>  		seq_printf(m, ",gid=%u",
>  				from_kgid_munged(&init_user_ns, opts->gid));
> +	/*
> +	 * Always show nostatfs (it makes statfs() report zero capacity, which
> +	 * is otherwise surprising); only show statfs where it is not the
> +	 * default, i.e. on PREEMPT_RT.
> +	 */
> +	if (opts->nostatfs)

[Severity: Low]
Does this code risk a data race and potential KCSAN warning?

Since opts->nostatfs can be updated concurrently by efivarfs_reconfigure()
using WRITE_ONCE(), reading it here without READ_ONCE() could result in
undefined behavior.

> +		seq_puts(m, ",nostatfs");
> +	else if (IS_ENABLED(CONFIG_PREEMPT_RT))
> +		seq_puts(m, ",statfs");
>  	return 0;
>  }

[ ... ]

> @@ -402,11 +421,17 @@ static int efivarfs_get_tree(struct fs_context *fc)
>  
>  static int efivarfs_reconfigure(struct fs_context *fc)
>  {
> +	struct efivarfs_fs_info *sfi = fc->root->d_sb->s_fs_info;
> +	struct efivarfs_fs_info *new_sfi = fc->s_fs_info;
> +
>  	if (!efivar_supports_writes() && !(fc->sb_flags & SB_RDONLY)) {
>  		pr_err("Firmware does not support SetVariableRT. Can not remount with rw\n");
>  		return -EINVAL;
>  	}
>  
> +	/* statfs() reads nostatfs locklessly (no s_umount); pair with WRITE_ONCE. */
> +	WRITE_ONCE(sfi->mount_opts.nostatfs, new_sfi->mount_opts.nostatfs);

[Severity: Medium]
This is a pre-existing issue, but does this function silently discard newly
parsed uid and gid mount options during a remount?

It appears that efivarfs_reconfigure() only copies the nostatfs option into
the active superblock context, ignoring any changes to the uid and gid
options that were parsed into new_sfi.

> +
>  	return 0;
>  }
>  
> @@ -524,8 +549,19 @@ static int efivarfs_init_fs_context(struct fs_context *fc)
>  	if (!sfi)
>  		return -ENOMEM;
>  
> -	sfi->mount_opts.uid = GLOBAL_ROOT_UID;
> -	sfi->mount_opts.gid = GLOBAL_ROOT_GID;
> +	if (fc->purpose == FS_CONTEXT_FOR_RECONFIGURE) {
> +		/* Preserve options not respecified on remount. */
> +		struct efivarfs_fs_info *old = fc->root->d_sb->s_fs_info;
> +
> +		sfi->mount_opts.uid = old->mount_opts.uid;
> +		sfi->mount_opts.gid = old->mount_opts.gid;

[Severity: Medium]
This is a pre-existing issue, but is this intended to carry over the uid and
gid options so they can be modified by the parser?

Because efivarfs_reconfigure() never applies the parsed uid and gid back to
the live superblock, the preservation here acts as dead code. Administrators
who run "mount -o remount,uid=1000" will find their requested ownership ignored
when creating new variables.

> +		sfi->mount_opts.nostatfs = READ_ONCE(old->mount_opts.nostatfs);
> +	} else {
> +		sfi->mount_opts.uid = GLOBAL_ROOT_UID;
> +		sfi->mount_opts.gid = GLOBAL_ROOT_GID;
> +		/* QueryVariableInfo() stalls the CPU; default nostatfs on PREEMPT_RT. */
> +		sfi->mount_opts.nostatfs = IS_ENABLED(CONFIG_PREEMPT_RT);
> +	}

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260925113145.7396-1-singhpra@juniper.net?part=1

Re: [PATCH v3] efivarfs: add nostatfs mount option to skip QueryVariableInfo()

[Ard Biesheuvel]

Hello Prashant,

Thanks for respinning this.

