linux-kernel - Re: [PATCH v2 04/19] mm/hugetlb: Introduce nr_free_vmemmap

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Date:   Tue, 27 Oct 2020 15:03:30 -0700
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Muchun Song <songmuchun@...edance.com>, corbet@....net,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, x86@...nel.org,
        hpa@...or.com, dave.hansen@...ux.intel.com, luto@...nel.org,
        peterz@...radead.org, viro@...iv.linux.org.uk,
        akpm@...ux-foundation.org, paulmck@...nel.org,
        mchehab+huawei@...nel.org, pawan.kumar.gupta@...ux.intel.com,
        rdunlap@...radead.org, oneukum@...e.com, anshuman.khandual@....com,
        jroedel@...e.de, almasrymina@...gle.com, rientjes@...gle.com,
        willy@...radead.org
Cc:     duanxiongchun@...edance.com, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v2 04/19] mm/hugetlb: Introduce nr_free_vmemmap_pages in
 the struct hstate

On 10/26/20 7:50 AM, Muchun Song wrote:
> If the size of hugetlb page is 2MB, we need 512 struct page structures
> (8 pages) to be associated with it. As far as I know, we only use the
> first 4 struct page structures.

Use of first 4 struct page structures comes from HUGETLB_CGROUP_MIN_ORDER.
You could point that out here.

I thought about creating a HUGETLB_MIN_ORDER definition that could be used
to calculate RESERVE_VMEMMAP_NR.  However, I think a hard coded value of
2U as in the patch is OK.

> For tail pages, the value of compound_dtor is the same. So we can reuse
> first page of tail page structs. We map the virtual addresses of the
> remaining 6 pages of tail page structs to the first tail page struct,
> and then free these 6 pages. Therefore, we need to reserve at least 2
> pages as vmemmap areas.
> 
> So we introduce a new nr_free_vmemmap_pages field in the hstate to
> indicate how many vmemmap pages associated with a hugetlb page that we
> can free to buddy system.
> 
> Signed-off-by: Muchun Song <songmuchun@...edance.com>
> ---
>  include/linux/hugetlb.h |  3 +++
>  mm/hugetlb.c            | 35 +++++++++++++++++++++++++++++++++++
>  2 files changed, 38 insertions(+)

Patch looks fine with updated commit message.
Acked-by: Mike Kravetz <mike.kravetz@...cle.com>

-- 
Mike Kravetz