linux-kernel - Re: [PATCH 5/8] hugetlb: document the demote sysfs interfaces

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Message-ID: <87bl4m9gsw.fsf@linux.ibm.com>
Date:   Tue, 21 Sep 2021 19:22:47 +0530
From:   "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>
To:     Mike Kravetz <mike.kravetz@...cle.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Cc:     David Hildenbrand <david@...hat.com>,
        Michal Hocko <mhocko@...e.com>,
        Oscar Salvador <osalvador@...e.de>, Zi Yan <ziy@...dia.com>,
        Muchun Song <songmuchun@...edance.com>,
        Naoya Horiguchi <naoya.horiguchi@...ux.dev>,
        David Rientjes <rientjes@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mike Kravetz <mike.kravetz@...cle.com>
Subject: Re: [PATCH 5/8] hugetlb: document the demote sysfs interfaces

Mike Kravetz <mike.kravetz@...cle.com> writes:

> Describe demote and demote_size interfaces.
>
> Signed-off-by: Mike Kravetz <mike.kravetz@...cle.com>
> ---
>  Documentation/admin-guide/mm/hugetlbpage.rst | 29 ++++++++++++++++++--
>  1 file changed, 27 insertions(+), 2 deletions(-)
>
> diff --git a/Documentation/admin-guide/mm/hugetlbpage.rst b/Documentation/admin-guide/mm/hugetlbpage.rst
> index 8abaeb144e44..902059a0257b 100644
> --- a/Documentation/admin-guide/mm/hugetlbpage.rst
> +++ b/Documentation/admin-guide/mm/hugetlbpage.rst
> @@ -234,8 +234,12 @@ will exist, of the form::
>  
>  	hugepages-${size}kB
>  
> -Inside each of these directories, the same set of files will exist::
> +Inside each of these directories, the set of files contained in ``/proc``
> +will exist.  In addition, two additional interfaces for demoting huge
> +pages will exist::
>  
> +        demote
> +        demote_size
>  	nr_hugepages
>  	nr_hugepages_mempolicy
>  	nr_overcommit_hugepages
> @@ -243,7 +247,28 @@ Inside each of these directories, the same set of files will exist::
>  	resv_hugepages
>  	surplus_hugepages
>  
> -which function as described above for the default huge page-sized case.
> +The demote interfaces provide the ability to split a huge page into
> +smaller huge pages.  For example, the x86 architecture supports both
> +1GB and 2MB huge pages sizes.  A 1GB huge page can be split into 512
> +2MB huge pages.  The demote interfaces are:
> +
> +demote_size
> +        is the size of demoted pages.  When a page is demoted a corresponding
> +        number of huge pages of demote_size will be created.  For huge pages
> +        of the smallest supported size (2MB on x86), demote_size will be the
> +        system page size (PAGE_SIZE).  If demote_size is the system page size
> +        then demoting a page will simply free the huge page.  demote_size is
> +        a read only interface.

That is an alternate interface for nr_hugepages. Will it be better to
return EINVAL on write to 'demote' file below
/sys/kernel/mm/hugepages/hugepages-2048kB ?

Or may be not expose demote possibility within 2M hugepage directory at all?


> +
> +demote
> +        is used to demote a number of huge pages.  A user with root privileges
> +        can write to this file.  It may not be possible to demote the
> +        requested number of huge pages.  To determine how many pages were
> +        actually demoted, compare the value of nr_hugepages before and after
> +        writing to the demote interface.  demote is a write only interface.
> +
> +The interfaces which are the same as in ``/proc`` function as described
> +above for the default huge page-sized case.
>  
>  .. _mem_policy_and_hp_alloc:
>  
> -- 
> 2.31.1