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 288513E49F7; Wed, 5 Aug 2026 08:59:39 +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=1785920381; cv=none; b=u3MSFVEPNc34JFQM+H2MQr1lyYWqt/EJLiOTxCE8XkNnWns+nOGGXNOJVG+vrM7EeN/uaNeHlGfw38VkHkH1FHvsv6YePCb+LFOwcVCs/3XN1ztJ0RMxt4iUEAJlH7uxYSxGOYj0ZOUbuMVdutkz70Zhr6cQIjnUl4aBopYJYBY= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785920381; c=relaxed/simple; bh=1Zpye7X5O3XNwKDO5om4yeP7kri9nwvCpDEG6m/bpWw=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=nTCMeqo+4GWQzzBJPyIEPZlQsg6nsWQEaR7vQWb57k5L3FlWxOJ1/QqYOcp7Dsee11+SuTsjzYA8t7BYgexk5CdgvS1aU4mSyAZ1+NgEbZuHweHqlIazLVBUUCp3CR+WVxcedgPuItQOLsIqcdnydbCR1lduW5fubu5Dl165ifM= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=K+1Rt2ao; 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="K+1Rt2ao" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 018091F000E9; Wed, 5 Aug 2026 08:59:32 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1785920379; bh=1yRpRg/YjcbX7C2paRrZom5mhwTrr0QvF5ECsYRO/+w=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=K+1Rt2aozvXZ6W3FHxtBV97CQTGhnvf409HxCGwFj1X7JitOXG86TUEDPv2k6/x1G 9O21hqeZLoY80QDCALep3ZAzkMnjq2H4JgVfTCNCL3LSyWqgHDPxpQkvn6OEdCzSTP nFTo5dTvUmevSjaaFR31pHViXEAjraEDTdHwz8ddfwS+yWdKF9/yRqpRB/xSbt/oU/ 9gsWhKTBxLM2/PQ9fPfZu+Y5waWH5MhkyPQergkWmtzoLZgE1MUZ+ljzvSOkksvxg9 P3PI82Af/rplRRKV01PYALUmNe+GNe41JF0++hzkt1334sQRaqV8XOvJxYe66oAdn7 Fn3FDMZalFhGg== Date: Wed, 5 Aug 2026 09:59:17 +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 , linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-kselftest@vger.kernel.org Subject: Re: [PATCH v3 06/15] mm: propagate VMA anonymous page offset on map, remap, split + merge Message-ID: References: <20260729-b4-scalable-cow-virt-pgoff-v3-0-e8ecfefea812@kernel.org> <20260729-b4-scalable-cow-virt-pgoff-v3-6-e8ecfefea812@kernel.org> <4ad4b17b-bae6-4169-8649-bff4a29e7fde@kernel.org> <6ea7cdad-afb1-45af-a63c-30b760fe160d@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: <6ea7cdad-afb1-45af-a63c-30b760fe160d@kernel.org> On Wed, Aug 05, 2026 at 09:35:56AM +0200, David Hildenbrand (Arm) wrote: > On 8/3/26 15:46, Lorenzo Stoakes (ARM) wrote: > > On Mon, Aug 03, 2026 at 12:52:42PM +0200, David Hildenbrand (Arm) wrote: > >> On 7/29/26 18:48, Lorenzo Stoakes (ARM) wrote: > >>> We must correctly update VMA anonymous page offset state on all VMA > >>> operations that would result in it changing, with special attention given > >>> to remapping. > >>> > >>> We cover most cases by simply updating vma_set_range() to do so (with a new > >>> anonymous page offset parameter), but also notably must update the merging > >>> and mapping logic to propagate this parameter correctly. > >>> > >>> The remap logic remains the same - we may update the anonymous page offset > >>> if the VMA is unfaulted, but now this applies to MAP_PRIVATE file-backed > >>> mappings too, so we update the code to reflect this. > >>> > >>> Note that we use __linear_anon_page_index() upon remap as the VMA may be > >>> shared, in order that we update the field consistently regardless of VMA > >>> type. > >>> > >>> Similarly, pass through anon page offset to the merge logic, updating the > >>> vma_merge_struct struct to propagate it, and also use > >>> __linear_anon_page_index() to obtain the anonymous page index so it can be > >>> safely used for both shared and MAP_PRIVATE file-backed mappings. > >>> > >>> Finally, we update insert_vm_struct() to correctly set the anonymous page > >>> offset on insertion of a VMA. > >>> > >>> We simply ensure state