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Date:   Mon, 4 Apr 2022 14:56:17 +0530
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Subject: Re: [PATCH v4 2/2] arm64: mm: hugetlb: Enable
 HUGETLB_PAGE_FREE_VMEMMAP for arm64
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To:     Muchun Song <songmuchun@bytedance.com>, will@kernel.org,
        akpm@linux-foundation.org, david@redhat.com, bodeddub@amazon.com,
        osalvador@suse.de, mike.kravetz@oracle.com, rientjes@google.com,
        mark.rutland@arm.com, catalin.marinas@arm.com, james.morse@arm.com,
        21cnbao@gmail.com
Cc:     linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, duanxiongchun@bytedance.com,
        fam.zheng@bytedance.com, smuchun@gmail.com
References: <20220331065640.5777-1-songmuchun@bytedance.com>
 <20220331065640.5777-2-songmuchun@bytedance.com>
From:   Anshuman Khandual <anshuman.khandual@arm.com>
In-Reply-To: <20220331065640.5777-2-songmuchun@bytedance.com>
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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