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=-2.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 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 E2D52C4BA12 for ; Wed, 26 Feb 2020 15:07:02 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id B6F1F24688 for ; Wed, 26 Feb 2020 15:07:02 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1582729622; bh=QT6GUyLzIUdRMVSaNAxDiNdKzTo0kQ+EQU+2BK797Sw=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:List-ID: From; b=BwMJcnC1rDE68pxW7Vaf7NjGtNirhTSq4JyQFGwg6N//HOMxtU1EmohhX+BMAXuLC H/xhPWUU14I9BX090FXsvydOgBS6LuWftRQ7wo3TT2Ayg8oGLG2E5/z+7EBiTm1EHd r9DUbpnAx2ebjCmzfgKjAhj3wvSJTshI6BXu5Z5Q= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727877AbgBZPG6 (ORCPT ); Wed, 26 Feb 2020 10:06:58 -0500 Received: from mail.kernel.org ([198.145.29.99]:49932 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726990AbgBZPG6 (ORCPT ); Wed, 26 Feb 2020 10:06:58 -0500 Received: from paulmck-ThinkPad-P72.home (199-192-87-166.static.wiline.com [199.192.87.166]) (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 41E122467D; Wed, 26 Feb 2020 15:06:57 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1582729617; bh=QT6GUyLzIUdRMVSaNAxDiNdKzTo0kQ+EQU+2BK797Sw=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=FFkA0N8gusvWYmfVUiPUaZKzj8nUanEMpm09zuCMQphmDbzFDySX7V8AW8zC4dMbj L1pOu3+jZuFEldIfaMbsKANrPIeQxVLBfl9Ecsgqxre6DfLfwizpPveoN5KwIKCRWZ Sxr/hu+ZExeF+XTdQnCSJ2tk2GbIKQ4RDLecOr1w= Received: by paulmck-ThinkPad-P72.home (Postfix, from userid 1000) id D1D253521EAF; Wed, 26 Feb 2020 07:06:56 -0800 (PST) Date: Wed, 26 Feb 2020 07:06:56 -0800 From: "Paul E. McKenney" To: Uladzislau Rezki Cc: Joel Fernandes , "Theodore Y. Ts'o" , Ext4 Developers List , Suraj Jitindar Singh , LKML Subject: Re: [PATCH RFC] ext4: fix potential race between online resizing and write operations Message-ID: <20200226150656.GB2935@paulmck-ThinkPad-P72> Reply-To: paulmck@kernel.org References: <20200221003035.GC2935@paulmck-ThinkPad-P72> <20200221131455.GA4904@pc636> <20200221202250.GK2935@paulmck-ThinkPad-P72> <20200222222415.GC191380@google.com> <20200223011018.GB2935@paulmck-ThinkPad-P72> <20200224174030.GA22138@pc636> <20200225020705.GA253171@google.com> <20200225185400.GA27919@pc636> <20200225224745.GX2935@paulmck-ThinkPad-P72> <20200226130440.GA30008@pc636> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200226130440.GA30008@pc636> User-Agent: Mutt/1.9.4 (2018-02-28) Sender: linux-ext4-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-ext4@vger.kernel.org On Wed, Feb 26, 2020 at 02:04:40PM +0100, Uladzislau Rezki wrote: > On Tue, Feb 25, 2020 at 02:47:45PM -0800, Paul E. McKenney wrote: > > On Tue, Feb 25, 2020 at 07:54:00PM +0100, Uladzislau Rezki wrote: > > > > > > > I was thinking a 2 fold approach (just thinking out loud..): > > > > > > > > > > > > > > If kfree_call_rcu() is called in atomic context or in any rcu reader, then > > > > > > > use GFP_ATOMIC to grow an rcu_head wrapper on the atomic memory pool and > > > > > > > queue that. > > > > > > > > > > > > I am not sure if that is acceptable, i mean what to do when GFP_ATOMIC > > > > > gets failed in atomic context? Or we can just consider it as out of > > > > > memory and another variant is to say that headless object can be called > > > > > from preemptible context only. > > > > > > > > Yes that makes sense, and we can always put disclaimer in the API's comments > > > > saying if this object is expected to be freed a lot, then don't use the > > > > headless-API to be extra safe. > > > > > > > Agree. > > > > > > > BTW, GFP_ATOMIC the documentation says if GFP_ATOMIC