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

Mike Kravetz
2021-08-17T01:05:11Z

On 8/16/21 4:28 PM, Andrew Morton wrote:
> On Mon, 16 Aug 2021 15:49:50 -0700 Mike Kravetz <mike.kravetz@oracle.com> wrote:
>
>> Describe demote and demote_size interfaces.
>>
>> Signed-off-by: Mike Kravetz <mike.kravetz@oracle.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.
>> +
>> +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.
>
> Are these new demote interfaces duplicated in /proc?
> Documentation/admin-guide/mm/hugetlbpage.rst says "The ``/proc``
> interfaces discussed above have been retained for backwards
> compatibility.", so new interfaces need not appear in /proc?
>

The new demote interfaces are only in sysfs, they are not /proc.

--
Mike Kravetz