From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754833AbcLNDCq (ORCPT ); Tue, 13 Dec 2016 22:02:46 -0500 Received: from mail-vk0-f51.google.com ([209.85.213.51]:33446 "EHLO mail-vk0-f51.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753016AbcLNDCo (ORCPT ); Tue, 13 Dec 2016 22:02:44 -0500 MIME-Version: 1.0 In-Reply-To: <3eb4bfa4-ff94-bb5f-095a-3d17d68bf411@virtuozzo.com> References: <87lgvlzp34.fsf@yhuang-dev.intel.com> <1481553981-3856-1-git-send-email-aryabinin@virtuozzo.com> <20161213101254.GC10498@dhcp22.suse.cz> <20161213172441.GA22610@dhcp22.suse.cz> <3eb4bfa4-ff94-bb5f-095a-3d17d68bf411@virtuozzo.com> From: Andy Lutomirski Date: Tue, 13 Dec 2016 19:02:23 -0800 Message-ID: Subject: Re: [PATCH] mm-add-vfree_atomic-fix To: Andrey Ryabinin Cc: Michal Hocko , Johannes Weiner , Andrew Morton , Huang Ying , Stephen Rothwell , Christoph Hellwig , Joel Fernandes , Jisheng Zhang , Chris Wilson , John Dias , Thomas Gleixner , "H. Peter Anvin" , Ingo Molnar , "linux-kernel@vger.kernel.org" , LKP Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Dec 13, 2016 at 11:21 AM, Andrey Ryabinin wrote: > On 12/13/2016 09:15 PM, Andy Lutomirski wrote: > > >>>> >>>> But not quite acked by me. What happened to the vfree code that >>>> causes vfree_deferred to be called in a preemptable context? That >>>> sounds like a bug. >>> >>> Not sure I understand but the above stack points to a preemptible >>> context (copy_process). My stack was different and it looks preemptible as well. >>> free_thread_stack calls vfree_atomic unconditionally. So I am not sure >>> why do you think this is a bug? >>> >>>> (This code doesn't exist in Linus' tree. What tree does this apply to.) >>> >>> Anyway, now that I am looking at Andrew's tree I can see [1] which >>> doesn't have this_cpu_ptr. So I am not sure where this this_cpu_ptr came >>> from. Maybe the previous version of the patch which has shown up in the >>> linux-next and Andrew has picked up [2] in the meantime. /me confused >>> >>> [1] http://www.ozlabs.org/~akpm/mmotm/broken-out/mm-add-vfree_atomic.patch >>> [2] http://lkml.kernel.org/r/1481553981-3856-1-git-send-email-aryabinin@virtuozzo.com >> >> The underlying issue seems to be that we have this shiny new function >> vfree_atomic() which doesn't work in *non-atomic* context and that we > > It does work non-atomic context. It's fixed now. > >> have "kernel/fork: use vfree_atomic() to free thread stack" that calls >> vfree_atomic() from non-atomic context. > > From both context actually. Usually task stack is freed from atomic context: > http://lkml.kernel.org/r/20161019111541.GQ29358@nuc-i3427.alporthouse.com > http://lkml.kernel.org/r/CALCETrVqjejgpQVUdem8RK3uxdEgfOZy4cOJqJQjCLtBDnJfyQ@mail.gmail.com > > On rare occasions it can be freed from non-atomic context, e.g. error path in copy_process(). > >> I'm not sure what the motivation of the latter patch was, but ISTM we >> should revert it. TBH I'm not quite sure what the purpose of >> splitting vfree() and vfree_atomic() was, but I'm not seeing any >> reason that the common case of freeing stacks from non-atomic context >> should defer the free instead of just doing it right away. >> >> Andrey, Johannes, why should task stack freeing use vfree_atomic() in >> the first place? > > Because vfree() now can sleep and task stack freeing usually done in atomic context. > > Fair enough. -- Andy Lutomirski AMA Capital Management, LLC