[PATCH v3 4/6] docs: transhuge: document process level THP controls

[Usama Arif <usamaarif642@gmail.com>]

This includes the PR_SET_THP_DISABLE/PR_GET_THP_DISABLE pair of
prctl calls as well the newly introduced PR_THP_DISABLE_EXCEPT_ADVISED
flag for the PR_SET_THP_DISABLE prctl call.

Signed-off-by: Usama Arif <usamaarif642@gmail.com>
---
 Documentation/admin-guide/mm/transhuge.rst | 38 ++++++++++++++++++++++
 1 file changed, 38 insertions(+)

diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst
index 370fba113460..a36a04394ff5 100644
--- a/Documentation/admin-guide/mm/transhuge.rst
+++ b/Documentation/admin-guide/mm/transhuge.rst
@@ -225,6 +225,44 @@ to "always" or "madvise"), and it'll be automatically shutdown when
 PMD-sized THP is disabled (when both the per-size anon control and the
 top-level control are "never")
 
+process THP controls
+--------------------
+
+A process can control its own THP behaviour using the ``PR_SET_THP_DISABLE``
+and ``PR_GET_THP_DISABLE`` pair of prctl(2) calls. These calls support the
+following arguments::
+
+	prctl(PR_SET_THP_DISABLE, 1, 0, 0, 0):
+		This will set the MMF_DISABLE_THP_COMPLETELY mm flag which will
+		result in no THPs being faulted in or collapsed, irrespective
+		of global THP controls. This flag and hence the behaviour is
+		inherited across fork(2) and execve(2).
+
+	prctl(PR_SET_THP_DISABLE, 1, PR_THP_DISABLE_EXCEPT_ADVISED, 0, 0):
+		This will set the MMF_DISABLE_THP_EXCEPT_ADVISED mm flag which
+		will result in THPs being faulted in or collapsed only for
+		the following cases:
+		- Global THP controls are set to "always" or "madvise" and
+		  the process has madvised the region with either MADV_HUGEPAGE
+		  or MADV_COLLAPSE.
+		- Global THP controls is set to "never" and the process has
+		  madvised the region with MADV_COLLAPSE.
+		This flag and hence the behaviour is inherited across fork(2)
+		and execve(2).
+
+	prctl(PR_SET_THP_DISABLE, 0, 0, 0, 0):
+		This will clear the MMF_DISABLE_THP_COMPLETELY and
+		MMF_DISABLE_THP_EXCEPT_ADVISED mm flags. The process will
+		behave according to the global THP controls. This behaviour
+		will be inherited across fork(2) and execve(2).
+
+	prctl(PR_GET_THP_DISABLE, 0, 0, 0, 0):
+		This will return the THP disable mm flag status of the process
+		that was set by prctl(PR_SET_THP_DISABLE, ...). i.e.
+		- 1 if MMF_DISABLE_THP_COMPLETELY flag is set
+		- 3 if MMF_DISABLE_THP_EXCEPT_ADVISED flag is set
+		- 0 otherwise.
+
 Khugepaged controls
 -------------------
 
-- 
2.47.3