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

Message ID 20210721230511.201823-6-mike.kravetz@oracle.com (mailing list archive)
State New
Headers show
Series hugetlb: add demote/split page functionality | expand

Commit Message

Mike Kravetz July 21, 2021, 11:05 p.m. UTC
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(-)
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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::
-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
@@ -243,7 +247,28 @@  Inside each of these directories, the same set of files will exist::
-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:
+        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.
+        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: