Re: [PATCH 18/18] mm: memcontrol: update page->mem_cgroup stability rules

[Johannes Weiner]

On Tue, Apr 21, 2020 at 05:10:14PM +0800, Hillf Danton wrote:
> 
> On Mon, 20 Apr 2020 18:11:26 -0400 Johannes Weiner wrote:
> > 
> > The previous patches have simplified the access rules around
> > page->mem_cgroup somewhat:
> > 
> > 1. We never change page->mem_cgroup while the page is isolated by
> >    somebody else. This was by far the biggest exception to our rules
> >    and it didn't stop at lock_page() or lock_page_memcg().
> > 
> > 2. We charge pages before they get put into page tables now, so the
> >    somewhat fishy rule about "can be in page table as long as it's
> >    still locked" is now gone and boiled down to having an exclusive
> >    reference to the page.
> > 
> > Document the new rules. Any of the following will stabilize the
> > page->mem_cgroup association:
> > 
> > - the page lock
> > - LRU isolation
> > - lock_page_memcg()
> > - exclusive access to the page
> 
> Then rule-1 makes rule-3 no longer needed in mem_cgroup_move_account()?

Well, mem_cgroup_move_account() is the write side. It's the function
that changes page->mem_cgroup. So it needs to take all these locks in
order for the readside / fastpath to be okay with any one of them.