On Fri, 25 Sep 2026, at 13:31, Prashant Singh wrote:
> QueryVariableInfo() is an EFI runtime service that, on some firmware,
> takes tens of milliseconds and runs with preemption disabled, stalling
> the CPU that services it. efivarfs_statfs() calls it (rate-limited since
> commit b2326338dc68 ("efivarfs: Rate limit statfs() handler")) to report
> the variable-store used/available capacity, so any statfs(2) -- e.g.
> every "df" -- can inject that stall into unrelated latency-sensitive
> workloads on the same CPU.
>
> Add a negatable "nostatfs" mount option: with nostatfs, statfs(2) skips
> QueryVariableInfo() and reports zero used/available; with statfs it
> reports the capacity as before. It defaults to nostatfs on
> CONFIG_PREEMPT_RT so real-time kernels do not take the stall out of the
> box, but statfs can be passed there to force reporting back on -- e.g.
> for tools such as fwupd that need the efivars free space to update Secure
> Boot key databases.
>
> The option can also be toggled on a live mount via remount, so reporting
> can be enabled only for the duration of a firmware update without
> unmounting the boot-time efivarfs mount:
>
>   mount -o remount,statfs   /sys/firmware/efi/efivars   # reporting on
>   mount -o remount,nostatfs /sys/firmware/efi/efivars   # reporting off
>
> Tested on an Intel Xeon E5-2628L v4, 6.12 kernel, via
> "strace -T -e trace=statfs df" on the efivarfs mount.
>
> Cost of the firmware call (CONFIG_PREEMPT_RT not set, so reporting is
> enabled by default). Default mount calls QueryVariableInfo() and returns
> the real store capacity, ~66-129 ms per call:
>
>   statfs("/sys/firmware/efi/efivars", {f_type=EFIVARFS_MAGIC,
>       f_bsize=1, f_blocks=90024, f_bfree=33387, f_bavail=28267, ...})
>       = 0 <0.129124>
>
> With -o nostatfs the firmware call is skipped, capacity reported as
> zero, ~11 us per call:
>
>   statfs("/sys/firmware/efi/efivars", {f_type=EFIVARFS_MAGIC,
>       f_bsize=1, f_blocks=0, f_bfree=0, f_bavail=0, ...}) = 0 <0.000011>
>
> PREEMPT_RT default and live remount toggle (CONFIG_PREEMPT_RT=y). The
> boot-time mount defaults to nostatfs (mount shows nostatfs); no firmware
> call, capacity zero:
>
>   statfs(... f_blocks=0, f_bfree=0, f_bavail=0, ...) = 0 <0.000018>
>
> Enabling reporting in place with "mount -o remount,statfs" (mount now
> shows statfs) takes the ~70 ms firmware call and returns the real
> capacity, without unmounting:
>
>   statfs(... f_blocks=90024, f_bfree=30315, f_bavail=25195, ...)
>       = 0 <0.070293>
>
> Disabling it again with "mount -o remount,nostatfs" returns to the fast,
> zero-capacity path:
>
>   statfs(... f_blocks=0, f_bfree=0, f_bavail=0, ...) = 0 <0.000014>
>
> An unrelated remount that does not respecify the option preserves it: a
> "mount -o remount,rw" after "remount,statfs" leaves the mount showing
> statfs.
>
> Suggested-by: Ard Biesheuvel <ardb@kernel.org>
> Suggested-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> Signed-off-by: Prashant Singh <singhpra@juniper.net>
> ---
> Changes since v2:
> - Make the flag negatable (fsparam_flag_no): "statfs" forces reporting
>   back on, "nostatfs" skips QueryVariableInfo(). Default stays nostatfs
>   on PREEMPT_RT. This lets fwupd get the efivars free space it needs for
>   Secure Boot key (KEK/DB/DBX) updates on an RT kernel by mounting with
>   -o statfs (Luis Claudio R. Goncalves, Steve McIntyre).
> - Support toggling the option on a live mount via remount, so reporting
>   can be enabled just for a firmware update without unmounting efivarfs
>   (Sebastian Andrzej Siewior). Options not respecified on remount are
>   preserved.
> - Read/write nostatfs with READ_ONCE()/WRITE_ONCE(): statfs() reads it
>   without s_umount while remount can update it.

Why? There is no memory ordering or tearing issue here, but only a
fundamental race where a remount and a statfs() might occur at the
same time.

