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

[Hiroyuki Kamezawa <kamezawa.hiroyuki@gmail.com>]

On Wed, May 2, 2012 at 7:28 AM, Suleiman Souhlal <suleiman@google.com> wrote:
> 2012/4/26 KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>:
>> 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@jp.fujitsu.com>
>> ---
>>  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?
>
see whole series of patch series, 7 & 8 is against that probelm.
But they will be dropped and this race will remain. And this patch's
title will be
changed to be "remove -EINTR" rather than "remove failure of pre_destroy*.
pre_destrou() will continue to fail until cgroup core is fixed.

Thanks,
-Kame

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