Re: [PATCH bpf-next] bpf, x86: Use global buffer for trampoline size generation

["Alexei Starovoitov" <alexei.starovoitov@gmail.com>]

On Mon Sep 7, 2026 at 9:05 AM PDT, Jiri Olsa wrote:
> Currently arch_bpf_trampoline_size allocates and frees a temporary
> trampoline buffer on every invocation. The buffer is only used as a
> scratch space while __arch_prepare_bpf_trampoline() calculates the
> required size, and the generated trampoline is discarded.
>
> Allocating a writable scratch page during kernel initialization and
> reusing it for all size calculations. This improves tracing_multi
> attachment time.
>
> With current code:
>
>   # ./test_progs -t tracing_multi_bench_attach -v
>   ...
>   serial_test_tracing_multi_bench_attach: found 55227 functions
>   serial_test_tracing_multi_bench_attach: attached in   1.563s
>   serial_test_tracing_multi_bench_attach: detached in   0.256s
>
> With the fix:
>
>   # ./test_progs -t tracing_multi_bench_attach -v
>   ...
>   serial_test_tracing_multi_bench_attach: found 55235 functions
>   serial_test_tracing_multi_bench_attach: attached in   0.798s
>   serial_test_tracing_multi_bench_attach: detached in   0.258s
>
> Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> ---
> was "bpf, x86: Add support for jit dry run",
> - doing this by having single scratch page instead as suggested by Alexei
>
>  arch/x86/net/bpf_jit_comp.c | 30 +++++++++++++++---------------
>  1 file changed, 15 insertions(+), 15 deletions(-)
>
> diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
> index bba351944202..13ef0d53ca29 100644
> --- a/arch/x86/net/bpf_jit_comp.c
> +++ b/arch/x86/net/bpf_jit_comp.c
> @@ -9,6 +9,7 @@
>  #include <linux/filter.h>
>  #include <linux/if_vlan.h>
>  #include <linux/bitfield.h>
> +#include <linux/init.h>
>  #include <linux/bpf.h>
>  #include <linux/bpf_verifier.h>
>  #include <linux/memory.h>
> @@ -35,6 +36,15 @@ void __asan_store8(void *p);
>  
>  static bool all_callee_regs_used[4] = {true, true, true, true};
>  
> +static void *trampoline_size_image;
> +
> +static int __init init_trampoline_size_image(void)
> +{
> +	trampoline_size_image = execmem_alloc(EXECMEM_MODULE_DATA, PAGE_SIZE);

I think I asked it earlier... why does it have to be execmem ?
Can it be normal page?

If so then alloc it and free it every time. No need to keep one page in reserve.

pw-bot: cr