linux-kernel - Re: [RFC PATCH 03/24] mm/hugetlb: Introduce a new config HUGETLB_PAGE_FREE

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Date:   Tue, 29 Sep 2020 16:41:41 -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
Cc:     linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, linux-fsdevel@...r.kernel.org
Subject: Re: [RFC PATCH 03/24] mm/hugetlb: Introduce a new config
 HUGETLB_PAGE_FREE_VMEMMAP

On 9/15/20 5:59 AM, Muchun Song wrote:
> The purpose of introducing HUGETLB_PAGE_FREE_VMEMMAP is to configure
> whether to enable the feature of freeing unused vmemmap associated
> with HugeTLB pages.
> 
> Signed-off-by: Muchun Song <songmuchun@...edance.com>
> ---
>  fs/Kconfig | 15 +++++++++++++++
>  1 file changed, 15 insertions(+)
> 
> diff --git a/fs/Kconfig b/fs/Kconfig
> index 976e8b9033c4..61e9c08096ca 100644
> --- a/fs/Kconfig
> +++ b/fs/Kconfig
> @@ -245,6 +245,21 @@ config HUGETLBFS
>  config HUGETLB_PAGE
>  	def_bool HUGETLBFS
>  
> +config HUGETLB_PAGE_FREE_VMEMMAP
> +	bool "Free unused vmemmap associated with HugeTLB pages"
> +	default n
> +	depends on HUGETLB_PAGE
> +	depends on SPARSEMEM_VMEMMAP
> +	depends on HAVE_BOOTMEM_INFO_NODE
> +	help
> +	  There are many struct page structure associated with each HugeTLB
> +	  page. But we only use a few struct page structure. In this case,
> +	  it waste some memory. It is better to free the unused struct page
> +	  structures to buddy system which can save some memory. For
> +	  architectures that support it, say Y here.
> +
> +	  If unsure, say N.
> +

I could be wrong, but I believe the convention is introduce the config
option at the same time code which depends on the option is introduced.
Therefore, it might be better to combine with the next patch.

Also, it looks like most of your development was done on x86.  Should
this option be limited to x86 only for now?
-- 
Mike Kravetz