Re: [PATCH 02/16] mm/huge_memory: access vm_page_prot with READ_ONCE in remove_migration_pmd

[Zach O'Keefe]

On 23 Jun 20:03, Miaohe Lin wrote:
> 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.

IMO adding the READ_ONCE() as proposed in fine. Adding a helper to be called
dependent on locking context still requires the caller / dev to know what the
locking context is - so I don't think it provides much benefit.