From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 3FBCB348880 for ; Fri, 5 Dec 2025 19:44:14 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1764963854; cv=none; b=SbJHDpQyAKfRAZgwgfDA5l9cbSdlprBcZhxneyARNYe3ho+a/zyUZmL8ktGB2C9tJy+VdzCv1G7jmGHOaEcGHByICoNvqtCmZWwj363UKVAQjj2HQpuDh0hK3/rAWA2Wpe6EH9Z3q7Xh13gD2naJGBJ/mVFZhAfax7DQydUKGwQ= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1764963854; c=relaxed/simple; bh=qHJKjreeIB8u6+GC7KtXgFelYE26+ueZzxyId6y/BHA=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version:Content-Type; b=DFFeN3RDBcoI5dT+a5+Jjn4QKWH9AhqgQw5oWWzZK122TVnSNauBfY7VVfDIcZZi8r9PfAys7orG/K8viiCzkYy3XcgBscuqlX6bTqtkZWa0wV0qPkXOhYoCGunnZ19e+FQBQtbP1E6E9SJJ3RVZvs4oJYv6eExRVkjL5u6NCSc= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=aVyMDVUn; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="aVyMDVUn" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 4621BC19424; Fri, 5 Dec 2025 19:44:12 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1764963853; bh=qHJKjreeIB8u6+GC7KtXgFelYE26+ueZzxyId6y/BHA=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=aVyMDVUnqvDhSDHXNC1m2wArU7R1MUDFvsEPnrTbUKjqQt1T5H8KHPX3g+qQ5GcXO XdrTtneqi0WkYXwY55LbOrndMb8Z7l1ounMEiplPr0Lwt0MbH1oM0p8kx6EaYg6rxC ZWNh6oHU+VTewK8gnXlbGvi9aaMtBZzDzk8fwBoGgc4FgHCWAn1lKz9xIIYb1ozKPJ cW/OWFyD1n+d51NUaVEuLVVj40S7MdhJLWpkc0KyPP9UG2qOGftPwbLqzJArruzo1w PbrktfwvyRgeJrxhSxHuh2d01dlLnnXygtrhJ7/o+DyyYWunowyGp9jW2aobtZmKGA GM3ZX4AR37/gg== Received: from phl-compute-01.internal (phl-compute-01.internal [10.202.2.41]) by mailfauth.phl.internal (Postfix) with ESMTP id 91C13F40070; Fri, 5 Dec 2025 14:44:11 -0500 (EST) Received: from phl-mailfrontend-02 ([10.202.2.163]) by phl-compute-01.internal (MEProxy); Fri, 05 Dec 2025 14:44:11 -0500 X-ME-Sender: X-ME-Received: X-ME-Proxy-Cause: gggruggvucftvghtrhhoucdtuddrgeefgedrtddtgdelvdehucetufdoteggodetrfdotf fvucfrrhhofhhilhgvmecuhfgrshhtofgrihhlpdfurfetoffkrfgpnffqhgenuceurghi lhhouhhtmecufedttdenucesvcftvggtihhpihgvnhhtshculddquddttddmnecujfgurh ephffvvefufffkofgjfhggtgfgsehtkeertdertdejnecuhfhrohhmpefmihhrhihlucfu hhhuthhsvghmrghuuceokhgrsheskhgvrhhnvghlrdhorhhgqeenucggtffrrghtthgvrh hnpeffkefffedugfeiudejheefleehteevtefgvefhveetheehkefhjeefhefgleejveen ucevlhhushhtvghrufhiiigvpedtnecurfgrrhgrmhepmhgrihhlfhhrohhmpehkihhrih hllhdomhgvshhmthhprghuthhhphgvrhhsohhnrghlihhthidqudeiudduiedvieehhedq vdekgeeggeejvdekqdhkrghspeepkhgvrhhnvghlrdhorhhgsehshhhuthgvmhhovhdrnh grmhgvpdhnsggprhgtphhtthhopeduledpmhhouggvpehsmhhtphhouhhtpdhrtghpthht oheprghkphhmsehlihhnuhigqdhfohhunhgurghtihhonhdrohhrghdprhgtphhtthhope hmuhgthhhunhdrshhonhhgsehlihhnuhigrdguvghvpdhrtghpthhtohepuggrvhhiuges khgvrhhnvghlrdhorhhgpdhrtghpthhtohepohhsrghlvhgrughorhesshhushgvrdguvg dprhgtphhtthhopehrphhptheskhgvrhhnvghlrdhorhhgpdhrtghpthhtohepvhgsrggs khgrsehsuhhsvgdrtgiipdhrtghpthhtoheplhhorhgvnhiiohdrshhtohgrkhgvshesoh hrrggtlhgvrdgtohhmpdhrtghpthhtohepfihilhhlhiesihhnfhhrrgguvggrugdrohhr ghdprhgtphhtthhopeiiihihsehnvhhiughirgdrtghomh X-ME-Proxy: Feedback-ID: i10464835:Fastmail Received: by mail.messagingengine.com (Postfix) with ESMTPA; Fri, 5 Dec 2025 14:44:11 -0500 (EST) From: Kiryl Shutsemau To: Andrew Morton , Muchun Song Cc: David Hildenbrand , Oscar Salvador , Mike Rapoport , Vlastimil Babka , Lorenzo Stoakes , Matthew Wilcox , Zi Yan , Baoquan He , Michal Hocko , Johannes Weiner , Jonathan Corbet , Usama Arif , kernel-team@meta.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, Kiryl Shutsemau Subject: [PATCH 11/11] hugetlb: Update vmemmap_dedup.rst Date: Fri, 5 Dec 2025 19:43:47 +0000 Message-ID: <20251205194351.1646318-12-kas@kernel.org> X-Mailer: git-send-email 2.51.2 In-Reply-To: <20251205194351.1646318-1-kas@kernel.org> References: <20251205194351.1646318-1-kas@kernel.