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 0388C3451BA; Thu, 18 Dec 2025 15:10:20 +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=1766070620; cv=none; b=ZJwZJmVXwv9RcIYllrVlsIEh1pDemM0Gj71FOZDa5S2VqY0zTMmNdXKb8At9XY0+I3B9Kbkdl579qkKe82BDdJAdjlD6Y2f6uOGdeswn4sExhN9nbyx/otT7ioYY78O8r4j8/F+ZFQXg6xMAvFLMfW+ID5To7++7dqLaMSHK0dw= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1766070620; 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=qfv9eFtP3l6sRyDFoVU+O3fNx+f9YA6tdMMI2fjKjCSUVOHZUPy23BnoYMljDKe0IaIHo9tL/SRf1+W4XLZIiP7YqzA1si073b4wQhU1KYjkZCZ3ehOzzbISpLQ3KYLhc/AHRMRHi4MP+HuVabXa86ldYrsqRHCk142llaiV3ms= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=j5pE5e8i; 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="j5pE5e8i" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 236BEC116C6; Thu, 18 Dec 2025 15:10:19 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1766070619; bh=qHJKjreeIB8u6+GC7KtXgFelYE26+ueZzxyId6y/BHA=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=j5pE5e8i3AKjkR8X26UUaRqpDhXaxWIJP0ehbs1RS+1zq9BnrSEf286kgh7Qu6/Ls XGLlHSahXuhHL34g9WL1KNM+oKpopVdzyztmbr0ALoN4bwBvU259mk89csSgIsmD8E 4XUOEGSVDm3pbksY+UU7y+RKy9Ar0GUNtETuMWvxUBsGPF1v9RChz52Rc8D8Y+2zvw wFHsfOzJcBilraBa/s8XcIO8NYrqm8j3BN+7GWkk6DPsI+C8oF0KBHrluvtazBeEuW EfIRTGeQhbZPMShfvqfCL74EOOqGguuvQ6D/V29n1yUnLOsOii7tKov1bDRmwu7sjM 6YCwEEsnf/vlQ== Received: from phl-compute-05.internal (phl-compute-05.internal [10.202.2.45]) by mailfauth.phl.internal (Postfix) with ESMTP id 73C68F4007D; Thu, 18 Dec 2025 10:10:18 -0500 (EST) Received: from phl-frontend-03 ([10.202.2.162]) by phl-compute-05.internal (MEProxy); Thu, 18 Dec 2025 10:10:18 -0500 X-ME-Sender: X-ME-Received: X-ME-Proxy-Cause: gggruggvucftvghtrhhoucdtuddrgeefgedrtddtgdegheejgecutefuodetggdotefrod ftvfcurfhrohhfihhlvgemucfhrghsthforghilhdpuffrtefokffrpgfnqfghnecuuegr ihhlohhuthemuceftddtnecusecvtfgvtghiphhivghnthhsucdlqddutddtmdenucfjug hrpefhvfevufffkffojghfgggtgfesthekredtredtjeenucfhrhhomhepmfhirhihlhcu ufhhuhhtshgvmhgruhcuoehkrghssehkvghrnhgvlhdrohhrgheqnecuggftrfgrthhtvg hrnhepffekffefudfgiedujeehfeelheetveetgfevhfevteehheekhfejfeehgfeljeev necuvehluhhsthgvrhfuihiivgeptdenucfrrghrrghmpehmrghilhhfrhhomhepkhhirh hilhhlodhmvghsmhhtphgruhhthhhpvghrshhonhgrlhhithihqdduieduudeivdeiheeh qddvkeeggeegjedvkedqkhgrsheppehkvghrnhgvlhdrohhrghesshhhuhhtvghmohhvrd hnrghmvgdpnhgspghrtghpthhtohepvddtpdhmohguvgepshhmthhpohhuthdprhgtphht thhopegrkhhpmheslhhinhhugidqfhhouhhnuggrthhiohhnrdhorhhgpdhrtghpthhtoh epmhhutghhuhhnrdhsohhngheslhhinhhugidruggvvhdprhgtphhtthhopegurghvihgu sehkvghrnhgvlhdrohhrghdprhgtphhtthhopeifihhllhihsehinhhfrhgruggvrggurd horhhgpdhrtghpthhtohepuhhsrghmrggrrhhifheigedvsehgmhgrihhlrdgtohhmpdhr tghpthhtohepfhhvughlsehgohhoghhlvgdrtghomhdprhgtphhtthhopehoshgrlhhvrg guohhrsehsuhhsvgdruggvpdhrtghpthhtoheprhhpphhtsehkvghrnhgvlhdrohhrghdp rhgtphhtthhopehvsggrsghkrgesshhushgvrdgtii X-ME-Proxy: Feedback-ID: i10464835:Fastmail Received: by mail.messagingengine.com (Postfix) with ESMTPA; Thu, 18 Dec 2025 10:10:17 -0500 (EST) From: Kiryl Shutsemau To: Andrew Morton , Muchun Song , David Hildenbrand , Matthew Wilcox , Usama Arif , Frank van der Linden Cc: Oscar Salvador , Mike Rapoport , Vlastimil Babka , Lorenzo Stoakes , Zi Yan , Baoquan He , Michal Hocko , Johannes Weiner , Jonathan Corbet , kernel-team@meta.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, Kiryl Shutsemau Subject: [PATCHv2 14/14] hugetlb: Update vmemmap_dedup.rst Date: Thu, 18 Dec 2025 15:09:45 +0000 Message-ID: <20251218150949.721480-15-kas@kernel.org> X-Mailer: git-send-email 2.51.2 In-Reply-To: <20251218150949.721480-1-kas@kernel.org> References: <20251218150949.721480-1-kas@kernel.org> Precedence: bulk X-Mailing-List: linux-doc@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