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

[sashiko-bot@kernel.org]

> 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@google.com>

Sashiko has reviewed this patch and found no issues.

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Sashiko AI review · https://sashiko.dev/#/patchset/20261026215226.3713941-1-jthoughton@google.com?part=10