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=-2.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_2 autolearn=no 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 33F30C3A5A9 for ; Mon, 4 May 2020 19:10:11 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 1099120746 for ; Mon, 4 May 2020 19:10:11 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727788AbgEDTKK (ORCPT ); Mon, 4 May 2020 15:10:10 -0400 Received: from mail.kernel.org ([198.145.29.99]:60780 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725981AbgEDTKJ (ORCPT ); Mon, 4 May 2020 15:10:09 -0400 Received: from gandalf.local.home (cpe-66-24-58-225.stny.res.rr.com [66.24.58.225]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 447F8206C0; Mon, 4 May 2020 19:10:08 +0000 (UTC) Date: Mon, 4 May 2020 15:10:06 -0400 From: Steven Rostedt To: Joerg Roedel Cc: Mathieu Desnoyers , linux-kernel , Ingo Molnar , Thomas Gleixner , Peter Zijlstra , Borislav Petkov , Andrew Morton , Shile Zhang , Andy Lutomirski , "Rafael J. Wysocki" , Dave Hansen , Tzvetomir Stoyanov Subject: Re: [PATCH] percpu: Sync vmalloc mappings in pcpu_alloc() and free_percpu() Message-ID: <20200504151006.69d2a16c@gandalf.local.home> In-Reply-To: <20200504183832.GL8135@suse.de> References: <20200429082854.6e1796b5@oasis.local.home> <20200429100731.201312a9@gandalf.local.home> <20200430141120.GA8135@suse.de> <20200430121136.6d7aeb22@gandalf.local.home> <20200430191434.GC8135@suse.de> <20200430211308.74a994dc@oasis.local.home> <1902703609.78863.1588300015661.JavaMail.zimbra@efficios.com> <20200430223919.50861011@gandalf.local.home> <20200504151236.GI8135@suse.de> <20200504134042.178409c3@gandalf.local.home> <20200504183832.GL8135@suse.de> X-Mailer: Claws Mail 3.17.3 (GTK+ 2.24.32; x86_64-pc-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, 4 May 2020 20:38:32 +0200 Joerg Roedel wrote: > Fair point, but this just shows how problematic it is to call something > like vmalloc_sync_mappings() from a lower-level kernel API function. > The obvious fix for this would be to make pgd_lock irq-safe, but this is > getting more and more ridiculous. Yeah, it's just getting more deeper into the rabbit hole. > > I know you don't like to have a vmalloc_sync_mappings() call in the > tracing code, but can you live with it until we get rid of this broken > interface? I'm fine with adding it to the tracing code (with that ridiculous comment! ;-) I'll even tag is as stable, but again, it's uncertain what commit that it "fixes". > > My plan for this is to use a small bitmap to track in the vmalloc and > the (x86-)ioremap code at which levels of the page-tables the code made > changes and combine that with an architecture-dependend mask to decide > whether anything needs to be synced. > > On x86-64 the sync would be necessary at most 64 times after boot, so I > think this will only have a very small performance impact, even with > VMAP_STACKS. And as a bonus it would also get rid of vmalloc faulting on > x86, fixing the issue with tracing too. Just because tracing get's a workaround, let's hopefully not get distracted and forgot to add this. It looks like the best solution is to not need the vmalloc_sync_mappings() call in the future. But, as we are about to release trace-cmd with a feature that reliably triggers the bug, I can't wait for the proper fix. -- Steve