Re: [v3 PATCH 2/4] mm: move mem_cgroup_uncharge out of __page_cache_release()

[Yang Shi <yang.shi@linux.alibaba.com>]

On 6/13/19 4:39 AM, Kirill A. Shutemov wrote:
> On Thu, Jun 13, 2019 at 05:56:47AM +0800, Yang Shi wrote:
>> The later patch would make THP deferred split shrinker memcg aware, but
>> it needs page->mem_cgroup information in THP destructor, which is called
>> after mem_cgroup_uncharge() now.
>>
>> So, move mem_cgroup_uncharge() from __page_cache_release() to compound
>> page destructor, which is called by both THP and other compound pages
>> except HugeTLB.  And call it in __put_single_page() for single order
>> page.
>
> If I read the patch correctly, it will change behaviour for pages with
> NULL_COMPOUND_DTOR. Have you considered it? Are you sure it will not break
> anything?

So far a quick search shows NULL_COMPOUND_DTOR is not used by any type 
of compound page. The HugeTLB code sets destructor to NULL_COMPOUND_DTOR 
when freeing hugetlb pages via hugetlb specific destructor.

The prep_new_page() would call prep_compound_page() if __GFP_COMP is 
used, which sets dtor to COMPOUND_PAGE_DTOR by default.  Just hugetlb 
and THP set their specific dtors.

And, it looks __put_compound_page() doesn't check if dtor is NULL or not 
at all.

>