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 B6AB458123F for ; Wed, 9 Sep 2026 16:36:45 +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=1788971806; cv=none; b=MOI/9Oo6pUf4HSJRhvDWfOQTQo8e58QeDLde6PPUL6p7ftAuTo/U6f6OR5SwG5Ib/TrnbLaJdfbpgVEJkWbUyXL7xgHZ+RhtF/6mHgyGLgw9LRDRCv+3fcCCPK7utTV30BtqS+rwsxEsJc/EwlYgYyJ/eF1tV3SjiRrbgcTc/68= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788971806; c=relaxed/simple; bh=gzUNQn07qEf6tHgnalV7kcZShXzuwXfjXh+7ezu0Iws=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=elhJ5TP4mA/anXUSKy0m1lwml0SQm6AE6xL2+xCQf9SRqSbQS8jNRLONbBkYLZ5AAHkWS2HWyFcNfRSHp2cXPg5e77WGkHH8d+H0nStsUOJJryD2YXZnGJpkYRgGDxqfnAx0b0nHDGQvlujor96MMhS+2nHIOess8vdB0jfTDqQ= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=oBKesYFn; 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="oBKesYFn" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 9B3F31F000FF; Wed, 9 Sep 2026 16:36:37 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1788971805; bh=VP//58Wn8k2qcOTJ9qxFhG0WtvAq1P5Aq/CZfspgCd4=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=oBKesYFnWiOL6rtCA1WvhldX565pHoDow6cst5O+yiMJV8riZzIgENxvieESVooHj 9uMYjfMx9fCGNW+BWfKgL3GFDiiKJbSTmDPwkfvB/kwVg+OII+EN7eKID2LqBJptbD Ry5keUUsOTMZ4CUaw9ebSqpsZRULbOse7VVROGG2UZP2AGoKSvWDgoh6QKUb1MzGQI Pg00ASMSf3TbwzPBbeGqpzKbY9lOurMrc9ZfoPdnkRwn71wsI2xaFsWejZQy3UDtdJ 5DUoLtwEVttzeKCuqBW5+mc0gP6U3G6Ok8ram3hl+LQhgAiMoex5zhuV8FZJVDFaop NpDwRU0SpznOw== Date: Wed, 9 Sep 2026 17:36:34 +0100 From: "Lorenzo Stoakes (ARM)" To: "David Hildenbrand (Arm)" Cc: Andrew Morton , Suren Baghdasaryan , "Liam R. Howlett" , Vlastimil Babka , Shakeel Butt , Zi Yan , Baolin Wang , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Lance Yang , Usama Arif , Kiryl Shutsemau , Mike Rapoport , Michal Hocko , Xu Xin , Chengming Zhou , Jann Horn , Pedro Falcato , Rik van Riel , Harry Yoo , Chris Li , Kairui Song , Kemeng Shi , Nhat Pham , Baoquan He , Youngjun Park , Peter Xu , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Guilherme Giacomo Simoes Subject: Re: [PATCH] mm: implement and use vma_is_faulted(), silence KCSAN Message-ID: References: <20260909-vma-is-faulted-v1-1-3a701f48984a@kernel.org> <3a261c3e-4c26-41b3-b55f-5800ae1081f3@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: <3a261c3e-4c26-41b3-b55f-5800ae1081f3@kernel.org> On Wed, Sep 09, 2026 at 06:24:52PM +0200, David Hildenbrand (Arm) wrote: > On 9/9/26 18:14, Lorenzo Stoakes (ARM) wrote: > > Provide a function to abstract the common task of checking whether > > a VMA is faulted in or not. > > > > A VMA or mmap lock must be held when calling this function. For an attached > > VMA the transitions between unfaulted/faulted state are: > > > > Transition | VMA/mmap Lock state > > ------------------------|----------------------------------------------- > > unfaulted to faulted | write lock OR read lock + mm->page_table_lock > > faulted to unfaulted | write lock > > > > So vma_is_faulted() never provides a false positive (the lock precludes > > it), but if only a read lock is held, a negative result must be re-checked > > with mm->page_table_lock held. > > > > Detached VMAs cannot be concurrently manipulated as they are removed from > > the maple tree so require no guarantees. > > > > Use data_race() to silence KCSAN about non-existent data races between > > concurrent vma->anon_vma read/write on optimistic fault tests. > > > > Also while here, const-ify vma_is_attached(), vma_assert_stabilised() and > > dependants. > > > > Finally, update the core VMA merge/split, rmap, mremap, KSM and fault > > preparation callers which test vma->anon_vma directly to use > > vma_is_faulted() instead. > > > > Note that the lockless read in reusable_anon_vma() is doing more than > > checking whether the VMA is faulted - it is returning the anon_vma to be > > used on fault, so this check is not altered. > > > > There is one odd one out - file_backed_vma_is_retractable() - which holds > > neither a VMA nor mmap lock and is stabilised by the file rmap lock only. > > > > Therefore just add a comment to explain why the direct vma->anon_vma check > > is required. > > > > Reported-by: Guilherme Giacomo Simoes > > Closes: https://lore.kernel.org/all/20260829100034.423064-1-trintaeoitogc@gmail.com/ > > Closes: https://lore.kernel.org/all/20260909115723.528501-1-trintaeoitogc@gmail.com/ > > Signed-off-by: Lorenzo Stoakes (ARM) > > --- > > > Is vma_is_faulted() really the right thing to use when wanting to say that we > (likely) faulted in an anon page? > > I think that's highly confusing, considering just MAP_SHARED mappings where that > will never be true. Hmm, yeah. I mean there's nowhere you'd do this check where you weren't checking something that couldn't at least in theory be anon-faulted. But it's confusing vs. shared you're right. vma_is_anon_faulted()? > > -- > Cheers, > > David -- Cheers, Lorenzo