[PATCH v4 4/7] 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 | 37 ++++++++++++++++++++++
 1 file changed, 37 insertions(+)

diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst
index 370fba1134606..fa8242766e430 100644
--- a/Documentation/admin-guide/mm/transhuge.rst
+++ b/Documentation/admin-guide/mm/transhuge.rst
@@ -225,6 +225,43 @@ 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. The THP behaviour set using
+``PR_SET_THP_DISABLE`` is inherited across fork(2) and execve(2). These calls
+support the following arguments::
+
+	prctl(PR_SET_THP_DISABLE, 1, 0, 0, 0):
+		This will disable THPs completely for the process, irrespective
+		of global THP controls or MADV_COLLAPSE.
+
+	prctl(PR_SET_THP_DISABLE, 1, PR_THP_DISABLE_EXCEPT_ADVISED, 0, 0):
+		This will disable THPs for the process except when the usage of THPs is
+		advised. Consequently, THPs will only be used when:
+		- Global THP controls are set to "always" or "madvise" and
+		  the area either has VM_HUGEPAGE set (e.g., due do MADV_HUGEPAGE) or
+		  MADV_COLLAPSE is used.
+		- Global THP controls are set to "never" and MADV_COLLAPSE is used. This
+		  is the same behavior as if THPs would not be disabled on a process
+		  level.
+		Note that MADV_COLLAPSE is currently always rejected if VM_NOHUGEPAGE is
+		set on an area.
+
+	prctl(PR_SET_THP_DISABLE, 0, 0, 0, 0):
+		This will re-enabled THPs for the process, as if they would never have
+		been disabled. Whether THPs will actually be used depends on global THP
+		controls.
+
+	prctl(PR_GET_THP_DISABLE, 0, 0, 0, 0):
+		This returns a value whose bit indicate how THP-disable is configured:
+		Bits
+		 1 0  Value  Description
+		|0|0|   0    No THP-disable behaviour specified.
+		|0|1|   1    THP is entirely disabled for this process.
+		|1|1|   3    THP-except-advised mode is set for this process.
+
 Khugepaged controls
 -------------------
 
-- 
2.47.3