Re: [RFC PATCH 10/10] uaccess: Move the NULL mm warning out of line

[Arnd Bergmann]

On Mon, Oct 26, 2026, at 22:52, James Houghton wrote:
> uaccess_without_mm() is inlined into every user copy, including every
> copy_{to,from}_user() call site on architectures that define
> INLINE_COPY_USER, such as arm64. Each inlined WARN_ON_ONCE() adds its
> own __bug_table entry.
>
> Keep the NULL current->mm check inline, but move the warning into a
> single out-of-line, __cold function, uaccess_warn_without_mm().
>
> The trade-off is that only the first offending access in the system
> warns, rather than the first one from each call site. The stack trace
> still identifies the caller, and the access is refused either way.
>
> Compared to the base of this series, with defconfig + KVM:
>
>                      inline WARN_ON_ONCE()    out-of-line warning
>   x86_64 total       +5,884 B (+0.014%)       +9,714 B (+0.023%)
>   arm64  total      +40,948 B (+0.10%)       +24,609 B (+0.06%)
>
> On arm64, this removes the ~16 KB of __bug_table growth, while text
> size is unchanged, as a call is the same size as the inline brk. On
> x86_64, the call is larger than the inline ud2 and causes more .cold
> splitting, so this is a slight regression.
>
> Signed-off-by: James Houghton <jthoughton@xxxxxxxxxx>
> ---
> NOTE: This patch is included for reference only, to show the cost of the
>       inline warning.

I would suspect that the growth is larger on architectures that
don't implement __WARN_FLAGS() but rely on the generic WARN_ON().

This would mainly affect 32-bit arm and mips machines, some of which
are very memory limited, most of the 64-bit architectures have it.

      Arnd