Re: [RFC][PATCH 9/9 v2] memcg: never return error at pre_destroy()

[Suleiman Souhlal]

2012/4/26 KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>:
> When force_empty() called by ->pre_destroy(), no memory reclaim happens
> and it doesn't take very long time which requires signal_pending() check.
> And if we return -EINTR from pre_destroy(), cgroup.c show warning.
>
> This patch removes signal check in force_empty(). By this, ->pre_destroy()
> returns success always.
>
> Note: check for 'cgroup is empty' remains for force_empty interface.
>
> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
> ---
>  mm/hugetlb.c    |   10 +---------
>  mm/memcontrol.c |   14 +++++---------
>  2 files changed, 6 insertions(+), 18 deletions(-)
>
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 4dd6b39..770f1642 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -1922,20 +1922,12 @@ int hugetlb_force_memcg_empty(struct cgroup *cgroup)
>        int ret = 0, idx = 0;
>
>        do {
> +               /* see memcontrol.c::mem_cgroup_force_empty() */
>                if (cgroup_task_count(cgroup)
>                        || !list_empty(&cgroup->children)) {
>                        ret = -EBUSY;
>                        goto out;
>                }
> -               /*
> -                * If the task doing the cgroup_rmdir got a signal
> -                * we don't really need to loop till the hugetlb resource
> -                * usage become zero.
> -                */
> -               if (signal_pending(current)) {
> -                       ret = -EINTR;
> -                       goto out;
> -               }
>                for_each_hstate(h) {
>                        spin_lock(&hugetlb_lock);
>                        list_for_each_entry(page, &h->hugepage_activelist, lru) {
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 2715223..ee350c5 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -3852,8 +3852,6 @@ static int mem_cgroup_force_empty_list(struct mem_cgroup *memcg,
>                pc = lookup_page_cgroup(page);
>
>                ret = mem_cgroup_move_parent(page, pc, memcg, GFP_KERNEL);
> -               if (ret == -ENOMEM || ret == -EINTR)
> -                       break;
>
>                if (ret == -EBUSY || ret == -EINVAL) {
>                        /* found lock contention or "pc" is obsolete. */
> @@ -3863,7 +3861,7 @@ static int mem_cgroup_force_empty_list(struct mem_cgroup *memcg,
>                        busy = NULL;
>        }
>
> -       if (!ret && !list_empty(list))
> +       if (!loop)
>                return -EBUSY;
>        return ret;
>  }
> @@ -3893,11 +3891,12 @@ static int mem_cgroup_force_empty(struct mem_cgroup *memcg, bool free_all)
>  move_account:
>        do {
>                ret = -EBUSY;
> +               /*
> +                * This never happens when this is called by ->pre_destroy().
> +                * But we need to take care of force_empty interface.
> +                */
>                if (cgroup_task_count(cgrp) || !list_empty(&cgrp->children))
>                        goto out;

Are you sure this never happens when called by ->pre_destroy()?
Can't a task still get attached to the cgroup while ->pre_destroy() is running?

At least, I don't see anything in the cgroup code that prevents
someone from newly attaching a task at that point.
In fact, there is code that seems to handle the case when someone
attached to the cgroup after pre_destroy() has run: See the
cgroup_wakeup_rmdir_waiter() call in cgroup_attach_task().

-- Suleiman
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