linux-kernel - Re: [PATCH v4 2/2] arm64: mm: hugetlb: Enable HUGETLB_PAGE_FREE

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Date:   Mon, 4 Apr 2022 14:56:17 +0530
From:   Anshuman Khandual <anshuman.khandual@....com>
To:     Muchun Song <songmuchun@...edance.com>, will@...nel.org,
        akpm@...ux-foundation.org, david@...hat.com, bodeddub@...zon.com,
        osalvador@...e.de, mike.kravetz@...cle.com, rientjes@...gle.com,
        mark.rutland@....com, catalin.marinas@....com, james.morse@....com,
        21cnbao@...il.com
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, duanxiongchun@...edance.com,
        fam.zheng@...edance.com, smuchun@...il.com
Subject: Re: [PATCH v4 2/2] arm64: mm: hugetlb: Enable
 HUGETLB_PAGE_FREE_VMEMMAP for arm64

Hello Muchun,

On 3/31/22 12:26, Muchun Song wrote:
> The feature of minimizing overhead of struct page associated with each
> HugeTLB page aims to free its vmemmap pages (used as struct page) to
> save memory, where is ~14GB/16GB per 1TB HugeTLB pages (2MB/1GB type).

Enabling this feature saves us around 1.4/1.6 % memory but looking from
other way around, unavailability of vmemmap backing pages (~1.4GB) when
freeing up a corresponding HugeTLB page, could prevent ~1TB memory from
being used as normal page form (requiring their own struct pages), thus
forcing the HugeTLB page to remain as such ? Is not this problematic ?

These additional 1TB memory in normal pages, from a HugeTLB dissolution
could have eased the system's memory pressure without this feature being
enabled.

- Anshuman