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Mon, 14 Jul 2025 00:35:55 +0000 (UTC) Received: from h1.redhat.com (unknown [10.22.64.9]) by mx-prod-int-01.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 6CBC130001A1; Mon, 14 Jul 2025 00:35:42 +0000 (UTC) From: Nico Pache To: linux-mm@kvack.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-trace-kernel@vger.kernel.org Cc: david@redhat.com, ziy@nvidia.com, baolin.wang@linux.alibaba.com, lorenzo.stoakes@oracle.com, Liam.Howlett@oracle.com, ryan.roberts@arm.com, dev.jain@arm.com, corbet@lwn.net, rostedt@goodmis.org, mhiramat@kernel.org, mathieu.desnoyers@efficios.com, akpm@linux-foundation.org, baohua@kernel.org, willy@infradead.org, peterx@redhat.com, wangkefeng.wang@huawei.com, usamaarif642@gmail.com, sunnanyong@huawei.com, vishal.moola@gmail.com, thomas.hellstrom@linux.intel.com, yang@os.amperecomputing.com, kirill.shutemov@linux.intel.com, aarcange@redhat.com, raquini@redhat.com, anshuman.khandual@arm.com, catalin.marinas@arm.com, tiwai@suse.de, will@kernel.org, dave.hansen@linux.intel.com, jack@suse.cz, cl@gentwo.org, jglisse@google.com, surenb@google.com, zokeefe@google.com, hannes@cmpxchg.org, rientjes@google.com, mhocko@suse.com, rdunlap@infradead.org, hughd@google.com, Bagas Sanjaya Subject: [PATCH v9 14/14] Documentation: mm: update the admin guide for mTHP collapse Date: Sun, 13 Jul 2025 18:32:07 -0600 Message-ID: <20250714003207.113275-15-npache@redhat.com> In-Reply-To: <20250714003207.113275-1-npache@redhat.com> References: <20250714003207.113275-1-npache@redhat.com> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.4 X-Rspam-User: X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: E1FB8140005 X-Stat-Signature: qfxkkaczysq7g18j48sun3y8donwd14s X-HE-Tag: 1752453395-554197 X-HE-Meta: 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 k9nGcrww jEVaYMx6OORS81lL4s+nBrHKuYugt1Z6DXjtVQxtdlntw1RZZLSemehBHyw0ZQj3dbX9QzP1PIkTgHFhRM37OvNhEbVxkeXbDOIXW1ry7E9DuTVRmoQcHuqTaTLg5Rgmi3IaZvR6ZvpEtW1yVpobL+/0bbh+FQDaMEMSGxEij+M1xZn6rTTVhvbXkxg2eviMU08e5NCC7Er/FuR1FBV7MKNQMGgkd5B1FoACgtLY6hq8exlwp4HVFedRVG5+UNtCbUWk7Oi9vpCtHv91FjgGG7yH0f6twq8ePCB+z7V2LpkrPEWzEF8GwUnsi10iMwrsQnp8a5MalarQLUKX52l6MEW0mw2isf9HUYAKGV7ff9ofhw7TBc7eVeYYDNfjafZzJsBf/YwWClxZMtSvcCZxrszUJBoRQnYDDMQZIQ/aFnFEqHWI= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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. Reviewed-by: Bagas Sanjaya Signed-off-by: Nico Pache --- Documentation/admin-guide/mm/transhuge.rst | 19 +++++++++++++------ 1 file changed, 13 insertions(+), 6 deletions(-) diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst index 28c8af61efba..bd49b46398c9 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 ` 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 the largest 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 @@ -221,11 +233,6 @@ top-level control are "never") Khugepaged controls ------------------- -.. note:: - khugepaged currently only searches for opportunities to collapse to - PMD-sized THP and no attempt is made to collapse to other THP - sizes. - khugepaged runs usually at low frequency so while one may not want to invoke defrag algorithms synchronously during the page faults, it should be worth invoking defrag at least in khugepaged. However it's -- 2.50.0