Re: [PATCH v2] efivarfs: add nostatfs mount option to skip QueryVariableInfo()

["Ard Biesheuvel" <ardb@kernel.org>]

On Sat, 19 Sep 2026, at 06:41, 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 "nostatfs" mount option: when set, statfs(2) skips
> QueryVariableInfo() entirely and reports zero used/available. It is set
> by default on CONFIG_PREEMPT_RT so real-time kernels do not take the
> stall out of the box.
>
> Tested on an Intel Xeon E5-2628L v4, 6.12 kernel with CONFIG_PREEMPT_RT
> not set (so reporting defaults to enabled and nostatfs must be passed
> explicitly), via "strace -T -e trace=statfs df" on the efivarfs mount.
>
> Default mount: statfs() 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: no firmware call, 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>
>
> Signed-off-by: Prashant Singh <singhpra@juniper.net>
> ---
> Changes since v1:
>   - Per review, dropped the fs.efivarfs sysctl / cached-capacity approach
>     entirely; instead add a simple "nostatfs" mount option that skips
>     QueryVariableInfo() in statfs() and reports zero, set by default on
>     CONFIG_PREEMPT_RT.
>
> v1: https://lore.kernel.org/all/20260917071600.5587-1-singhpra@juniper.net/
>
>  Documentation/filesystems/efivarfs.rst | 10 ++++++++++
>  fs/efivarfs/internal.h                 |  1 +
>  fs/efivarfs/super.c                    | 20 +++++++++++++++++---
>  3 files changed, 28 insertions(+), 3 deletions(-)
>
> diff --git a/Documentation/filesystems/efivarfs.rst 
> b/Documentation/filesystems/efivarfs.rst
> index f646c3f0980f..3ead638c796a 100644
> --- a/Documentation/filesystems/efivarfs.rst
> +++ b/Documentation/filesystems/efivarfs.rst
> @@ -37,6 +37,16 @@ accidentally.
>            |4_bytes_of_attributes + efivar_data|
>            +-----------------------------------+
> 
> +Mount options
> +=============
> +
> +nostatfs
> +      Do not report the variable-store used/available capacity in
> +      ``statfs(2)``; report zero instead. Obtaining the capacity requires the
> +      ``QueryVariableInfo()`` EFI runtime service, which on some firmware takes
> +      tens of milliseconds and runs with preemption disabled, stalling the
> +      calling CPU. This option is set by default on ``CONFIG_PREEMPT_RT``.
> +
>  *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..fbdaa4915d30 100644
> --- a/fs/efivarfs/super.c
> +++ b/fs/efivarfs/super.c
> @@ -74,6 +74,8 @@ 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));
> +	if (opts->nostatfs)
> +		seq_puts(m, ",nostatfs");
>  	return 0;
>  }
> 
> @@ -82,13 +84,19 @@ 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 (!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 +331,13 @@ static int efivarfs_callback(efi_char16_t 
> *name16, efi_guid_t vendor,
>  }
> 
>  enum {
> -	Opt_uid, Opt_gid,
> +	Opt_uid, Opt_gid, Opt_nostatfs,
>  };
> 
>  static const struct fs_parameter_spec efivarfs_parameters[] = {
>  	fsparam_uid("uid", Opt_uid),
>  	fsparam_gid("gid", Opt_gid),
> +	fsparam_flag("nostatfs", Opt_nostatfs),
>  	{},
>  };
> 
> @@ -350,6 +359,9 @@ static int efivarfs_parse_param(struct fs_context 
> *fc, struct fs_parameter *para
>  	case Opt_gid:
>  		opts->gid = result.gid;
>  		break;
> +	case Opt_nostatfs:
> +		opts->nostatfs = true;
> +		break;
>  	default:
>  		return -EINVAL;
>  	}
> @@ -526,6 +538,8 @@ static int efivarfs_init_fs_context(struct 
> fs_context *fc)
> 
>  	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;
>

This looks good to me but I'll leave some time for other folks to comment.