[PATCH v4] efivarfs: add nostatfs mount option to skip QueryVariableInfo()
From: Prashant Singh
Date: Mon Sep 28 2026 - 16:37:18 EST
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@xxxxxxxxxx>
Suggested-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
Signed-off-by: Prashant Singh <singhpra@xxxxxxxxxxx>
---
Changes since v3:
- Drop the show_options "statfs" branch; the absence of "nostatfs" now
indicates reporting is on (Ard Biesheuvel).
- Drop the uid/gid copies on the remount path; efivarfs_reconfigure()
applies only nostatfs, so they were dead code (Ard Biesheuvel).
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.
- 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@xxxxxxxxxxx/
v2: https://lore.kernel.org/all/20260919044124.8268-1-singhpra@xxxxxxxxxxx/
v3: https://lore.kernel.org/all/20260925113145.7396-1-singhpra@xxxxxxxxxxx/
Documentation/filesystems/efivarfs.rst | 16 +++++++++++
fs/efivarfs/internal.h | 1 +
fs/efivarfs/super.c | 38 ++++++++++++++++++++++----
3 files changed, 50 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..efeada6e314f 100644
--- a/fs/efivarfs/super.c
+++ b/fs/efivarfs/super.c
@@ -74,6 +74,9 @@ 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));
+ /* Absence of nostatfs means statfs() reports real capacity. */
+ if (opts->nostatfs)
+ seq_puts(m, ",nostatfs");
return 0;
}
@@ -82,13 +85,19 @@ 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 +332,13 @@ 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 +360,9 @@ 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 +415,17 @@ 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 without s_umount; mark the concurrent update. */
+ WRITE_ONCE(sfi->mount_opts.nostatfs, new_sfi->mount_opts.nostatfs);
+
return 0;
}
@@ -524,8 +543,17 @@ 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) {
+ /* nostatfs is writable via remount; keep it if not respecified. */
+ struct efivarfs_fs_info *old = fc->root->d_sb->s_fs_info;
+
+ 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