linux-kernel - [PATCH v5 12/12] Documentation: mm: update the admin guide for mTHP collapse

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20250428181218.85925-13-npache@redhat.com>
Date: Mon, 28 Apr 2025 12:12:18 -0600
From: Nico Pache <npache@...hat.com>
To: linux-mm@...ck.org,
	linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	linux-trace-kernel@...r.kernel.org
Cc: akpm@...ux-foundation.org,
	corbet@....net,
	rostedt@...dmis.org,
	mhiramat@...nel.org,
	mathieu.desnoyers@...icios.com,
	david@...hat.com,
	baohua@...nel.org,
	baolin.wang@...ux.alibaba.com,
	ryan.roberts@....com,
	willy@...radead.org,
	peterx@...hat.com,
	ziy@...dia.com,
	wangkefeng.wang@...wei.com,
	usamaarif642@...il.com,
	sunnanyong@...wei.com,
	vishal.moola@...il.com,
	thomas.hellstrom@...ux.intel.com,
	yang@...amperecomputing.com,
	kirill.shutemov@...ux.intel.com,
	aarcange@...hat.com,
	raquini@...hat.com,
	dev.jain@....com,
	anshuman.khandual@....com,
	catalin.marinas@....com,
	tiwai@...e.de,
	will@...nel.org,
	dave.hansen@...ux.intel.com,
	jack@...e.cz,
	cl@...two.org,
	jglisse@...gle.com,
	surenb@...gle.com,
	zokeefe@...gle.com,
	hannes@...xchg.org,
	rientjes@...gle.com,
	mhocko@...e.com,
	rdunlap@...radead.org,
	lorenzo.stoakes@...cle.com,
	Liam.Howlett@...cle.com
Subject: [PATCH v5 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 | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst
index dff8d5985f0f..5c63fe51b3ad 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 +144,18 @@ 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; When we collapse, we are
+bringing in new non-zero pages that will, on a subsequent scan, cause the
+max_ptes_none check of the +1 order to always be satisfied. By limiting
+this to less than half the current order, we make sure we don't cause this
+feedback loop. 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