linux-kernel - [RFC v2 5/5] mm: document mTHP defer setting

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Message-ID: <20250211004054.222931-6-npache@redhat.com>
Date: Mon, 10 Feb 2025 17:40:54 -0700
From: Nico Pache <npache@...hat.com>
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Subject: [RFC v2 5/5] mm: document mTHP defer setting

Now that we have mTHP support in khugepaged, lets add it to the
transhuge admin guide to provide proper guidance.

Signed-off-by: Nico Pache <npache@...hat.com>
---
 Documentation/admin-guide/mm/transhuge.rst | 22 ++++++++++++++++------
 1 file changed, 16 insertions(+), 6 deletions(-)

diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst
index b3b18573bbb4..99ba3763c1c4 100644
--- a/Documentation/admin-guide/mm/transhuge.rst
+++ b/Documentation/admin-guide/mm/transhuge.rst
@@ -63,7 +63,7 @@ often.
 THP can be enabled system wide or restricted to certain tasks or even
 memory ranges inside task's address space. Unless THP is completely
 disabled, there is ``khugepaged`` daemon that scans memory and
-collapses sequences of basic pages into PMD-sized huge pages.
+collapses sequences of basic pages into huge pages.
 
 The THP behaviour is controlled via :ref:`sysfs <thp_sysfs>`
 interface and using madvise(2) and prctl(2) system calls.
@@ -103,8 +103,8 @@ 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
+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 (ie. max_ptes_none=64).
@@ -119,11 +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), defered 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
@@ -155,6 +158,13 @@ hugepage sizes have enabled="never". If enabling multiple hugepage
 sizes, the kernel will select the most appropriate enabled size for a
 given allocation.
 
+khugepaged use max_ptes_none scaled to the order of the enabled mTHP size to
+determine collapses. When using mTHPs its recommended to set max_ptes_none low.
+Ideally less than HPAGE_PMD_NR / 2 (255 on 4k page size). This will prevent
+undesired "creep" behavior that leads to continously collapsing to a larger
+mTHP size. max_ptes_shared and max_ptes_swap have no effect when collapsing to a
+mTHP, and mTHP collapse will fail on shared or swapped out pages.
+
 It's also possible to limit defrag efforts in the VM to generate
 anonymous hugepages in case they're not immediately free to madvise
 regions or to never try to defrag memory and simply fallback to regular
@@ -318,7 +328,7 @@ 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