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

Mike Kravetz
2021-09-21T17:18:05Z

On 9/21/21 6:52 AM, Aneesh Kumar K.V wrote:
> Mike Kravetz <mike.kravetz@oracle.com> writes:
>
>> 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.
>
> 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?
>

Thanks for taking a look Aneesh!

You are right. If demote_size is PAGE_SIZE, then demote is just an
alternative to freeing huge pages via the nr_hugepages interface.
So, why even provide such an interface?
It certainly would be easy to just not display demote interfaces for the
smallest huge page size.

Based on other feedback, I am also making demote_size writable. It
makes little sense on x86 with only two huge page sizes. However, it
might be useful on other architectures with more sizes. demote_size can
only be a valid huge page size. Before your comment, I was going to
allow setting demote_size to PAGE_SIZE. Perhaps, that should not be
allowed.

Thanks for the suggestion, I think removing support for demote_size ==
PAGE_SIZE will make the code simpler.
--
Mike Kravetz