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.
