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 06BDA38B157; Thu, 19 Mar 2026 19:44:52 +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=1773949493; cv=none; b=AmJD+CuIRFyTJY8Mx5s6k4ABScqyNX3fjj/SvFLcLK+ZGrHuFlQQYV9L00mesu/1lTVvKr2CTwaVDFgmhqCvE7TCFN649vGdhh/DeMXo8zaIxZrFSZ0fKKXg/DNFK9hbdqMZb9sDwxjJE7zbu8fOiEpcXns9ONhpxNiCc+BBF1k= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1773949493; c=relaxed/simple; bh=k0gwJ1Ua/ckj2OmljPNiQb1F88aiM4Vf0+0bdFPe4DM=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=bY1liLX9vWV/T9DVAyywBOsco4WybH4I6ZCNruuCVB2YfzVXo4nRSY6R3tyCYUoApvNrQcxMAVdYC5wGeazu0KNKXtnD8yxTlOW8/u8yA8N4sS8KA2AKDeXRk0+dJ14aOLnNG4o2XK6aDh4ccup8q3Ac/xnbWFBtYQQEAi6itdU= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=BbWXpH+W; 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="BbWXpH+W" Received: by smtp.kernel.org (Postfix) with ESMTPSA id E9D6DC19424; Thu, 19 Mar 2026 19:44:50 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1773949492; bh=k0gwJ1Ua/ckj2OmljPNiQb1F88aiM4Vf0+0bdFPe4DM=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=BbWXpH+WNr49Z/mVCTxNJewhC+hgz9Og4EwtAXi/KWqVzSkkCUO5xo6tB/27HIPfu jvxkGFKpkrvn2sR4QxFzSZgftEPg7V9tagOE5Fj6pcsDBIfHADlxLUcmZBE4qBASt6 QEecyyDQtXjkIHYptdLxYZjB6esmqRoNfsRksAsY+MqsFmLCLG3rhY+9ZyU1JYfeVV wmE7CaSoHCppVr5oCnLc8meVYCwLNZNqtYSyBnxM4OVCkb3PUC7L94/NoDRiGAQm5k TglFNXuE401Gk3V1XcvNEHvinLJHzutaj0za+i/mgiZWJEsPCoAHKS8a73w97sh3E0 coeFyQl47QlfA== Message-ID: <03420838-dc1b-4a7b-93c4-8710fab58869@kernel.org> Date: Thu, 19 Mar 2026 20:44:49 +0100 Precedence: bulk X-Mailing-List: stable@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: Alex Williamson Cc: "Boone, Max" , 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> <45e50068-751c-4e8c-a6b0-62cf8d1e58e6@kernel.org> <20260319083039.03865989@shazbot.org> 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 15:30, Alex Williamson wrote: > On Thu, 19 Mar 2026 14:18:49 +0100 > "David Hildenbrand (Arm)" wrote: > >> On 3/19/26 09:36, Boone, Max wrote: >>> >>> >>> >>> 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'd say no, that's not a problem. It's really just that masking all > non-zero returns as -EAGAIN makes some assumptions about the other > error conditions that could change over time, so minimally those > dependencies should be clearly stated. Even better would be if we > didn't need to make those assumptions. I agree that follow_pfnmap_start() should be improved, to return -EINVAL on invalid parameters and -ENOENT on "no present entry"." And -ENOENT should be documented to be resolvable by triggering a fault. Looking at it, the retry label in the function also seems avoidable, and some other checks might be shaky ... Let me briefly see if I can rework that. -- Cheers, David