Mike Kravetz <mike.kravetz@xxxxxxxxxx> writes:
> Describe demote and demote_size interfaces.
>
> Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
> ---
> 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
