From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 7B460C5B572 for ; Tue, 11 Aug 2026 17:07:10 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 416EE6B007B; Tue, 11 Aug 2026 13:07:09 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 3C6FF6B009F; Tue, 11 Aug 2026 13:07:09 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 2B6A86B00A0; Tue, 11 Aug 2026 13:07:09 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0011.hostedemail.com [216.40.44.11]) by kanga.kvack.org (Postfix) with ESMTP id EB5516B007B for ; Tue, 11 Aug 2026 13:07:08 -0400 (EDT) Received: from smtpin09.hostedemail.com (lb01a-stub [10.200.18.249]) by unirelay04.hostedemail.com (Postfix) with ESMTP id 7EBBC1A0126 for ; Tue, 11 Aug 2026 17:07:08 +0000 (UTC) X-FDA: 85089618936.09.0406EE1 Received: from sea.source.kernel.org (sea.source.kernel.org [172.234.252.31]) by imf16.hostedemail.com (Postfix) with ESMTP id A4B43180011 for ; Tue, 11 Aug 2026 17:07:06 +0000 (UTC) Authentication-Results: imf16.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20260515 header.b=Bf5TjJfO; spf=pass (imf16.hostedemail.com: domain of ljs@kernel.org designates 172.234.252.31 as permitted sender) smtp.mailfrom=ljs@kernel.org; dmarc=pass (policy=quarantine) header.from=kernel.org ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1786468026; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=tzHgNprRUHzM+nWJ/u98+1Hlh2UAUyJ3Kbr/TZIbCxA=; b=XyJscGq3Lyjp4i8pvr5R14pbtCHgffvo8hZX973mcSQtnKYOs2lupp0nT9+85hig0WjjPH slNFw8ujUwo4JhlsjlEWTtSFc7PmoVGG/8Kyj0CcdzSqt5Ecaapxa/PBrdr24eFdcB7fkZ zYEPFp1a15gAgctTKvI+mwnIhv3zv2c= ARC-Authentication-Results: i=1; imf16.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20260515 header.b=Bf5TjJfO; spf=pass (imf16.hostedemail.com: domain of ljs@kernel.org designates 172.234.252.31 as permitted sender) smtp.mailfrom=ljs@kernel.org; dmarc=pass (policy=quarantine) header.from=kernel.org ARC-Seal: i=1; a=rsa-sha256; d=hostedemail.com; s=arc-20220608; cv=none; t=1786468026; b=3Qjbj+IxKcVLPjMFYGZMaGCSoYfAHMUW1hbr/V2oBODwJpzPYPldl0rUjnOWRXmvZQIDAA QlbhIubGMU2c53TCktEkNowwlpBC1mrw3W2wgLuitNEyRgN0Diq4+G6ZwTIhT5MlX18lr3 TZa3DY1iMQhfyl+8GVn61KAlepW8WOo= Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) by sea.source.kernel.org (Postfix) with ESMTP id D268D42E10; Tue, 11 Aug 2026 17:07:04 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 56DAB1F000E9; Tue, 11 Aug 2026 17:06:47 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1786468024; bh=tzHgNprRUHzM+nWJ/u98+1Hlh2UAUyJ3Kbr/TZIbCxA=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=Bf5TjJfOh3SVJsd4OtNdIG5B5d2xI11qP61qJcO2dcoDBDjurnJvmNPwolN5e/u0t B+s35+Ba3k0Ns93TcFR/N9sUve5/+B3Goi9qQ4RIFY2/TfV1P1ngNQrwNDWdpXcp/r 4q69xnZYEQ2sxu0tJvaoTKAAFegaeOn7niN+JtEhikAMpqH+Kg8C+KtvXPzvej/CwY f4+zv+hOyB74boEfRrhRV6IzRg4W734UjR7CsmOmmM/IuOBia8qp9ei8JtAxef+bGo fuAaB+9VFjwnNgmP/8bxFEXx+LF/kKv9tXOqV2pglDl8d/GMZ6T8XshRpJkqe6TIUs eq8QHs1RDNSBg== Date: Tue, 11 Aug 2026 18:06:44 +0100 From: "Lorenzo Stoakes (ARM)" To: "David Hildenbrand (Arm)" Cc: Andrew Morton , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Jann Horn , Pedro Falcato , "Matthew Wilcox (Oracle)" , Jan Kara , Miaohe Lin , Naoya Horiguchi , Rik van Riel , Harry Yoo , Lance Yang , Kees Cook , Zi Yan , Baolin Wang , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Usama Arif , Matthew Brost , Joshua Hahn , Rakie Kim , Byungchul Park , Gregory Price , Ying Huang , Alistair Popple , Peter Xu , Xu Xin , Chengming Zhou , Arnd Bergmann , Greg Kroah-Hartman , Christian Borntraeger , Janosch Frank , Claudio Imbrenda , Alexander Gordeev , Gerald Schaefer , Heiko Carstens , Vasily Gorbik , Sven Schnelle , Alex Deucher , Christian =?utf-8?B?S8O2bmln?