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Date: Wed, 16 Apr 2025 18:18:44 -0600
From: Nico Pache <npache@...hat.com>
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Subject: [PATCH v4 2/4] mm: document (m)THP defer usage

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

Signed-off-by: Nico Pache <npache@...hat.com>
---
 Documentation/admin-guide/mm/transhuge.rst | 31 ++++++++++++++++------
 1 file changed, 23 insertions(+), 8 deletions(-)

diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst
index 06814e05e1d5..38e1778d9eaa 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 (m)THP
+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 (i.e., max_ptes_none=64).
+
 .. _thp_sysfs:
 
 sysfs
@@ -109,11 +119,14 @@ Global THP controls
 
 Transparent Hugepage Support for anonymous memory can be entirely disabled
 (mostly for debugging purposes) or only enabled inside MADV_HUGEPAGE
-regions (to avoid the risk of consuming more memory resources) or enabled
-system wide. This can be achieved per-supported-THP-size with one of::
+regions (to avoid the risk of consuming more memory resources), deferred to
+khugepaged, or enabled system wide.
+
+This can be achieved per-supported-THP-size with one of::
 
 	echo always >/sys/kernel/mm/transparent_hugepage/hugepages-<size>kB/enabled
 	echo madvise >/sys/kernel/mm/transparent_hugepage/hugepages-<size>kB/enabled
+	echo defer >/sys/kernel/mm/transparent_hugepage/hugepages-<size>kB/enabled
 	echo never >/sys/kernel/mm/transparent_hugepage/hugepages-<size>kB/enabled
 
 where <size> is the hugepage size being addressed, the available sizes
@@ -136,6 +149,7 @@ 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
 
@@ -282,7 +296,8 @@ 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::
@@ -307,14 +322,14 @@ Boot parameters
 
 You can change the sysfs boot time default for the top-level "enabled"
 control by passing the parameter ``transparent_hugepage=always`` or
-``transparent_hugepage=madvise`` or ``transparent_hugepage=never`` to the
-kernel command line.
+``transparent_hugepage=madvise`` or ``transparent_hugepage=defer`` or
+``transparent_hugepage=never`` to the kernel command line.
 
 Alternatively, each supported anonymous THP size can be controlled by
 passing ``thp_anon=<size>[KMG],<size>[KMG]:<state>;<size>[KMG]-<size>[KMG]:<state>``,
 where ``<size>`` is the THP size (must be a power of 2 of PAGE_SIZE and
 supported anonymous THP)  and ``<state>`` is one of ``always``, ``madvise``,
-``never`` or ``inherit``.
+``defer``, ``never`` or ``inherit``.
 
 For example, the following will set 16K, 32K, 64K THP to ``always``,
 set 128K, 512K to ``inherit``, set 256K to ``madvise`` and 1M, 2M
-- 
2.48.1

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