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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 --- 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; base-commit: 9b87fdc9af2fbfcdb5c24a64139685ef80f6573f -- 2.34.1