Re: [PATCH v5 13/38] bpf: arena: mark arena_map_mmap() mappings VM_MIXEDMAP

On 10/12/26 17:55, Lorenzo Stoakes (ARM) wrote:

On Mon, Oct 12, 2026 at 05:08:09PM +0200, David Hildenbrand (Arm) wrote:

quoted

On 10/9/26 20:04, Lorenzo Stoakes (ARM) wrote:

quoted

quoted

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;
 }

Historically, VM_MIXEDMAP really just means: can contain refcounted and
non-refcounted things. Everything with a struct page is refcounted.

I think we are now changing the semantics to mean "maps kernel pages". I think

Yes.

It was sort of implicit before given insert_page() etc. but still really meaning
that it _might_ be both.

Yeah, it's in the name. Mixed: pfn and/or ordinary.

quoted

this is fine, but we really have to update the documentation (and possibly
rename this thing to something more expressive?)

Yeah I completely agree.

I will rename it, but I think that's best for 7.5 obviously! :)

Yes.

VM_PFNMAP / VM_MIXEDMAP - These contain kernel-mapped data which cannot be
			  reasonably reconstructed on page fault.

the description is already pretty clear. just not that kernel-mapped pages
*must* use one of these.

(again, the ->fault thing returning allcoated kernel memory is nasty)



-- 
Cheers,

David