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

[Mike Kravetz]

On 9/21/21 6:52 AM, Aneesh Kumar K.V wrote:
> 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?
> 

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