reserves are depleted, > > > > the kernel can even panic some times, so if GFP_ATOMIC allocation fails, then > > > > there seems to be bigger problems in the system any way. I would say let us > > > > write a patch to allocate there and see what the -mm guys think. > > > > > > > OK. It might be that they can offer something if they do not like our > > > approach. I will try to compose something and send the patch to see. > > > The tree.c implementation is almost done, whereas tiny one is on hold. > > > > > > I think we should support batching as well as bulk interface there. > > > Another way is to workaround head-less object, just to attach the head > > > dynamically using kmalloc() and then call_rcu() but then it will not be > > > a fair headless support :) > > > > > > What is your view? > > > > > > > > > > Otherwise, grow an rcu_head on the stack of kfree_call_rcu() and call > > > > > > > synchronize_rcu() inline with it. > > > > > > > > > > > > > > > > > > > What do you mean here, Joel? "grow an rcu_head on the stack"? > > > > > > > > By "grow on the stack", use the compiler-allocated rcu_head on the > > > > kfree_rcu() caller's stack. > > > > > > > > I meant here to say, if we are not in atomic context, then we use regular > > > > GFP_KERNEL allocation, and if that fails, then we just use the stack's > > > > rcu_head and call synchronize_rcu() or even synchronize_rcu_expedited since > > > > the allocation failure would mean the need for RCU to free some memory is > > > > probably great. > > > > > > > Ah, i got it. I thought you meant something like recursion and then > > > unwinding the stack back somehow :) > > > > > > > > > > Use preemptible() andr task_struct's rcu_read_lock_nesting to differentiate > > > > > > > between the 2 cases. > > > > > > > > > > > > If the current context is preemptable then we can inline synchronize_rcu() > > > > > together with freeing to handle such corner case, i mean when we are run > > > > > out of memory. > > > > > > > > Ah yes, exactly what I mean. > > > > > > > OK. > > > > > > > > As for "task_struct's rcu_read_lock_nesting". Will it be enough just > > > > > have a look at preempt_count of current process? If we have for example > > > > > nested rcu_read_locks: > > > > > > > > > > > > > > > rcu_read_lock() > > > > > rcu_read_lock() > > > > > rcu_read_lock() > > > > > > > > > > > > > > > the counter would be 3. > > > > > > > > No, because preempt_count is not incremented during rcu_read_lock(). RCU > > > > reader sections can be preempted, they just cannot goto sleep in a reader > > > > section (unless the kernel is RT). > > > > > > > So in CONFIG_PREEMPT kernel we can identify if we are in atomic or not by > > > using rcu_preempt_depth() and in_atomic(). When it comes to !CONFIG_PREEMPT > > > then we skip it and consider as atomic. Something like: > > > > > > > > > static bool is_current_in_atomic() > > > { > > > #ifdef CONFIG_PREEMPT_RCU > > > > If possible: if (IS_ENABLED(CONFIG_PREEMPT_RCU)) > > > > Much nicer than #ifdef, and I -think- it should work in this case. > > > OK. Thank you, Paul! > > There is one point i would like to highlight it is about making caller > instead to be responsible for atomic or not decision. Like how kmalloc() > works, it does not really know the context it runs on, so it is up to > caller to inform. > > The same way: > > kvfree_rcu(p, atomic = true/false); > > in this case we could cover !CONFIG_PREEMPT case also. Understood, but couldn't we instead use IS_ENABLED() to work out the actual situation at runtime and relieve the caller of this burden? Or am I missing a corner case? Thanx, Paul