From: Kirill Tkhai <ktkhai@virtuozzo.com>
To: Michal Hocko <mhocko@kernel.org>
Cc: akpm@linux-foundation.org, hannes@cmpxchg.org,
vdavydov.dev@gmail.com, cgroups@vger.kernel.org,
linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] memcg: Remove memcg_cgroup::id from IDR on mem_cgroup_css_alloc() failure
Date: Fri, 13 Apr 2018 14:49:32 +0300 [thread overview]
Message-ID: <8a81c801-35c8-767d-54b0-df9f1ca0abc0@virtuozzo.com> (raw)
In-Reply-To: <20180413113855.GI17484@dhcp22.suse.cz>
On 13.04.2018 14:38, Michal Hocko wrote:
> On Fri 13-04-18 14:29:11, Kirill Tkhai wrote:
>> On 13.04.2018 14:20, Michal Hocko wrote:
>>> On Fri 13-04-18 14:06:40, Kirill Tkhai wrote:
>>>> On 13.04.2018 14:02, Michal Hocko wrote:
>>>>> On Fri 13-04-18 12:35:22, Kirill Tkhai wrote:
>>>>>> On 13.04.2018 11:55, Michal Hocko wrote:
>>>>>>> On Thu 12-04-18 17:52:04, Kirill Tkhai wrote:
>>>>>>> [...]
>>>>>>>> @@ -4471,6 +4477,7 @@ mem_cgroup_css_alloc(struct cgroup_subsys_state *parent_css)
>>>>>>>>
>>>>>>>> return &memcg->css;
>>>>>>>> fail:
>>>>>>>> + mem_cgroup_id_remove(memcg);
>>>>>>>> mem_cgroup_free(memcg);
>>>>>>>> return ERR_PTR(-ENOMEM);
>>>>>>>> }
>>>>>>>
>>>>>>> The only path which jumps to fail: here (in the current mmotm tree) is
>>>>>>> error = memcg_online_kmem(memcg);
>>>>>>> if (error)
>>>>>>> goto fail;
>>>>>>>
>>>>>>> AFAICS and the only failure path in memcg_online_kmem
>>>>>>> memcg_id = memcg_alloc_cache_id();
>>>>>>> if (memcg_id < 0)
>>>>>>> return memcg_id;
>>>>>>>
>>>>>>> I am not entirely clear on memcg_alloc_cache_id but it seems we do clean
>>>>>>> up properly. Or am I missing something?
>>>>>>
>>>>>> memcg_alloc_cache_id() may allocate a lot of memory, in case of the system reached
>>>>>> memcg_nr_cache_ids cgroups. In this case it iterates over all LRU lists, and double
>>>>>> size of every of them. In case of memory pressure it can fail. If this occurs,
>>>>>> mem_cgroup::id is not unhashed from IDR and we leak this id.
>>>>>
>>>>> OK, my bad I was looking at the bad code path. So you want to clean up
>>>>> after mem_cgroup_alloc not memcg_online_kmem. Now it makes much more
>>>>> sense. Sorry for the confusion on my end.
>>>>>
>>>>> Anyway, shouldn't we do the thing in mem_cgroup_free() to be symmetric
>>>>> to mem_cgroup_alloc?
>>>>
>>>> We can't, since it's called from mem_cgroup_css_free(), which doesn't have a deal
>>>> with idr freeing. All the asymmetry, we see, is because of the trick to unhash ID
>>>> earlier, then from mem_cgroup_css_free().
>>>
>>> Are you sure. It's been some time since I've looked at the quite complex
>>> cgroup tear down code but from what I remember, css_free is called on
>>> the css release (aka when the reference count drops to zero). mem_cgroup_id_put_many
>>> seems to unpin the css reference so we should have idr_remove by the
>>> time when css_free is called. Or am I still wrong and should go over the
>>> brain hurting cgroup removal code again?
>>
>> mem_cgroup_id_put_many() unpins css, but this may be not the last reference to the css.
>> Thus, we release ID earlier, then all references to css are freed.
>
> Right and so what. If we have released the idr then we are not going to
> do that again in css_free. That is why we have that memcg->id.id > 0
> check before idr_remove and memcg->id.id = 0 for the last memcg ref.
> count. So again, why cannot we do the clean up in mem_cgroup_free and
> have a less confusing code? Or am I just not getting your point and
> being dense here?
We can, but mem_cgroup_free() called from mem_cgroup_css_alloc() is unlikely case.
The likely case is mem_cgroup_free() is called from mem_cgroup_css_free(), where
this idr manipulations will be a noop. Noop in likely case looks more confusing
for me.
Less confusing will be to move
memcg->id.id = idr_alloc(&mem_cgroup_idr, NULL,
1, MEM_CGROUP_ID_MAX,
GFP_KERNEL);
into mem_cgroup_css_alloc(). How are you think about this?
Kirill
next prev parent reply other threads:[~2018-04-13 11:49 UTC|newest]
Thread overview: 23+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-04-12 14:52 [PATCH] memcg: Remove memcg_cgroup::id from IDR on mem_cgroup_css_alloc() failure Kirill Tkhai
2018-04-13 8:55 ` Michal Hocko
2018-04-13 9:35 ` Kirill Tkhai
2018-04-13 11:02 ` Michal Hocko
2018-04-13 11:06 ` Kirill Tkhai
2018-04-13 11:20 ` Michal Hocko
2018-04-13 11:29 ` Kirill Tkhai
2018-04-13 11:38 ` Michal Hocko
2018-04-13 11:49 ` Kirill Tkhai [this message]
2018-04-13 11:54 ` Michal Hocko
2018-04-13 12:07 ` Kirill Tkhai
2018-04-13 12:14 ` Michal Hocko
2018-04-13 12:51 ` Michal Hocko
2018-07-26 23:25 ` Andrew Morton
2018-07-27 19:31 ` Johannes Weiner
2018-07-29 19:26 ` Vladimir Davydov
2018-07-30 15:31 ` Johannes Weiner
2018-07-31 23:39 ` Andrew Morton
2018-08-01 15:55 ` Johannes Weiner
2018-08-01 16:22 ` Vladimir Davydov
2018-08-02 8:03 ` Kirill Tkhai
2018-08-02 8:13 ` [PATCH] memcg: Add comment to mem_cgroup_css_online() Kirill Tkhai
2018-08-01 16:16 ` [PATCH] memcg: Remove memcg_cgroup::id from IDR on mem_cgroup_css_alloc() failure Vladimir Davydov
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=8a81c801-35c8-767d-54b0-df9f1ca0abc0@virtuozzo.com \
--to=ktkhai@virtuozzo.com \
--cc=akpm@linux-foundation.org \
--cc=cgroups@vger.kernel.org \
--cc=hannes@cmpxchg.org \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-mm@kvack.org \
--cc=mhocko@kernel.org \
--cc=vdavydov.dev@gmail.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).