[Devel] Re: [PATCH] memcg: lock-free clear page writeback (Was Re: [PATCH 04/10] memcg: disable local interrupts in lock_page_cgroup()

[Minchan Kim]

Hi Kame,

On Thu, Oct 7, 2010 at 3:24 PM, KAMEZAWA Hiroyuki
<kamezawa.hiroyu at jp.fujitsu.com> wrote:
> Greg, I think clear_page_writeback() will not require _any_ locks with this patch.
> But set_page_writeback() requires it...
> (Maybe adding a special function for clear_page_writeback() is better rather than
>  adding some complex to switch() in update_page_stat())
>
> ==
> From: KAMEZAWA Hiroyuki <kamezawa.hiroyu at jp.fujitsu.com>
>
> Now, at page information accounting, we do lock_page_cgroup() if pc->mem_cgroup
> points to a cgroup where someone is moving charges from.
>
> At supporing dirty-page accounting, one of troubles is writeback bit.
> In general, writeback can be cleared via IRQ context. To update writeback bit
> with lock_page_cgroup() in safe way, we'll have to disable IRQ.
> ....or do something.
>
> This patch waits for completion of writeback under lock_page() and do
> lock_page_cgroup() in safe way. (We never got end_io via IRQ context.)
>
> By this, writeback-accounting will never see race with account_move() and
> it can trust pc->mem_cgroup always _without_ any lock.
>
> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu at jp.fujitsu.com>
> ---
>  mm/memcontrol.c |   18 ++++++++++++++++++
>  1 file changed, 18 insertions(+)
>
> Index: mmotm-0928/mm/memcontrol.c
> ===================================================================
> --- mmotm-0928.orig/mm/memcontrol.c
> +++ mmotm-0928/mm/memcontrol.c
> @@ -2183,17 +2183,35 @@ static void __mem_cgroup_move_account(st
>  /*
>  * check whether the @pc is valid for moving account and call
>  * __mem_cgroup_move_account()
> + * Don't call this under pte_lock etc...we'll do lock_page() and wait for
> + * the end of I/O.
>  */
>  static int mem_cgroup_move_account(struct page_cgroup *pc,
>                struct mem_cgroup *from, struct mem_cgroup *to, bool uncharge)
>  {
>        int ret = -EINVAL;
> +
> +       /*
> +        * We move severl flags and accounting information here. So we need to
> +        * avoid the races with update_stat routines. For most of routines,
> +        * lock_page_cgroup() is enough for avoiding race. But we need to take
> +        * care of IRQ context. If flag updates comes from IRQ context, This
> +        * "move account" will be racy (and cause deadlock in lock_page_cgroup())
> +        *
> +        * Now, the only race we have is Writeback flag. We wait for it cleared
> +        * before starting our jobs.
> +        */
> +
> +       lock_page(pc->page);
> +       wait_on_page_writeback(pc->page);
> +
>        lock_page_cgroup(pc);
>        if (PageCgroupUsed(pc) && pc->mem_cgroup == from) {
>                __mem_cgroup_move_account(pc, from, to, uncharge);
>                ret = 0;
>        }
>        unlock_page_cgroup(pc);
> +       unlock_page(pc->page);
>        /*
>         * check events
>         */
>
>

Looks good to me.
But let me ask a question.
Why do only move_account need this logic?
Is deadlock candidate is only this place?
How about mem_cgroup_prepare_migration?

unmap_and_move
lock_page
mem_cgroup_prepare_migration
lock_page_cgroup
...
softirq happen
lock_page_cgroup


If race happens only where move_account and writeback, please describe
it as comment.
It would help to review the code in future.

-- 
Kind regards,
Minchan Kim
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