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

Alex Shi
2020-04-21T09:22:11Z

在 2020/4/21 上午6:11, Johannes Weiner 写道:
> 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
>
> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>

Reviewed-by: Alex Shi <alex.shi@linux.alibaba.com>

> ---
> mm/memcontrol.c | 21 +++++++--------------
> 1 file changed, 7 insertions(+), 14 deletions(-)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index a8cce52b6b4d..7b63260c9b57 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -1201,9 +1201,8 @@ int mem_cgroup_scan_tasks(struct mem_cgroup *memcg,
> * @page: the page
> * @pgdat: pgdat of the page
> *
> - * This function is only safe when following the LRU page isolation
> - * and putback protocol: the LRU lock must be held, and the page must
> - * either be PageLRU() or the caller must have isolated/allocated it.
> + * This function relies on page->mem_cgroup being stable - see the
> + * access rules in commit_charge().
> */
> struct lruvec *mem_cgroup_page_lruvec(struct page *page, struct pglist_data *pgdat)
> {
> @@ -2605,18 +2604,12 @@ static void commit_charge(struct page *page, struct mem_cgroup *memcg)
> {
> VM_BUG_ON_PAGE(page->mem_cgroup, page);
> /*
> - * Nobody should be changing or seriously looking at
> - * page->mem_cgroup at this point:
> - *
> - * - the page is uncharged
> - *
> - * - the page is off-LRU
> - *
> - * - an anonymous fault has exclusive page access, except for
> - * a locked page table
> + * Any of the following ensures page->mem_cgroup stability:
> *
> - * - a page cache insertion, a swapin fault, or a migration
> - * have the page locked
> + * - the page lock
> + * - LRU isolation
> + * - lock_page_memcg()
> + * - exclusive reference
> */
> page->mem_cgroup = memcg;
> }
>