> - Add PREEMPT_RT + remount test data to the commit message.
>
> Changes since v1:
> - Replace the sysctl with a mount option named "nostatfs", per review
>   (Ard Biesheuvel).
>
> v1: https://lore.kernel.org/all/20260917071600.5587-1-singhpra@juniper.net/
> v2: https://lore.kernel.org/all/20260919044124.8268-1-singhpra@juniper.net/
>
>  Documentation/filesystems/efivarfs.rst | 16 +++++++++
>  fs/efivarfs/internal.h                 |  1 +
>  fs/efivarfs/super.c                    | 46 +++++++++++++++++++++++---
>  3 files changed, 58 insertions(+), 5 deletions(-)
>
> diff --git a/Documentation/filesystems/efivarfs.rst 
> b/Documentation/filesystems/efivarfs.rst
> index f646c3f0980f..cd81ee84115b 100644
> --- a/Documentation/filesystems/efivarfs.rst
> +++ b/Documentation/filesystems/efivarfs.rst
> @@ -37,6 +37,22 @@ accidentally.
>            |4_bytes_of_attributes + efivar_data|
>            +-----------------------------------+
> 
> +Mount options
> +=============
> +
> +statfs / nostatfs
> +      Control whether ``statfs(2)`` reports the variable-store used/available
> +      capacity. Obtaining it requires the ``QueryVariableInfo()`` EFI runtime
> +      service, which on some firmware takes tens of milliseconds and runs with
> +      preemption disabled, stalling the calling CPU. With ``nostatfs`` the call
> +      is skipped and ``statfs(2)`` reports zero. The default is to report,
> +      except on ``CONFIG_PREEMPT_RT`` where it defaults to ``nostatfs``; pass
> +      ``statfs`` there to force reporting back on (e.g. for tools such as fwupd
> +      that need the free space for firmware updates). The option can also be
> +      flipped on a live mount with ``mount -o remount,statfs`` /
> +      ``mount -o remount,nostatfs``, so reporting can be enabled only for the
> +      duration of a firmware update without unmounting efivarfs.
> +
>  *See also:*
> 
>  - Documentation/admin-guide/acpi/ssdt-overlays.rst
> diff --git a/fs/efivarfs/internal.h b/fs/efivarfs/internal.h
> index f913b6824289..0cb053884b4b 100644
> --- a/fs/efivarfs/internal.h
> +++ b/fs/efivarfs/internal.h
> @@ -11,6 +11,7 @@
>  struct efivarfs_mount_opts {
>  	kuid_t uid;
>  	kgid_t gid;
> +	bool nostatfs;		/* skip QueryVariableInfo() in statfs() */
>  };
> 
>  struct efivarfs_fs_info {
> diff --git a/fs/efivarfs/super.c b/fs/efivarfs/super.c
> index 8d33f11db2a1..d2e1f31a72d1 100644
> --- a/fs/efivarfs/super.c
> +++ b/fs/efivarfs/super.c
> @@ -74,6 +74,15 @@ static int efivarfs_show_options(struct seq_file *m, 
> struct dentry *root)
>  	if (!gid_eq(opts->gid, GLOBAL_ROOT_GID))
>  		seq_printf(m, ",gid=%u",
>  				from_kgid_munged(&init_user_ns, opts->gid));
> +	/*
> +	 * Always show nostatfs (it makes statfs() report zero capacity, which
> +	 * is otherwise surprising); only show statfs where it is not the
> +	 * default, i.e. on PREEMPT_RT.
> +	 */
> +	if (opts->nostatfs)
> +		seq_puts(m, ",nostatfs");
> +	else if (IS_ENABLED(CONFIG_PREEMPT_RT))
> +		seq_puts(m, ",statfs");

Please drop this else branch - it should be sufficient for the 'nostatfs'
to disappear from the mount options to understand what is going on.