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Update the documentation regarding vmemmap optimization for hugetlb to reflect the changes in how the kernel maps the tail pages. Fake heads no longer exist. Remove their description. Signed-off-by: Kiryl Shutsemau --- Documentation/mm/vmemmap_dedup.rst | 60 +++++++++++++----------------- 1 file changed, 26 insertions(+), 34 deletions(-) diff --git a/Documentation/mm/vmemmap_dedup.rst b/Documentation/mm/vmemmap_dedup.rst index 1863d88d2dcb..a0c4c79d6922 100644 --- a/Documentation/mm/vmemmap_dedup.rst +++ b/Documentation/mm/vmemmap_dedup.rst @@ -124,33 +124,35 @@ Here is how things look before optimization:: | | +-----------+ -The value of page->compound_info is the same for all tail pages. The first -page of ``struct page`` (page 0) associated with the HugeTLB page contains the 4 -``struct page`` necessary to describe the HugeTLB. The only use of the remaining -pages of ``struct page`` (page 1 to page 7) is to point to page->compound_info. -Therefore, we can remap pages 1 to 7 to page 0. Only 1 page of ``struct page`` -will be used for each HugeTLB page. This will allow us to free the remaining -7 pages to the buddy allocator. +The first page of ``struct page`` (page 0) associated with the HugeTLB page +contains the 4 ``struct page`` necessary to describe the HugeTLB. The remaining +pages of ``struct page`` (page 1 to page 7) are tail pages. + +The optimization is only applied when the size of the struct page is a power-of-2 +In this case, all tail pages of the same order are identical. See +compound_head(). This allows us to remap the tail pages of the vmemmap to a +shared, read-only page. The head page is also remapped to a new page. This +allows the original vmemmap pages to be freed. Here is how things look after remapping:: - HugeTLB struct pages(8 pages) page frame(8 pages) - +-----------+ ---virt_to_page---> +-----------+ mapping to +-----------+ - | | | 0 | -------------> | 0 | - | | +-----------+ +-----------+ - | | | 1 | ---------------^ ^ ^ ^ ^ ^ ^ - | | +-----------+ | | | | | | - | | | 2 | -----------------+ | | | | | - | | +-----------+ | | | | | - | | | 3 | -------------------+ | | | | - | | +-----------+ | | | | - | | | 4 | ---------------------+ | | | - | PMD | +-----------+ | | | - | level | | 5 | -----------------------+ | | - | mapping | +-----------+ | | - | | | 6 | -------------------------+ | - | | +-----------+ | - | | | 7 | ---------------------------+ + HugeTLB struct pages(8 pages) page frame + +-----------+ ---virt_to_page---> +-----------+ mapping to +----------------+ + | | | 0 | -------------> | 0 | + | | +-----------+ +----------------+ + | | | 1 | ------┐ + | | +-----------+ | + | | | 2 | ------┼ +----------------+ + | | +-----------+ | | vmemmap_tail | + | | | 3 | ------┼------> | shared for the | + | | +-----------+ | | struct hstate | + | | | 4 | ------┼ +----------------+ + | | +-----------+ | + | | | 5 | ------┼ + | PMD | +-----------+ | + | level | | 6 | ------┼ + | mapping | +-----------+ | + | | | 7 | ------┘ | | +-----------+ | | | | @@ -172,16 +174,6 @@ The contiguous bit is used to increase the mapping size at the pmd and pte (last) level. So this type of HugeTLB page can be optimized only when its size of the ``struct page`` structs is greater than **1** page. -Notice: The head vmemmap page is not freed to the buddy allocator and all -tail vmemmap pages are mapped to the head vmemmap page frame. So we can see -more than one ``struct page`` struct with ``PG_head`` (e.g. 8 per 2 MB HugeTLB -page) associated with each HugeTLB page. The ``compound_head()`` can handle -this correctly. There is only **one** head ``struct page``, the tail -``struct page`` with ``PG_head`` are fake head ``struct page``. We need an -approach to distinguish between those two different types of ``struct page`` so -that ``compound_head()`` can return the real head ``struct page`` when the -parameter is the tail ``struct page`` but with ``PG_head``. - Device DAX ========== -- 2.51.2