Re: [PATCH v4 13/38] bpf: arena: mark arena_map_mmap() mappings VM_MIXEDMAP

[Suren Baghdasaryan]

On Sat, Oct 3, 2026 at 6:37 PM Lorenzo Stoakes (ARM) <ljs@xxxxxxxxxx> wrote:
>
> The bpf_map->ops->map_mmap callback invoked by bpf_map_mmap() can be set to
> one of ringbuf_map_mmap_kern(), ringbuf_map_mmap_user(), array_map_mmap()
> or arena_map_mmap().
>
> It is convention in mm to mark mappings whose pages the kernel manages
> itself with VM_MIXEDMAP, so core mm knows not to treat them as ordinary
> page cache or anonymous memory.
>
> The map_mmap callbacks ringbuf_map_mmap_kern() and ringbuf_map_mmap_user()
> use remap_vmalloc_range(), which ultimately invokes vm_insert_page() and so
> marks the ranges VM_MIXEDMAP, and array_map_mmap() sets VM_MIXEDMAP
> explicitly.
>
> However, the exception to this is arena_map_mmap(), which doesn't set the
> flag.
>
> This patch corrects this and updates the comment to reflect it.
>
> The pages are refcounted and vm_normal_page() finds them regardless of the
> flag, and VM_DONTEXPAND remains set (marking the memory as VM_SPECIAL and
> thus unmergeable). The one effect is that NUMA balancing now skips these
> VMAs, as it already does for the other bpf map mappings, which is the
> reason array_map_mmap() gives for setting the flag.
>
> The intent of this patch is to be able to establish the invariant that only
> PFN-mapped or mixed map ranges may clear the VM_MAYWRITE flag, as is done
> in bpf_map_mmap().
>
> Reviewed-by: Emil Tsalapatis <emil@xxxxxxxxxxxxxxx>
> Signed-off-by: Lorenzo Stoakes (ARM) <ljs@xxxxxxxxxx>

Reviewed-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>


> ---
>  kernel/bpf/arena.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/bpf/arena.c b/kernel/bpf/arena.c
> index 7b6847200b43..b69fe5e34339 100644
> --- a/kernel/bpf/arena.c
> +++ b/kernel/bpf/arena.c
> @@ -620,8 +620,9 @@ static int arena_map_mmap(struct bpf_map *map, struct vm_area_struct *vma)
>          * clears VM_MAYEXEC. Set VM_DONTEXPAND to avoid potential change
>          * of user_vm_start. Set VM_DONTCOPY to prevent arena VMA from
>          * being copied into the child process on fork.
> +        * This is a kernel page so set VM_MIXEDMAP.
>          */
> -       vm_flags_set(vma, VM_DONTEXPAND | VM_DONTCOPY);
> +       vm_flags_set(vma, VM_MIXEDMAP | VM_DONTEXPAND | VM_DONTCOPY);
>         vma->vm_ops = &arena_vm_ops;
>         return 0;
>  }
>
> --
> 2.55.0
>