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

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@kernel.org>

Sashiko has reviewed this patch and found no issues. It looks great!

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260908-b4-mmap-prepare-vma-flag-sanify-v1-0-dacf19cce22b@kernel.org?part=13