linux-kernel - Re: [RFC PATCH 3/3] hugetlb: Lazy page table copies in fork()

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Message-ID: <Ypb3HhYU/bFL7yx7@FVFYT0MHHV2J.googleapis.com>
Date:   Wed, 1 Jun 2022 13:20:30 +0800
From:   Muchun Song <songmuchun@...edance.com>
To:     Mike Kravetz <mike.kravetz@...cle.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Michal Hocko <mhocko@...e.com>, Peter Xu <peterx@...hat.com>,
        Naoya Horiguchi <naoya.horiguchi@...ux.dev>,
        James Houghton <jthoughton@...gle.com>,
        Mina Almasry <almasrymina@...gle.com>,
        "Aneesh Kumar K . V" <aneesh.kumar@...ux.vnet.ibm.com>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [RFC PATCH 3/3] hugetlb: Lazy page table copies in fork()

On Fri, May 27, 2022 at 03:58:49PM -0700, Mike Kravetz wrote:
> Lazy page table copying at fork time was introduced with commit
> d992895ba2b2 ("Lazy page table copies in fork()").  At the time,
> hugetlb was very new and did not support page faulting.  As a result,
> it was excluded.  When full page fault support was added for hugetlb,
> the exclusion was not removed.
> 
> Simply remove the check that prevents lazy copying of hugetlb page
> tables at fork.  Of course, like other mappings this only applies to
> shared mappings.
> 
> Lazy page table copying at fork will be less advantageous for hugetlb
> mappings because:
> - There are fewer page table entries with hugetlb
> - hugetlb pmds can be shared instead of copied
> 
> In any case, completely eliminating the copy at fork time shold speed
> things up.
> 
> Signed-off-by: Mike Kravetz <mike.kravetz@...cle.com>

Acked-by: Muchun Song <songmuchun@...edance.com>

Thanks.