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

[Mike Kravetz]

On 8/16/21 4:28 PM, Andrew Morton wrote:
> On Mon, 16 Aug 2021 15:49:50 -0700 Mike Kravetz <mike.kravetz@xxxxxxxxxx> wrote:
> 
>> 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.
>> +
>> +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