linux-kernel - Re: [PATCH v2 2/4] hugetlb: add HPageCma flag and code to free non-gigantic pages in CMA

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Date:   Wed, 29 Sep 2021 12:42:05 -0700
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     David Hildenbrand <david@...hat.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Cc:     Michal Hocko <mhocko@...e.com>, Oscar Salvador <osalvador@...e.de>,
        Zi Yan <ziy@...dia.com>,
        Muchun Song <songmuchun@...edance.com>,
        Naoya Horiguchi <naoya.horiguchi@...ux.dev>,
        David Rientjes <rientjes@...gle.com>,
        "Aneesh Kumar K . V" <aneesh.kumar@...ux.ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v2 2/4] hugetlb: add HPageCma flag and code to free
 non-gigantic pages in CMA

On 9/24/21 2:36 AM, David Hildenbrand wrote:
> On 23.09.21 19:53, Mike Kravetz wrote:
>> When huge page demotion is fully implemented, gigantic pages can be
>> demoted to a smaller huge page size.  For example, on x86 a 1G page
>> can be demoted to 512 2M pages.  However, gigantic pages can potentially
>> be allocated from CMA.  If a gigantic page which was allocated from CMA
>> is demoted, the corresponding demoted pages needs to be returned to CMA.
>>
>> In order to track hugetlb pages that need to be returned to CMA, add the
>> hugetlb specific flag HPageCma.  Flag is set when a huge page is
>> allocated from CMA and transferred to any demoted pages.  Non-gigantic
>> huge page freeing code checks for the flag and takes appropriate action.
> 
> Do we really need that flag or couldn't we simply always try cma_release() and fallback to out ordinary freeing-path?
> 
> IIRC, cma knows exactly if something was allocated via a CMA are and can be free via it. No need for additional tracking usually.
> 

Yes, I think this is possible.
Initially, I thought the check for whether pages were part of CMA
involved a mutex or some type of locking.  But, it really is
lightweight.  So, should not be in issue calling in every case.

I will most likely add a !CMA_CONFIG stub for cma_release() and remove
some of the #ifdefs in hugetlb.c

Thanks, I will update in next version.
-- 
Mike Kravetz