[PATCH 13/39] bpf: arena: mark arena_map_mmap() mappings VM_MIXEDMAP

[Lorenzo Stoakes (ARM)]

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 the 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().

Signed-off-by: Lorenzo Stoakes (ARM) <ljs@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