linux-kernel - Re: [PATCH v20 4/9] mm: hugetlb: free the vmemmap pages associated with each HugeTLB page

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Date:   Fri, 16 Apr 2021 14:10:39 -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,
        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,
        osalvador@...e.de, mhocko@...e.com, song.bao.hua@...ilicon.com,
        david@...hat.com, naoya.horiguchi@....com,
        joao.m.martins@...cle.com
Cc:     duanxiongchun@...edance.com, fam.zheng@...edance.com,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
        Chen Huang <chenhuang5@...wei.com>,
        Bodeddula Balasubramaniam <bodeddub@...zon.com>
Subject: Re: [PATCH v20 4/9] mm: hugetlb: free the vmemmap pages associated
 with each HugeTLB page

On 4/15/21 1:40 AM, Muchun Song wrote:
> Every HugeTLB has more than one struct page structure. We __know__ that
> we only use the first 4 (__NR_USED_SUBPAGE) struct page structures
> to store metadata associated with each HugeTLB.
> 
> There are a lot of struct page structures associated with each HugeTLB
> page. For tail pages, the value of compound_head is the same. So we can
> reuse first page of tail page structures. We map the virtual addresses
> of the remaining pages of tail page structures to the first tail page
> struct, and then free these page frames. Therefore, we need to reserve
> two pages as vmemmap areas.
> 
> When we allocate a HugeTLB page from the buddy, we can free some vmemmap
> pages associated with each HugeTLB page. It is more appropriate to do it
> in the prep_new_huge_page().
> 
> The free_vmemmap_pages_per_hpage(), which indicates how many vmemmap
> pages associated with a HugeTLB page can be freed, returns zero for
> now, which means the feature is disabled. We will enable it once all
> the infrastructure is there.
> 
> Signed-off-by: Muchun Song <songmuchun@...edance.com>
> Reviewed-by: Oscar Salvador <osalvador@...e.de>
> Tested-by: Chen Huang <chenhuang5@...wei.com>
> Tested-by: Bodeddula Balasubramaniam <bodeddub@...zon.com>
> Acked-by: Michal Hocko <mhocko@...e.com>

There may need to be some trivial rebasing due to Oscar's changes
when they go in.

Reviewed-by: Mike Kravetz <mike.kravetz@...cle.com>
-- 
Mike Kravetz