linux-kernel - [PATCH v4 12/12] Documentation: mm: update the admin guide for mTHP collapse

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Message-ID: <20250417000238.74567-13-npache@redhat.com>
Date: Wed, 16 Apr 2025 18:02:38 -0600
From: Nico Pache <npache@...hat.com>
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Subject: [PATCH v4 12/12] Documentation: mm: update the admin guide for mTHP collapse

Now that we can collapse to mTHPs lets update the admin guide to
reflect these changes and provide proper guidence on how to utilize it.

Signed-off-by: Nico Pache <npache@...hat.com>
---
 Documentation/admin-guide/mm/transhuge.rst | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst
index dff8d5985f0f..06814e05e1d5 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.
@@ -144,6 +144,14 @@ hugepage sizes have enabled="never". If enabling multiple hugepage
 sizes, the kernel will select the most appropriate enabled size for a
 given allocation.
 
+khugepaged uses max_ptes_none scaled to the order of the enabled mTHP size to
+determine collapses. When using mTHPs it's 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 continuously 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
-- 
2.48.1