[PATCH v5 4/7] docs: transhuge: document process level THP controls

[Usama Arif]

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@xxxxxxxxx>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
Reviewed-by: Zi Yan <ziy@xxxxxxxxxx>
---
 Documentation/admin-guide/mm/transhuge.rst | 36 ++++++++++++++++++++++
 1 file changed, 36 insertions(+)

diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst
index 370fba1134606..a16a04841b960 100644
--- a/Documentation/admin-guide/mm/transhuge.rst
+++ b/Documentation/admin-guide/mm/transhuge.rst
@@ -225,6 +225,42 @@ 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 madvise(..., MADV_COLLAPSE) being used.
+
+	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
+		  madvise(..., MADV_HUGEPAGE) or madvise(..., MADV_COLLAPSE) is used.
+		- Global THP controls are set to "never" and madvise(..., 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
+		madvise(..., MADV_NOHUGEPAGE) is set on an area.
+
+	prctl(PR_SET_THP_DISABLE, 0, 0, 0, 0):
+		This will re-enable THPs for the process, as if they were never disabled.
+		Whether THPs will actually be used depends on global THP controls and
+		madvise() calls.
+
+	prctl(PR_GET_THP_DISABLE, 0, 0, 0, 0):
+		This returns a value whose bits 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