From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-5.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED, USER_AGENT_MUTT autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 86652C43613 for ; Thu, 20 Jun 2019 19:47:18 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 67DC3206BA for ; Thu, 20 Jun 2019 19:47:18 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726521AbfFTTrS (ORCPT ); Thu, 20 Jun 2019 15:47:18 -0400 Received: from mga04.intel.com ([192.55.52.120]:26025 "EHLO mga04.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726293AbfFTTrS (ORCPT ); Thu, 20 Jun 2019 15:47:18 -0400 X-Amp-Result: UNKNOWN X-Amp-Original-Verdict: FILE UNKNOWN X-Amp-File-Uploaded: False Received: from fmsmga008.fm.intel.com ([10.253.24.58]) by fmsmga104.fm.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 20 Jun 2019 12:47:17 -0700 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.63,398,1557212400"; d="scan'208";a="160774653" Received: from mudigirx-mobl1.gar.corp.intel.com (HELO localhost) ([10.252.61.12]) by fmsmga008.fm.intel.com with ESMTP; 20 Jun 2019 12:47:14 -0700 Date: Thu, 20 Jun 2019 22:47:14 +0300 From: Jarkko Sakkinen To: Sean Christopherson Cc: linux-sgx@vger.kernel.org Subject: Re: [RFC PATCH] x86/sgx: Use mmu_notifier.release() instead of per-vma refcounting Message-ID: <20190620194714.GD15383@linux.intel.com> References: <20190619182216.5684-1-sean.j.christopherson@intel.com> <20190620194519.GC15383@linux.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190620194519.GC15383@linux.intel.com> Organization: Intel Finland Oy - BIC 0357606-4 - Westendinkatu 7, 02160 Espoo User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-sgx-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-sgx@vger.kernel.org On Thu, Jun 20, 2019 at 10:45:19PM +0300, Jarkko Sakkinen wrote: > On Wed, Jun 19, 2019 at 11:22:16AM -0700, Sean Christopherson wrote: > > Using per-vma refcounting to track mm_structs associated with an enclave > > requires hooking .vm_close(), which in turn prevents the mm from merging > > vmas (precisely to allow refcounting). > > > > Avoid refcounting encl_mm altogether by registering an mmu_notifier at > > .mmap(), removing the dying encl_mm at mmu_notifier.release() and > > protecting mm_list during reclaim via a per-enclave SRCU. > > > > Removing refcounting/vm_close() allows merging of enclave vmas, at the > > cost of delaying removal of encl_mm structs from mm_list, i.e. an mm is > > disassociated from an enclave when the mm exits or the enclave dies, as > > opposed to when the last vma (in a given mm) is closed. > > > > The impact of delying encl_mm removal is its memory footprint and > > whatever overhead is incurred during EPC reclaim (to walk an mm's vmas). > > Practically speaking, a stale encl_mm will exist for a meaningful amount > > of time if and only if the enclave is mapped in a long-lived process and > > then passed off to another long-lived process. It is expected that the > > vast majority of use cases will not encounter this condition, e.g. even > > using a daemon to build enclaves should not result in a stale encl_mm as > > the builder should never need to mmap() the enclave. > > > > Even if there are scenarios that lead to defunct encl_mms, the cost is > > likely far outweighed by the benefits of reducing the number of vmas > > across all enclaves. > > > > Note, using SRCU to protect mm_list is not strictly necessary, i.e. the > > existing walker with encl_mm refcounting could be massaged to work with > > mmu_notifier.release(), but the resulting code is subtle and fragile (I > > never actually got it working). The primary issue is that an encl_mm > > can't be moved off the list until its refcount goes to zero, otherwise > > the custom walker goes off into the weeds. The refcount requirement > > then prevents using mm_list to identify if an mmu_notifier.release() > > has fired, i.e. another mechanism is needed to guard against races > > between exit_mmap() and sgx_release(). > > > > Cc: Dave Hansen > > Cc: Andy Lutomirski > > Signed-off-by: Sean Christopherson > > OK, so what this needs is to split two patches. That can be seen from > your last paragraph (you are implying it with your note). Then this > can be sanely reviewed. > > And no, this does not fight by any means with the changes that I'm > doing. OK, so please ignore what I just said. You're right what I asked could not be sanely done :-) /Jarkko