>  	return 0;
>  }
> 
> @@ -82,13 +91,19 @@ static int efivarfs_statfs(struct dentry *dentry, 
> struct kstatfs *buf)
>  	const u32 attr = EFI_VARIABLE_NON_VOLATILE |
>  			 EFI_VARIABLE_BOOTSERVICE_ACCESS |
>  			 EFI_VARIABLE_RUNTIME_ACCESS;
> +	struct efivarfs_fs_info *sfi = dentry->d_sb->s_fs_info;
>  	u64 storage_space, remaining_space, max_variable_size;
>  	u64 id = huge_encode_dev(dentry->d_sb->s_dev);
>  	efi_status_t status;
> 
> -	/* Some UEFI firmware does not implement QueryVariableInfo() */
> +	/*
> +	 * Some UEFI firmware does not implement QueryVariableInfo(); the
> +	 * nostatfs mount option also disables this (preempt-disabled,
> +	 * potentially slow) call entirely, reporting zero used/available.
> +	 */
>  	storage_space = remaining_space = 0;
> -	if (efi_rt_services_supported(EFI_RT_SUPPORTED_QUERY_VARIABLE_INFO)) {
> +	if (!READ_ONCE(sfi->mount_opts.nostatfs) &&
> +	    efi_rt_services_supported(EFI_RT_SUPPORTED_QUERY_VARIABLE_INFO)) {
>  		static DEFINE_RATELIMIT_STATE(_rs, 2 * HZ, 5);
>  		static u64 storage, remaining;
>  		static DEFINE_SPINLOCK(lock);
> @@ -323,12 +338,13 @@ static int efivarfs_callback(efi_char16_t 
> *name16, efi_guid_t vendor,
>  }
> 
>  enum {
> -	Opt_uid, Opt_gid,
> +	Opt_uid, Opt_gid, Opt_statfs,
>  };
> 
>  static const struct fs_parameter_spec efivarfs_parameters[] = {
>  	fsparam_uid("uid", Opt_uid),
>  	fsparam_gid("gid", Opt_gid),
> +	fsparam_flag_no("statfs", Opt_statfs),
>  	{},
>  };
> 
> @@ -350,6 +366,9 @@ static int efivarfs_parse_param(struct fs_context 
> *fc, struct fs_parameter *para
>  	case Opt_gid:
>  		opts->gid = result.gid;
>  		break;
> +	case Opt_statfs:
> +		opts->nostatfs = result.negated;
> +		break;
>  	default:
>  		return -EINVAL;
>  	}
> @@ -402,11 +421,17 @@ static int efivarfs_get_tree(struct fs_context 
> *fc)
> 
>  static int efivarfs_reconfigure(struct fs_context *fc)
>  {
> +	struct efivarfs_fs_info *sfi = fc->root->d_sb->s_fs_info;
> +	struct efivarfs_fs_info *new_sfi = fc->s_fs_info;
> +
>  	if (!efivar_supports_writes() && !(fc->sb_flags & SB_RDONLY)) {
>  		pr_err("Firmware does not support SetVariableRT. Can not remount with rw\n");
>  		return -EINVAL;
>  	}
> 
> +	/* statfs() reads nostatfs locklessly (no s_umount); pair with WRITE_ONCE. */
> +	WRITE_ONCE(sfi->mount_opts.nostatfs, new_sfi->mount_opts.nostatfs);
> +
>  	return 0;
>  }
> 
> @@ -524,8 +549,19 @@ static int efivarfs_init_fs_context(struct fs_context *fc)
>  	if (!sfi)
>  		return -ENOMEM;
> 
> -	sfi->mount_opts.uid = GLOBAL_ROOT_UID;
> -	sfi->mount_opts.gid = GLOBAL_ROOT_GID;
> +	if (fc->purpose == FS_CONTEXT_FOR_RECONFIGURE) {
> +		/* Preserve options not respecified on remount. */
> +		struct efivarfs_fs_info *old = fc->root->d_sb->s_fs_info;
> +
> +		sfi->mount_opts.uid = old->mount_opts.uid;
> +		sfi->mount_opts.gid = old->mount_opts.gid;
> +		sfi->mount_opts.nostatfs = READ_ONCE(old->mount_opts.nostatfs);
> +	} else {
> +		sfi->mount_opts.uid = GLOBAL_ROOT_UID;
> +		sfi->mount_opts.gid = GLOBAL_ROOT_GID;
> +		/* QueryVariableInfo() stalls the CPU; default nostatfs on PREEMPT_RT. */
> +		sfi->mount_opts.nostatfs = IS_ENABLED(CONFIG_PREEMPT_RT);
> +	}
> 

Why do we need this? Does remount otherwise forget its options? Does this
mean that before this patch, remounting with only a different guid would
reset the uid specified at mount time to the default? If so, please fix
that in a separate (preceding) patch.