From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (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 BF3C23B6C0C for ; Sun, 27 Sep 2026 05:51:54 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1790488316; cv=none; b=NwoMwQoO+DE6ba+sGABMTj5YMrQfHuyMJSpleMWEOgQEeMnAXtWtJGkOElZU1XTGz0Eak3h4ucxQabI3bP/VRP2iPkumjRTeEWYSrfpQ5kdcrMRedlxJiGr4q31dkJw7RSqeA3qUPPF+k3sA8hmchmaMEZLVGpHojqRdF3/CWHQ= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1790488316; c=relaxed/simple; bh=a25nHWLGwS/GJLDSzM2p/vQIYk9Crk1Nk115/tz9cEw=; h=Date:To:From:Subject:Message-Id; b=BFbkCrIYKWZ0PB7Uu2kqk+L5YuQcd3BF64bc2Er3OoekOI3yv8TWbZzUtBj/ohi7rxPaCe+V6hdQ2wwZOLL3KgN2X5k+DpWIl8NghBYbjoxPaaS8a3QrDNM2IuIriqHCiH5R68AACNPaM0R8msw4QPkUJGol6BcY203TrirU+e4= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux-foundation.org header.i=@linux-foundation.org header.b=nwrkP94U; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux-foundation.org header.i=@linux-foundation.org header.b="nwrkP94U" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 93A2D1F000FF; Sun, 27 Sep 2026 05:51:54 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux-foundation.org; s=korg; t=1790488314; bh=/JF80SJ57g2c6dn75j/qLUf3i6pWfUkR4jMGD+rm3tg=; h=Date:To:From:Subject; b=nwrkP94UV6Lk6TGPSP0pxX/DQLlIkGiOR18X7txY4bBL2M0lQGtbs3vpzXM4OhZfK nnl3IQXJaelATlgVCM4xENhtOobid5licT2uN+20ab3xFRih1yFfEZPNniGsnclXsG ItaKcCIX1DnloNctQHhnUH9ViEWQXhChwzZ1n85A= Date: Sat, 26 Sep 2026 22:51:54 -0700 To: mm-commits@vger.kernel.org,muchun.song@linux.dev,akpm@linux-foundation.org From: Andrew Morton Subject: + documentation-mm-update-dax-vmemmap-deduplication-docs.patch added to mm-unstable branch Message-Id: <20260927055154.93A2D1F000FF@smtp.kernel.org> Precedence: bulk X-Mailing-List: mm-commits@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: The patch titled Subject: Documentation/mm: update DAX vmemmap deduplication docs has been added to the -mm mm-unstable branch. Its filename is documentation-mm-update-dax-vmemmap-deduplication-docs.patch This patch will shortly appear at https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/documentation-mm-update-dax-vmemmap-deduplication-docs.patch This patch will later appear in the mm-unstable branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm Before you just go and hit "reply", please: a) Consider who else should be cc'ed b) Prefer to cc a suitable mailing list as well c) Ideally: find the original patch on the mailing list and do a reply-to-all to that, adding suitable additional cc's *** Remember to use Documentation/process/submit-checklist.rst when testing your code *** The -mm tree is included into linux-next via various branches at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm and is updated there most days ------------------------------------------------------ From: Muchun Song Subject: Documentation/mm: update DAX vmemmap deduplication docs Date: Sun, 27 Sep 2026 10:54:41 +0800 Device DAX now uses the common per-zone shared tail page for vmemmap deduplication. The old documentation still described a DAX-specific layout with a separately populated tail vmemmap page and half the HugeTLB savings. Update the generic and powerpc documentation to describe the shared layout. In the powerpc document, keep the radix and 64K-specific details, drop the duplicated 4K PUD arithmetic, and replace the repeated device-dax diagrams with a single parameterized PMD/PUD diagram. Link: https://lore.kernel.org/20260927025441.741633-13-songmuchun@bytedance.com Signed-off-by: Muchun Song Signed-off-by: Andrew Morton Acked-by: Qi Zheng Cc: David Hildenbrand Cc: Oscar Salvador Cc: Madhavan Srinivasan Cc: Michael Ellerman Cc: Jonathan Corbet Cc: Lorenzo Stoakes Cc: Mike Rapoport Cc: Nicholas Piggin Cc: Christophe Leroy Cc: Randy Dunlap Cc: Lance Yang --- Documentation/arch/powerpc/vmemmap_dedup.rst | 90 +++-------------- Documentation/mm/vmemmap_dedup.rst | 32 ------ 2 files changed, 21 insertions(+), 101 deletions(-) --- a/Documentation/arch/powerpc/vmemmap_dedup.rst~documentation-mm-update-dax-vmemmap-deduplication-docs +++ a/Documentation/arch/powerpc/vmemmap_dedup.rst @@ -19,82 +19,28 @@ With 1G PUD level mapping, we require 16 vmemmap page can contain 1024 struct pages (64K/sizeof(struct page)). Hence we require 16 64K pages in vmemmap to map the struct page for 1G PUD level mapping. -Here's how things look like on device-dax after the sections are populated:: - +-----------+ ---virt_to_page---> +-----------+ mapping to +-----------+ - | | | 0 | -------------> | 0 | - | | +-----------+ +-----------+ - | | | 1 | -------------> | 1 | - | | +-----------+ +-----------+ - | | | 2 | ----------------^ ^ ^ ^ ^ ^ - | | +-----------+ | | | | | - | | | 3 | ------------------+ | | | | - | | +-----------+ | | | | - | | | 4 | --------------------+ | | | - | PUD | +-----------+ | | | - | level | | . | ----------------------+ | | - | mapping | +-----------+ | | - | | | . | ------------------------+ | - | | +-----------+ | - | | | 15 | --------------------------+ - | | +-----------+ - | | - | | - | | - +-----------+ - - With 4K page size, 2M PMD level mapping requires 512 struct pages and a single 4K vmemmap page contains 64 struct pages(4K/sizeof(struct page)). Hence we require 8 4K pages in vmemmap to map the struct page for 2M pmd level mapping. -Here's how things look like on device-dax after the sections are populated:: - - +-----------+ ---virt_to_page---> +-----------+ mapping to +-----------+ - | | | 0 | -------------> | 0 | - | | +-----------+ +-----------+ - | | | 1 | -------------> | 1 | - | | +-----------+ +-----------+ - | | | 2 | ----------------^ ^ ^ ^ ^ ^ - | | +-----------+ | | | | | - | | | 3 | ------------------+ | | | | - | | +-----------+ | | | | - | | | 4 | --------------------+ | | | - | PMD | +-----------+ | | | - | level | | 5 | ----------------------+ | | - | mapping | +-----------+ | | - | | | 6 | ------------------------+ | - | | +-----------+ | - | | | 7 | --------------------------+ - | | +-----------+ - | | - | | - | | - +-----------+ - -With 1G PUD level mapping, we require 262144 struct pages and a single 4K -vmemmap page can contain 64 struct pages (4K/sizeof(struct page)). Hence we -require 4096 4K pages in vmemmap to map the struct pages for 1G PUD level -mapping. - -Here's how things look like on device-dax after the sections are populated:: - - +-----------+ ---virt_to_page---> +-----------+ mapping to +-----------+ - | | | 0 | -------------> | 0 | - | | +-----------+ +-----------+ - | | | 1 | -------------> | 1 | - | | +-----------+ +-----------+ - | | | 2 | ----------------^ ^ ^ ^ ^ ^ - | | +-----------+ | | | | | - | | | 3 | ------------------+ | | | | - | | +-----------+ | | | | - | | | 4 | --------------------+ | | | - | PUD | +-----------+ | | | - | level | | . | ----------------------+ | | - | mapping | +-----------+ | | - | | | . | ------------------------+ | - | | +-----------+ | - | | | 4095 | --------------------------+ - | | +-----------+ +Here's how things look on device-dax after vmemmap-optimized sections are +populated. ``N`` is the number of vmemmap pages required by the DAX mapping +above:: + + Device DAX vmemmap pages (N pages) backing page frames + +-----------+ ---virt_to_page---> +-----------+ mapping to +-------------+ + | | | 0 | -------------> | 0 | + | | +-----------+ +-------------+ + | | | 1 | ------+ + | | +-----------+ | + | | | 2 | ------+ + | | +-----------+ | + | | | . | ------+ +-------------+ + | PMD/PUD | +-----------+ | | A single, | + | level | | . | ------+------> | per-zone | + | mapping | +-----------+ | | shared tail | + | | | N - 1 | ------+ | page | + | | +-----------+ +-------------+ | | | | | | --- a/Documentation/mm/vmemmap_dedup.rst~documentation-mm-update-dax-vmemmap-deduplication-docs +++ a/Documentation/mm/vmemmap_dedup.rst @@ -1,4 +1,3 @@ - .. SPDX-License-Identifier: GPL-2.0 ========================================= @@ -192,32 +191,7 @@ to 4 on HugeTLB pages. There's no remapping of vmemmap given that device-dax memory is not part of System RAM ranges initialized at boot. Thus the tail page deduplication -happens at a later stage when we populate the sections. HugeTLB reuses the -the head vmemmap page representing, whereas device-dax reuses the tail -vmemmap page. This results in only half of the savings compared to HugeTLB. - -Deduplicated tail pages are not mapped read-only. +happens at a later stage when we populate the sections. -Here's how things look like on device-dax after the sections are populated:: - - +-----------+ ---virt_to_page---> +-----------+ mapping to +-----------+ - | | | 0 | -------------> | 0 | - | | +-----------+ +-----------+ - | | | 1 | -------------> | 1 | - | | +-----------+ +-----------+ - | | | 2 | ----------------^ ^ ^ ^ ^ ^ - | | +-----------+ | | | | | - | | | 3 | ------------------+ | | | | - | | +-----------+ | | | | - | | | 4 | --------------------+ | | | - | PMD | +-----------+ | | | - | level | | 5 | ----------------------+ | | - | mapping | +-----------+ | | - | | | 6 | ------------------------+ | - | | +-----------+ | - | | | 7 | --------------------------+ - | | +-----------+ - | | - | | - | | - +-----------+ +Deduplicated tail pages are not mapped read-only. The mapping layout is the same +as HugeTLB. _ Patches currently in -mm which might be from muchun.song@linux.dev are mm-sparse-vmemmap-factor-out-shared-vmemmap-tail-page-allocation.patch mm-sparse-vmemmap-allocate-shared-tail-page-array-dynamically.patch mm-sparse-vmemmap-introduce-config_vmemmap_optimization.patch mm-sparse-vmemmap-open-code-init_compound_tail.patch mm-sparse-vmemmap-prepare-dax-vmemmap-population-for-compound-page-orders.patch mm-sparse-vmemmap-set-compound-page-order-for-device-dax.patch mm-sparse-vmemmap-switch-device-dax-to-shared-tail-vmemmap-pages.patch mm-sparse-vmemmap-move-vmemmap-optimization-helpers-to-a-public-header.patch powerpc-mm-switch-device-dax-to-shared-tail-vmemmap-pages.patch mm-sparse-vmemmap-drop-the-extra-tail-page-from-device-dax-reservation.patch mm-sparse-vmemmap-drop-unused-section_nr_vmemmap_pages-arguments.patch documentation-mm-update-dax-vmemmap-deduplication-docs.patch