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smtp.subspace.kernel.org; dkim=pass header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=cIBWz7ft; arc=none smtp.remote-ip=100.103.45.18 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 8CBED606A31D; Wed, 28 Oct 2026 20:34:05 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key, unprotected) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=cIBWz7ft Received: by smtp.kernel.org (Postfix) with ESMTPSA id E8B341F01825; Wed, 28 Oct 2026 20:34:02 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1793219645; bh=5HMKZuJ5mzuvxrnS1ESvLr/009YKienGm5nWikk9ws4=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=cIBWz7ftIdOwqDs027jvbsvqfjLC7DFt594wmqtGfW3hHL7XAk4RfMM3E/c6GxnLz M+32Ms3Yplm33eju/AyD5QrKJ5c36zTKSdZQkNiXmI2OdbwRRx2vh22uQpoZ+tx6jy cWYZ2hOiEp9gOPEnO6AcWWioGmfDctCgxK3BmgZx1V7JmLWNUjzT1quMRZT/yYUtt2 96aDlR58KXqhUwNUS+dsm/bVVTDrjnzrVIMXtpzG6+GIOnjPjIaMNNrPWRPkY/nDAT 8GXnNgRTSCneh9SNPYxBNwHikmsbbhB9CNuKUwtS9wxZzrt+rNAhhSofzl8N1IPKCZ y44AKjUVHOC4g== Message-ID: <44a5aa3e-173c-41fd-bbb5-92ae9d1cdc8f@kernel.org> Date: Wed, 28 Oct 2026 21:34:00 +0100 X-Mailing-List: linux-rdma@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] RDMA/uverbs: Don't return the shared zero page from rdma_umap_fault() To: Zi Yan , Jason Gunthorpe Cc: Leon Romanovsky , linux-mm@kvack.org, Lorenzo Stoakes , Lance Yang , Kiryl Shutsemau , linux-rdma@vger.kernel.org, linux-kernel@vger.kernel.org References: <20261025-fix-rdma_umap_fault-zero-page-v1-1-186255624f1f@nvidia.com> <78802ed9-9cc6-41c4-b131-7c7d38f7ff39@kernel.org> <829c448d-e6e2-4874-a166-7624e87489ef@kernel.org> <5E86E76B-F97A-4998-9EDB-8E450470FA4A@nvidia.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 10/28/26 18:49, Zi Yan wrote: > On 28 Oct 2026, at 4:43, David Hildenbrand (Arm) wrote: > >>> >>> For writable path, a new vmf_insert_pfn_mkwrite() is needed, so it was >>> not fixed. I can fix that part as well. >> >> We should do that; returning kernel allocations from this function in ->fault is >> bad. >> >> How urgent is this fix here? (can it actually be triggered?Specific hardware?) >> > > Not urgent. Like Jason said in another email, RDMA only runs on arch > with special PTE, so refcount, mapcount, rss are balanced. With DEBUG_VM, > zero_page checks in rmap will fire on the returned zero_page. > >>> >>> The read-only zero page part might need more attention, since >>> we just prevented vmf_insert_pfn_prot() from using zero_page. Do we want >>> to add the support for rdma_umap_fault()? >> >> We just have to make sure that we cannot install it writable etc. But that's not >> a concern. >> >> Just have to be careful :) > > There is one wrinkle, it does not work on > s390 + mm_forbids_zeropage() == true, where zero page is not present, > and a private zero page is needed. Yes, see vm_mixed_zeropage_allowed() I added. In a VM_PFNMAP it doesn't matter. It's only a problem if we can GUP involved IIRC, and VM_PFNMAP just disallows it entirely. Similar for s390, it doesn't support any form of memory deduplication (incl. shared zeropage) for some KVM use cases. IIRC, PFNMAP does not really apply there. (have to make sure that gup-fast doesn't try pinning the shared zeropage) (of course, the shared zeropage must never get mapped writable) -- Cheers, David