Re: [PATCH v4] efivarfs: add nostatfs mount option to skip QueryVariableInfo()

[Ard Biesheuvel]

On Mon, 28 Sep 2026, at 22:34, 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@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).
>

Please don't respond to questions by respinning the patch without
having any discussion at all.

I asked you an honest question, and it seems Sashiko spotted an
issue here too.

Is there a problem with how the current code deals with uid and gid
on a remount?