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

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

On Sat, 19 Sep 2026, at 16:51, Ard Biesheuvel wrote:
> 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.

Queued up in efi/next now -- thanks.