From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (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 4570B3A1E70; Thu, 19 Mar 2026 13:18:52 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1773926333; cv=none; b=L2oA71FQ9wW0pIbJ6mjkq1Ee7L1DBQKPP//CoAK3RMrIO0M8RJB35g0Zg2DWkv/XAdYsPmyG7/tXQJw+lA0KSpYK9uN6d/gQSsPa0NV0azEctKyFUZtoyvse9Sqp9h3tgWNbapf3Gz/3lxw/wzpj32KQbODB+GXbg2EjlWNhXzA= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1773926333; c=relaxed/simple; bh=DBeq6ynyMBT7SGVxMF7nNxXLpd+5i5Ws71dP8HR4daE=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=IVz6fBQULGO9diqb96sUJllNJ3XnA39Yrf/pBAMhPOfgiMhPFoOJFDXAMEuyJnY6qG/WrZsQCCDhXsHKbQL/3RNTmQA01eXWYV48wHnpRpv4u7OcJpCy7YOiezNg+l27YttYj85aIxgZ8Oudj64l9xd46KVNA9mHde1V2qiu7vw= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=ZKlmN21x; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="ZKlmN21x" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 5FE4DC19424; Thu, 19 Mar 2026 13:18:51 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1773926332; bh=DBeq6ynyMBT7SGVxMF7nNxXLpd+5i5Ws71dP8HR4daE=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=ZKlmN21xzTYbuxpDehDoEyHf4slhheV6fzre8D17e+zmj+nbm6dXD9//rN+rOTWG2 9T7UHBnqj7YnFqSeFYXxIeDR7Qyq/hHWZl/RbQVrLWHlZzX3XUa1ie1azTEIHc5GtO 6B9SxCO1V2B0ODFWnt/27jJZAH8jdL9i2UViBnnKx99Ul0V/cWBU5l8IJgaL2WFEU0 ISYAGNcqJmLc8nCPHG0baUMxvFqi++YOznl/oWtbYSeY8xco3Spwb5UZwIcfjijgBi QezSdk3a4jzBNqAGmjyUWddC2BtuYJLlmr81MRyLix+9oNqrtnhL3aVxNgiMTj1Tvo d7vbbuX9c0OEw== Message-ID: <45e50068-751c-4e8c-a6b0-62cf8d1e58e6@kernel.org> Date: Thu, 19 Mar 2026 14:18:49 +0100 Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] vfio/type1: Retry follow_pfnmap_start() when PFNMAP is zapped To: "Boone, Max" , Alex Williamson Cc: Max Boone via B4 Relay , "kvm@vger.kernel.org" , "linux-kernel@vger.kernel.org" , "stable@vger.kernel.org" References: <20260317-retry-pin-on-reclaimed-pud-v1-1-1f0d0a23f78d@akamai.com> <20260318152249.43eb81f6@shazbot.org> <3C8F924E-CA2D-4368-83DF-3CCCD4BA49FF@akamai.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 8bit On 3/19/26 09:36, Boone, Max wrote: > > >> On Mar 18, 2026, at 10:22 PM, Alex Williamson wrote: >> >> […] >> >>> + /* >>> + * follow_pfnmap_start() returns -EINVAL for >>> + * invalid parameters and non-present entries. >>> + * If that happens here after a successful >>> + * fixup_user_fault(), it is likely that the >>> + * pfnmap has been zapped. Retry instead of >>> + * failing. >>> + */ >> >> It's a little stronger than that, right? We're betting that the only >> remaining non-zero return is due to a race and we can introduce what >> appears to be potential for an infinite loop here because -EAGAIN will >> get kicked out to redo the vma_lookup() and fixup_user_fault() should >> return a genuine error if we're completely in the weeds. Should we >> make this a little stronger and more specific? Thanks, > > I’d say that the best case would be to have follow_pfnmap_start() return > -EINVAL or -ENOENT w.r.t. which of the two return values it is. But then > again, we could theoretically run into an infinite loop I guess - as the zap > and faulting could run in lockstep (the race window is extremely small > though). Well, in theory :) To hit that race repeatedly, you'd really have to be quite lucky I guess. But the real question is: if user space triggered the pinning, and user space keeps hurting itself to make progress, is that a real problem? I guess the crucial part would be to a) Have some cond_resched(() in there? b) Checking for fatal signals somewhere? c) Possibly drop locks (mmap lock?) every now and then? For GUP, a) and b) are in place in __get_user_pages(). c) might be done, but I think it's less deterministic. > > We could make the retry above bounded, and bubble up a -EBUSY such > that users of the ioctl can decide to retry instead of fail? Would that be a possible ABI break? You'd really have to only do that in a case where user space does stupid things, I guess. > > David, you mentioned that gup already has retry logic that we don’t have > with follow_fault_pfn() -> follow_pfnmap_start(). Would we potentially run > into an infinite loop with this change? GUP triggers page faults through faultin_page(). If handle_mm_fault() returns * VM_FAULT_COMPLETED we return -EAGAIN * VM_FAULT_ERROR we return the error * VM_FAULT_RETRY we return -EBUSY * Otherwise 0 In the caller __get_user_pages(), we * Retry immediately with ret == 0 * Return to the GUP caller (letting it retry) with -EBUSY/-EAGAIN Having at least a) and b) sounds reasonable. Not sure about having c), might be tricky if we are not allowed to drop the lock. -- Cheers, David