On 2022/6/23 11:14, Kirill A. Shutemov wrote: > On Thu, Jun 23, 2022 at 01:06:13AM +0800, Miaohe Lin wrote: >> vma->vm_page_prot is read lockless from the rmap_walk, it may be updated >> concurrently. Using READ_ONCE to prevent the risk of reading intermediate >> values. > > Have you checked all other vm_page_prot reads that they hold mmap_lock? I took a glance when I made this patch. > > I think the right fix would be to provide a helper to read vm_page_prot > which does READ_ONCE() and use it everywhere. This seems more sustainable. > This patch is inspired from the below commit 6d2329f8872f ("mm: vm_page_prot: update with WRITE_ONCE/READ_ONCE") It changed all the places that need to use READ_ONCE. But remove_migration_pmd is missed due to it's introduced later. It looks fine to add a helper to read vm_page_prot which does READ_ONCE() but READ_ONCE is unneeded while under the mmap_lock, so might it be a little overkill to add a helper because the helper is used iff mmap_lock is not held? Thanks.