[RFC 2/2] mm: document transparent_hugepage=defer usage

[Nico Pache <npache@redhat.com>]

The new transparent_hugepage=defer option allows for a more conservative
approach to THPs. Document its usage in the transhuge admin-guide.

Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Barry Song <baohua@kernel.org>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Lance Yang <ioworker0@gmail.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Zi Yan <ziy@nvidia.com>
Cc: Rafael Aquini <aquini@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Nico Pache <npache@redhat.com>
---
 Documentation/admin-guide/mm/transhuge.rst | 18 +++++++++++++++---
 1 file changed, 15 insertions(+), 3 deletions(-)

diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst
index 058485daf186..1946fbb789b2 100644
--- a/Documentation/admin-guide/mm/transhuge.rst
+++ b/Documentation/admin-guide/mm/transhuge.rst
@@ -88,8 +88,9 @@ In certain cases when hugepages are enabled system wide, application
 may end up allocating more memory resources. An application may mmap a
 large region but only touch 1 byte of it, in that case a 2M page might
 be allocated instead of a 4k page for no good. This is why it's
-possible to disable hugepages system-wide and to only have them inside
-MADV_HUGEPAGE madvise regions.
+possible to disable hugepages system-wide, only have them inside
+MADV_HUGEPAGE madvise regions, or defer them away from the page fault
+handler to khugepaged.
 
 Embedded systems should enable hugepages only inside madvise regions
 to eliminate any risk of wasting any precious byte of memory and to
@@ -99,6 +100,15 @@ Applications that gets a lot of benefit from hugepages and that don't
 risk to lose memory by using hugepages, should use
 madvise(MADV_HUGEPAGE) on their critical mmapped regions.
 
+Applications that would like to benefit from THPs but would still like a
+more memory conservative approach can choose 'defer'. This avoids
+inserting THPs at the page fault handler unless they are MADV_HUGEPAGE.
+Khugepaged will then scan the mappings for potential collapses into PMD
+sized pages. Admins using this the 'defer' setting should consider
+tweaking khugepaged/max_ptes_none. The current default of 511 may
+aggressively collapse your PTEs into PMDs. Lower this value to conserve
+more memory (ie. max_ptes_none=64).
+
 .. _thp_sysfs:
 
 sysfs
@@ -136,6 +146,7 @@ The top-level setting (for use with "inherit") can be set by issuing
 one of the following commands::
 
 	echo always >/sys/kernel/mm/transparent_hugepage/enabled
+	echo defer >/sys/kernel/mm/transparent_hugepage/enabled
 	echo madvise >/sys/kernel/mm/transparent_hugepage/enabled
 	echo never >/sys/kernel/mm/transparent_hugepage/enabled
 
@@ -264,7 +275,8 @@ of small pages into one large page::
 A higher value leads to use additional memory for programs.
 A lower value leads to gain less thp performance. Value of
 max_ptes_none can waste cpu time very little, you can
-ignore it.
+ignore it. Consider lowering this value when using
+``transparent_hugepage=defer``
 
 ``max_ptes_swap`` specifies how many pages can be brought in from
 swap when collapsing a group of pages into a transparent huge page::
-- 
2.45.2