Re: [PATCH] riscv: mm: Pre-allocate module page tables for execmem

From: Mike Rapoport <rppt@kernel.org>
To: Karl Mehltretter <kmehltretter@gmail.com>
Cc: "Paul Walmsley" <pjw@kernel.org>,
	"Palmer Dabbelt" <palmer@dabbelt.com>,
	"Albert Ou" <aou@eecs.berkeley.edu>,
	"Alexandre Ghiti" <alex@ghiti.fr>,
	"Luis Chamberlain" <mcgrof@kernel.org>,
	"Björn Töpel" <bjorn@kernel.org>,
	linux-riscv@lists.infradead.org, bpf@vger.kernel.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH] riscv: mm: Pre-allocate module page tables for execmem
Date: Thu, 15 Oct 2026 11:16:11 +0300	[thread overview]
Message-ID: <atCLy-3RZbf3fGRp@kernel.org> (raw)
In-Reply-To: <20261015033643.84972-1-kmehltretter@gmail.com>

On Thu, Oct 15, 2026 at 05:36:43AM +0200, Karl Mehltretter wrote:
> RV64 pre-allocates page-table levels for late kernel mappings. The module
> range is guarded by CONFIG_MODULES, although execmem can use it without
> modules.
> 
> With CONFIG_BPF_JIT=y and CONFIG_MODULES=n, execmem adds a mapping to
> init_mm while the current process retains an empty PGD entry. Initializing
> a BPF program pack then faults:
> 
>   Unable to handle kernel paging request ...
>   [ffffffff78000000] pgd=0000000000000000
>   ...
>   bpf_jit_fill_hole_with_zero
>   bpf_prog_pack_alloc
> 
> Use CONFIG_EXECMEM for the module-range pre-allocation, as powerpc does
> for its execmem page-table setup. Both CONFIG_MODULES and CONFIG_BPF_JIT
> select it.

I think the commit message should mention that BPF_JIT_REGION is inside
MODULES region.

> Fixes: 2c9e5d4a0082 ("bpf: remove CONFIG_BPF_JIT dependency on CONFIG_MODULES of")
> Cc: stable@vger.kernel.org
> Assisted-by: LLM
> Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>

Acked-by: Mike Rapoport (Microsoft) <rppt@kernel.org>

> ---
> 
> Notes:
>     Tested on a BeagleV Ahead with matched LLVM 18.1.3 builds based on
>     v7.3-rc7:
>     
>     - unpatched: a native XDP attach triggered the Oops above
>     - patched: native XDP attach and detach completed without an Oops
>
>  arch/riscv/mm/init.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
> index fb37b0b67efe..d92342c162e2 100644
> --- a/arch/riscv/mm/init.c
> +++ b/arch/riscv/mm/init.c
> @@ -1437,7 +1437,7 @@ static void __init preallocate_pgd_pages_range(unsigned long start, unsigned lon
>  void __init pgtable_cache_init(void)
>  {
>  	preallocate_pgd_pages_range(VMALLOC_START, VMALLOC_END, "vmalloc");
> -	if (IS_ENABLED(CONFIG_MODULES))
> +	if (IS_ENABLED(CONFIG_EXECMEM))
>  		preallocate_pgd_pages_range(MODULES_VADDR, MODULES_END, "bpf/modules");
>  	if (IS_ENABLED(CONFIG_MEMORY_HOTPLUG)) {
>  		preallocate_pgd_pages_range(VMEMMAP_START, VMEMMAP_END, "vmemmap");
> -- 
> 2.43.0
> 

-- 
Sincerely yours,
Mike.

_______________________________________________
linux-riscv mailing list
linux-riscv@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-riscv