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 CFD6843D4EE; Tue, 11 Aug 2026 17:07:04 +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=1786468026; cv=none; b=qd7IY4tHQgTtifLWFt9jC3cX/8Z4Uqm8AetRQiRpJdrMGqOtWV7AZEbkaK0ivmnMYM8R3gKMGY/hCcnQmb/8GvT3YCLVgMmegoUi9zw6skx9Kus9MOjJDiauI+9wqZqCUd6zG1+bTlIPTEGN/l0oxjwNeoyIg1IYEwXAHoJ/NF0= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1786468026; c=relaxed/simple; bh=tzHgNprRUHzM+nWJ/u98+1Hlh2UAUyJ3Kbr/TZIbCxA=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=ZZOIhdLPzj1DC/xCO2FzkdV2lcDv7zXO1uWQV7zkklocbOOlmMdyJca8spVJ1DsDzX30MqA6Zrv7bqG6SlSOi611nWUZkh6sr17NFiW9lhLclrVX49J0vDeLtauLmyQvRGiMCkiN6mj5Yb0KKkprXPPTBvc0UxkOlBUtYLs/zbQ= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=Bf5TjJfO; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="Bf5TjJfO" 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> 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=us-ascii Content-Disposition: inline In-Reply-To: <20f5b9eb-c4ad-4241-9556-e9bfce9ac715@kernel.org> 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