Re: [PATCH bpf-next v3 4/5] bpf, x86: make sure allocation in arch_bpf_trampoline_size() is writable

[Jiri Olsa <olsajiri@gmail.com>]

On Sun, Aug 16, 2026 at 11:57:35AM +0300, Mike Rapoport wrote:

SNIP

> From 14dff78529a9204408a5c14036954df5f2f76431 Mon Sep 17 00:00:00 2001
> From: "Mike Rapoport (Microsoft)" <rppt@kernel.org>
> Date: Sun, 16 Aug 2026 11:27:32 +0300
> Subject: [PATCH] x86/bpf: make arch_bpf_trampoline_size allocate from
>  EXECMEM_MODULE_DATA
> 
> Jiri Olsa reports slowdown of tracing_multi benchmark that allocates huge
> number of trampolines [1].
> 
> The slowdown caused by extra protection changes in execmem_alloc_rw() and
> execmem_free().
> 
> With ROX caches enabled, all execmem allocations except EXECMEM_MODULE_DATA
> are ROX after the allocation. execmem_alloc_rw() temporarily sets them to
> W+NX and execmem_free() resets them back to ROX.
> 
> The only user of bpf_jit_alloc_exec_rw() is x86::arch_bpf_trampoline_size()
> that only needs a temporary writable buffer in the modules address space.
> 
> On x86 executable memory and module data are constrained to the same
> address range, so x86::arch_bpf_trampoline_size() can directly use
> execmem_alloc(EXECMEM_MODULE_DATA)
> 
> Replace the call to bpf_jit_alloc_exec_rw() with a call to
> execmem_alloc(EXECMEM_MODULE_DATA) in x86::arch_bpf_trampoline_size() and
> drop bpf_jit_alloc_exec_rw() helper.
> 
> Reported-by: Jiri Olsa <olsajiri@gmail.com>
> Link: https://lore.kernel.org/all/an8r7EODLIL-bZM3@krava
> Fixes: f0334294a428 ("bpf, x86: make sure allocation in arch_bpf_trampoline_size() is writable")
> Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>

looks good, thanks

serial_test_tracing_multi_bench_attach: found 55077 functions
serial_test_tracing_multi_bench_attach: attached in   1.470s
serial_test_tracing_multi_bench_attach: detached in   0.249s

Tested-by: Jiri Olsa <jolsa@kernel.org>

jirka


> ---
>  arch/x86/net/bpf_jit_comp.c | 8 +++++---
>  include/linux/filter.h      | 1 -
>  kernel/bpf/core.c           | 5 -----
>  3 files changed, 5 insertions(+), 9 deletions(-)
> 
> diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
> index b2feec81e231..7d064d3e2788 100644
> --- a/arch/x86/net/bpf_jit_comp.c
> +++ b/arch/x86/net/bpf_jit_comp.c
> @@ -13,6 +13,7 @@
>  #include <linux/bpf_verifier.h>
>  #include <linux/memory.h>
>  #include <linux/sort.h>
> +#include <linux/execmem.h>
>  #include <asm/extable.h>
>  #include <asm/ftrace.h>
>  #include <asm/set_memory.h>
> @@ -3706,15 +3707,16 @@ int arch_bpf_trampoline_size(const struct btf_func_model *m, u32 flags,
>  	 *
>  	 * We cannot use kvmalloc here, because we need image to be in
>  	 * module memory range.
> -	 * Since it must be writable use bpf_jit_alloc_exec_rw().
> +	 * Since it must be writable use execmem_alloc(EXECMEM_MODULE_DATA)
> +	 * that returns writable memory in the module address space.
>  	 */
> -	image = bpf_jit_alloc_exec_rw(PAGE_SIZE);
> +	image = execmem_alloc(EXECMEM_MODULE_DATA, PAGE_SIZE);
>  	if (!image)
>  		return -ENOMEM;
>  
>  	ret = __arch_prepare_bpf_trampoline(&im, image, image + PAGE_SIZE, image,
>  					    m, flags, tnodes, func_addr);
> -	bpf_jit_free_exec(image);
> +	execmem_free(image);
>  	return ret;
>  }
>  
> diff --git a/include/linux/filter.h b/include/linux/filter.h
> index 32d5297c557e..14acb2455746 100644
> --- a/include/linux/filter.h
> +++ b/include/linux/filter.h
> @@ -1333,7 +1333,6 @@ bpf_jit_binary_alloc(unsigned int proglen, u8 **image_ptr,
>  void bpf_jit_binary_free(struct bpf_binary_header *hdr);
>  u64 bpf_jit_alloc_exec_limit(void);
>  void *bpf_jit_alloc_exec(unsigned long size);
> -void *bpf_jit_alloc_exec_rw(unsigned long size);
>  void bpf_jit_free_exec(void *addr);
>  void bpf_jit_free(struct bpf_prog *fp);
>  struct bpf_binary_header *
> diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
> index e2076667b245..1b89c18cf246 100644
> --- a/kernel/bpf/core.c
> +++ b/kernel/bpf/core.c
> @@ -1128,11 +1128,6 @@ void *bpf_jit_alloc_exec(unsigned long size)
>  	return execmem_alloc(EXECMEM_BPF, size);
>  }
>  
> -void *bpf_jit_alloc_exec_rw(unsigned long size)
> -{
> -	return execmem_alloc_rw(EXECMEM_BPF, size);
> -}
> -
>  void bpf_jit_free_exec(void *addr)
>  {
>  	execmem_free(addr);
> -- 
> 2.53.0
> 
>  
> > thanks,
> > jirka
> 
> -- 
> Sincerely yours,
> Mike.