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

From: Mike Kravetz <mike.kravetz@oracle.com>
To: 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>,
	Mike Kravetz <mike.kravetz@oracle.com>
Subject: [PATCH 5/8] hugetlb: document the demote sysfs interfaces
Date: Wed, 21 Jul 2021 16:05:08 -0700	[thread overview]
Message-ID: <20210721230511.201823-6-mike.kravetz@oracle.com> (raw)
In-Reply-To: <20210721230511.201823-1-mike.kravetz@oracle.com>

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.
 
 .. _mem_policy_and_hp_alloc:
 
-- 
2.31.1

