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

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Date:   Tue, 11 Jun 2019 22:06:36 -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 6/11/19 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?

>