linux-kernel - Re: [PATCH 2/4] mm: thp: make deferred split shrinker memcg aware

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Date:   Wed, 12 Jun 2019 10:13:51 -0700
From:   Yang Shi <yang.shi@...ux.alibaba.com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     ktkhai@...tuozzo.com, kirill.shutemov@...ux.intel.com,
        hannes@...xchg.org, mhocko@...e.com, hughd@...gle.com,
        shakeelb@...gle.com, rientjes@...gle.com,
        akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/4] mm: thp: make deferred split shrinker memcg aware



On 6/12/19 3:09 AM, Kirill A. Shutemov wrote:
> On Tue, Jun 11, 2019 at 10:06:36PM -0700, Yang Shi wrote:
>>
>> On 6/11/19 7:47 PM, Kirill A. Shutemov wrote:
>>> On Fri, Jun 07, 2019 at 02:07:37PM +0800, Yang Shi wrote:
>>>> +	/*
>>>> +	 * The THP may be not on LRU at this point, e.g. the old page of
>>>> +	 * NUMA migration.  And PageTransHuge is not enough to distinguish
>>>> +	 * with other compound page, e.g. skb, THP destructor is not used
>>>> +	 * anymore and will be removed, so the compound order sounds like
>>>> +	 * the only choice here.
>>>> +	 */
>>>> +	if (PageTransHuge(page) && compound_order(page) == HPAGE_PMD_ORDER) {
>>> What happens if the page is the same order as THP is not THP? Why removing
>> It may corrupt the deferred split queue since it is never added into the
>> list, but deleted here.
>>
>>> of destructor is required?
>> Due to the change to free_transhuge_page() (extracted deferred split queue
>> manipulation and moved before memcg uncharge since page->mem_cgroup is
>> needed), it just calls free_compound_page(). So, it sounds pointless to
>> still keep THP specific destructor.
>>
>> It looks there is not a good way to tell if the compound page is THP in
>> free_page path or not, we may keep the destructor just for this?
> Other option would be to move mem_cgroup_uncharge(page); from
> __page_cache_release() to destructors. Destructors will be able to
> call it as it fits.

Yes, it is an option. Since __page_cache_release() is called by 
__put_single_page() too, so mem_cgroup_uncharge() has to be called in 
both __put_single_page() and the desctructor (free_compound_page() which 
is called by both THP and other compound page except HugeTLB). But, it 
sounds acceptable IMHO.

>