= , David Airlie , Simona Vetter , Maarten Lankhorst , Maxime Ripard , Thomas Zimmermann , Boris Brezillon , Steven Price , Liviu Dudau , Huang Rui , Matthew Auld , Thomas =?utf-8?Q?Hellstr=C3=B6m?= , Rodrigo Vivi , Masami Hiramatsu , Oleg Nesterov , Peter Zijlstra , Ingo Molnar , Arnaldo Carvalho de Melo , Namhyung Kim , Mark Rutland , Alexander Shishkin , Jiri Olsa , Ian Rogers , Adrian Hunter , James Clark , Jason Gunthorpe , John Hubbard , Muchun Song , Oscar Salvador , Chris Li , Kairui Song , Kemeng Shi , Nhat Pham , Baoquan He , Youngjun Park , linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-kselftest@vger.kernel.org, kvm@vger.kernel.org, linux-s390@vger.kernel.org, amd-gfx@lists.freedesktop.org, dri-devel@lists.freedesktop.org, intel-xe@lists.freedesktop.org, linux-perf-users@vger.kernel.org, linux-trace-kernel@vger.kernel.org Subject: Re: [PATCH v4 07/20] mm/vma: fix self-merge check in copy_vma() Message-ID: References: <20260806-b4-scalable-cow-virt-pgoff-v4-0-ab318a350404@kernel.org> <20260806-b4-scalable-cow-virt-pgoff-v4-7-ab318a350404@kernel.org> <3a0a9aaf-20f5-4f92-b848-00ab702e8fe2@kernel.org> <20f5b9eb-c4ad-4241-9556-e9bfce9ac715@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20f5b9eb-c4ad-4241-9556-e9bfce9ac715@kernel.org> X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: A4B43180011 X-Stat-Signature: c857hnpxmgreeaa7w1gah35igbg5epw6 X-Rspam-User: X-HE-Tag: 1786468026-398593 X-HE-Meta: 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 0tZF5yPV ZWtm+3qa7bxTEYFefMZ5M9Curg3Z7Sh6U7cVLbelek2HAKLVxhykgc0i34vxWJZ1ffQj6X/pKtu+3pat30yOeiJPVlR+5U56oVu8E0VH4XucsWqwCc1lr+APGKLVs/teIi/ghS7/YEr4UBoSbZgGWKOVWlOQwiFWcoPjzpeF+OnnLZ0yFrQvw1cun3EoM3+iOKi3nJPfUOFh8tpdXdyplyjzqTbY0ki8nF/2+7tD7FewBD+lJc61iMI+NM42U4PeowU8Gk4E6URIvYZPLFA4BOfMoVtVGq5CW6HZDY6u4ayzpHDEfhXaE6Um3GvFQzlbiJypFrMjTq3UpINrVsXFa9vtziAmXAG2n+OvbtN+xX7w3SipeGu7XdlkxY/My2QGefEYFtW4ogLLef+w= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Tue, Aug 11, 2026 at 06:53:05PM +0200, David Hildenbrand (Arm) wrote: > On 8/11/26 18:51, Lorenzo Stoakes (ARM) wrote: > > On Tue, Aug 11, 2026 at 06:44:01PM +0200, David Hildenbrand (Arm) wrote: > >>> > >>> Since copy_vma_and_data() which calls copy_vma() intends to reference the > >>> old VMA after the merge, it must have this pointer updated. > >>> > >>> This kind of self-merge is not possible with a succeeding merge, as the > >>> merge always prefers to expand the preceding VMA if possible. > >>> > >>> copy_vma() accounts for this by explicitly checking to see if a self-merge > >>> occurred and updating the vmap pointer if so. However it incorrect did so > >>> even for a subsequent merge (this is simply a noop so it had no impact). > >> > >> Took me longer than it should to find out that this is no real fix ;) > >> > >>> > >>> So change this to only check for the case which matters - a backwards > >>> merge - and rearrange the parameters to make it clearer we're doing that - > >>> i.e. check new_vma->vm_start < old_vma_start (having already renamed > >>> vma_start to old_vma_start to make it clear this is the previous VMA). > >>> > >>> Also update the existing wall-of-text comment to be a lot clearer. > >>> > >>> While we're here, replace the VM_BUG_ON_VMA() with a VM_WARN_ON_ONCE_VMA() > >>> and update the VMA userland tests accordingly. > >>> > >>> No functional change intended. > >>> > >>> Signed-off-by: Lorenzo Stoakes (ARM) > >>> --- > >> I skimmed over it, looks correct, but the merging stuff is way over my head to > >> confidentially give a RB tag her :) > > > > Well that's what A-b is for ;) > > Assisted-by? Half-assed-by? > > ;) A schloppy schplatch I like? ;) > > Acked-by: David Hildenbrand (Arm) Thanks! > > -- > Cheers, > > David -- Cheers, Lorenzo