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From: Mike Kravetz <mike.kravetz@oracle.com>
To: "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>,
	linux-mm@kvack.org, linux-kernel@vger.kernel.org
Cc: David Hildenbrand <david@redhat.com>,
	Michal Hocko <mhocko@suse.com>,
	Oscar Salvador <osalvador@suse.de>, Zi Yan <ziy@nvidia.com>,
	Muchun Song <songmuchun@bytedance.com>,
	Naoya Horiguchi <naoya.horiguchi@linux.dev>,
	David Rientjes <rientjes@google.com>,
	Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH 5/8] hugetlb: document the demote sysfs interfaces
Date: Tue, 21 Sep 2021 10:17:35 -0700	[thread overview]
Message-ID: <f602af2c-1055-1eff-3071-cca891d779b7@oracle.com> (raw)
In-Reply-To: <87bl4m9gsw.fsf@linux.ibm.com>

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

  reply	other threads:[~2021-09-21 17:18 UTC|newest]

Thread overview: 39+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-08-16 22:49 [PATCH RESEND 0/8] hugetlb: add demote/split page functionality Mike Kravetz
2021-08-16 22:49 ` [PATCH 1/8] hugetlb: add demote hugetlb page sysfs interfaces Mike Kravetz
2021-08-16 22:49 ` [PATCH 2/8] hugetlb: add HPageCma flag and code to free non-gigantic pages in CMA Mike Kravetz
2021-08-16 22:49 ` [PATCH 3/8] hugetlb: add demote bool to gigantic page routines Mike Kravetz
2021-08-16 22:49 ` [PATCH 4/8] hugetlb: add hugetlb demote page support Mike Kravetz
2021-08-16 22:49 ` [PATCH 5/8] hugetlb: document the demote sysfs interfaces Mike Kravetz
2021-08-16 23:28   ` Andrew Morton
2021-08-17  1:04     ` Mike Kravetz
2021-09-21 13:52   ` Aneesh Kumar K.V
2021-09-21 17:17     ` Mike Kravetz [this message]
2021-08-16 22:49 ` [PATCH 6/8] hugetlb: vmemmap optimizations when demoting hugetlb pages Mike Kravetz
2021-08-16 22:49 ` [PATCH 7/8] hugetlb: prepare destroy and prep routines for vmemmap optimized pages Mike Kravetz
2021-08-16 22:49 ` [PATCH 8/8] hugetlb: Optimized demote vmemmap optimizatized pages Mike Kravetz
2021-08-16 23:23 ` [PATCH RESEND 0/8] hugetlb: add demote/split page functionality Andrew Morton
2021-08-17  0:17   ` Mike Kravetz
2021-08-17  0:39     ` Andrew Morton
2021-08-17  0:58       ` Mike Kravetz
2021-08-16 23:27 ` Andrew Morton
2021-08-17  0:46   ` Mike Kravetz
2021-08-17  1:46     ` Andrew Morton
2021-08-17  7:30       ` David Hildenbrand
2021-08-17 16:19         ` Mike Kravetz
2021-08-17 18:49           ` David Hildenbrand
2021-08-24 22:08       ` Mike Kravetz
2021-08-27 17:22         ` Vlastimil Babka
2021-08-27 23:04           ` Mike Kravetz
2021-08-30 10:11             ` Vlastimil Babka
2021-09-02 18:17               ` Mike Kravetz
2021-09-06 14:40                 ` Vlastimil Babka
     [not found]                 ` <20210907085001.3773-1-hdanton@sina.com>
2021-09-08 21:00                   ` Mike Kravetz
2021-09-09 11:54                     ` Michal Hocko
2021-09-09 13:45                       ` Vlastimil Babka
2021-09-09 21:31                         ` Mike Kravetz
2021-09-10  8:20                         ` Michal Hocko
2021-09-11  0:11                           ` Mike Kravetz
2021-09-13 15:50                             ` Michal Hocko
2021-09-15 16:57                               ` Mike Kravetz
2021-09-17 20:44                                 ` Mike Kravetz
  -- strict thread matches above, loose matches on Subject: below --
2021-07-21 23:05 [PATCH " Mike Kravetz
2021-07-21 23:05 ` [PATCH 5/8] hugetlb: document the demote sysfs interfaces Mike Kravetz

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