From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-la0-f54.google.com (mail-la0-f54.google.com [209.85.215.54]) by kanga.kvack.org (Postfix) with ESMTP id 715E26B0038 for ; Mon, 15 Jun 2015 16:48:46 -0400 (EDT) Received: by laka10 with SMTP id a10so18228293lak.0 for ; Mon, 15 Jun 2015 13:48:45 -0700 (PDT) Received: from mail-lb0-f178.google.com (mail-lb0-f178.google.com. [209.85.217.178]) by mx.google.com with ESMTPS id ej7si11337010lad.149.2015.06.15.13.48.44 for (version=TLSv1.2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 15 Jun 2015 13:48:44 -0700 (PDT) Received: by lblr1 with SMTP id r1so34636881lbl.0 for ; Mon, 15 Jun 2015 13:48:44 -0700 (PDT) MIME-Version: 1.0 In-Reply-To: <20150615202856.GA13273@gmail.com> References: <1434188955-31397-1-git-send-email-mingo@kernel.org> <20150613185828.GA32376@redhat.com> <20150614075943.GA810@gmail.com> <20150614200623.GB19582@redhat.com> <87bnghit74.fsf@tassilo.jf.intel.com> <20150615202856.GA13273@gmail.com> From: Andy Lutomirski Date: Mon, 15 Jun 2015 13:48:23 -0700 Message-ID: Subject: Re: why do we need vmalloc_sync_all? Content-Type: text/plain; charset=UTF-8 Sender: owner-linux-mm@kvack.org List-ID: To: Ingo Molnar Cc: Andi Kleen , Oleg Nesterov , "linux-kernel@vger.kernel.org" , "linux-mm@kvack.org" , Andrew Morton , Denys Vlasenko , Brian Gerst , Peter Zijlstra , Borislav Petkov , "H. Peter Anvin" , Linus Torvalds , Thomas Gleixner , Waiman Long On Mon, Jun 15, 2015 at 1:28 PM, Ingo Molnar wrote: > > * Andy Lutomirski wrote: > >> On Sun, Jun 14, 2015 at 7:47 PM, Andi Kleen wrote: >> > Oleg Nesterov writes: >> >> >> >> But again, the kernel no longer does this? do_page_fault() does >> >> vmalloc_fault() without notify_die(). If it fails, I do not see how/why a >> >> modular DIE_OOPS handler could try to resolve this problem and trigger >> >> another fault. >> > >> > The same problem can happen from NMI handlers or machine check handlers. It's >> > not necessarily tied to page faults only. >> >> AIUI, the point of the one and only vmalloc_sync_all call is to prevent >> infinitely recursive faults when we call a notify_die callback. The only thing >> that it could realistically protect is module text or static non-per-cpu module >> data, since that's the only thing that's reliably already in the init pgd. I'm >> with Oleg: I don't see how that can happen, since do_page_fault fixes up vmalloc >> faults before it calls notify_die. > > Yes, but what I meant is that it can happen if due to an unrelated kernel bug and > unlucky timing we have installed this new handler just when that other unrelated > kernel bug triggers: say a #GPF crash in kernel code. I still don't see the problem. CPU A: crash and start executing do_page_fault CPU B: register_die_notifier CPU A: notify_die now we get a vmalloc fault, fix it up, and return to do_page_fault and print the oops. > > In any case it should all be mooted with the removal of lazy PGD instantiation. Agreed. --Andy -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org