is correctly propagated here, so no functional > >>> changes are intended. > >>> > >>> Also while we're here, replace a VM_BUG_ON_VMA() with a > >>> VM_WARN_ON_ONCE_VMA(). > >>> > >>> Also update VMA userland tests to reflect this change. > >>> > >>> Signed-off-by: Lorenzo Stoakes (ARM) > >> > >> > >> [...] > >> > >>> struct vm_area_struct *vma = *vmap; > >>> unsigned long vma_start = vma->vm_start; > >>> @@ -1919,11 +1929,14 @@ struct vm_area_struct *copy_vma(struct vm_area_struct **vmap, > >>> VMG_VMA_STATE(vmg, &vmi, NULL, vma, addr, addr + len); > >>> > >>> /* > >>> - * If anonymous vma has not yet been faulted, update new pgoff > >>> - * to match new location, to increase its chance of merging. > >>> + * If a vma has not yet been faulted, update its anonymous pgoff to > >>> + * match the new location to increase its chance of merging. > >>> */ > >>> - if (unlikely(vma_is_anonymous(vma) && !vma->anon_vma)) { > >>> - pgoff = addr >> PAGE_SHIFT; > >>> + if (!vma->anon_vma && !vma_test(vma, VMA_SHARED_BIT)) { > >> > >> Could we also use is_cow_mapping() ? > > > > No this would be incorrect. > > > > A read-only mapping would become unmergeable here. So this is something apart > > from the rmap aspect, > > I'd assume that we should never even consider anon_pgoff when merging > !is_cow_mapping(), it doesn't make any sense. > > No anon folios -> no anon_vma -> no anon_pgoff You can merge unfaulted ranges is the thing here. But anyway I actually wonder whether this whole branch shouldn't be: if (!vma->anon_vma) { ... } Because that way we keep anon_pgoff updated even for MAP_SHARED mappings. This isn't necessary and doesn't impact anything _except_ print_bad_page_map which outputs both pgoffs. But it'd be consistent, avoid any confusion about gating on VMA_SHARED, and simplify the code :) > > But I think I am missing one detail here: > > > and it is a contract that upon move of an unfaulted > > mapping (which for read-only anon would always be unfaulted) that vma->vm_pgoff > > is updated. > > "read-only anon": I assume you mean an anon mapping that does not have > VM_MAYWRITE set? A MAP_SHARED mapping of a read-only file becomes a MAP_PRIVATE !VMA_MAYWRITE_BIT mapping and must adhere to the same contract. Also mmap hooks can clear the VMA_MAYWRITE_BIT. However: - If you're a driver clearing VMA_MAYWRITE_BIT you should only be doing this for 'special' mappings anyway (I have a series I've not sent yet that establishes this as an invariant also) - and these are not mergeable anyway. - If you're a !VMA_MAYWRITE_BIT MAP_PRIVATE-file backed mappings you never set vma->anon_vma and always update anon pgoff so you always have alignment for purposes of merge. So I think also we can then change needs_adjacent_anon_pgoff() to: static bool needs_adjacent_anon_pgoff(const struct vma_merge_struct *vmg) { return vmg->file && is_cow_mapping(...); } [I have to create a vma_flags_t variant of is_cow_mapping()] With those two changes we gate on VMA_SHARED_BIT nowhere :) > > I recall that that's a combination that cannot be created. While you can create > something that does not have VM_WRITE set, IIRC VM_MAYWRITE is always set for > anon vmas. For pure anon yeah, see above for the MAP_SHARED->MAP_PRIVATE-file backed weird case. > > -- > Cheers, > > David (It's funny to me that if you want a truly read-only MAP_PRIVATE file-backed mapping (no idea why you would but anyway) you have to MAP_SHARED, but an actually MAP_PRIVATE file-backed mapping of a read-only file is writable [which makes sense obviously] :) -- Cheers, Lorenzo