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

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Date:   Mon, 16 Aug 2021 15:49:50 -0700
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     linux-mm@...ck.org, linux-kernel@...r.kernel.org
Cc:     David Hildenbrand <david@...hat.com>,
        Michal Hocko <mhocko@...e.com>,
        Oscar Salvador <osalvador@...e.de>, Zi Yan <ziy@...dia.com>,
        Muchun Song <songmuchun@...edance.com>,
        Naoya Horiguchi <naoya.horiguchi@...ux.dev>,
        David Rientjes <rientjes@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mike Kravetz <mike.kravetz@...cle.com>
Subject: [PATCH 5/8] hugetlb: document the demote sysfs interfaces

Describe demote and demote_size interfaces.

Signed-off-by: Mike Kravetz <mike.kravetz@...cle.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