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=-2.4 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,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 D66D8C52D54 for ; Thu, 27 Feb 2020 14:27:31 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id A5E7324656 for ; Thu, 27 Feb 2020 14:27:31 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=joelfernandes.org header.i=@joelfernandes.org header.b="WLmgqS5K" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2387928AbgB0O1b (ORCPT ); Thu, 27 Feb 2020 09:27:31 -0500 Received: from mail-qv1-f68.google.com ([209.85.219.68]:33668 "EHLO mail-qv1-f68.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2388303AbgB0OIx (ORCPT ); Thu, 27 Feb 2020 09:08:53 -0500 Received: by mail-qv1-f68.google.com with SMTP id ek2so1589707qvb.0 for ; Thu, 27 Feb 2020 06:08:53 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=joelfernandes.org; s=google; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to:user-agent; bh=bgYGSebMxipVx2p56pZyeVa4iTP2c2xGHpkLoDJFNy8=; b=WLmgqS5KhZmrbH0v82WS4EF7PbKONStLYOAaQ8PDF+YdyNRXlySssFTp+11S08p4g5 T16A4ua1a3eq/nPeJv8TQ7dz53YHwUqrg5WGZPRKGJ75UffhZdoao6tBvKRJy1foyCzt 0377kqJOVr8WS7Z7xlFEl7M1D+2gAkywDCz/c= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to:user-agent; bh=bgYGSebMxipVx2p56pZyeVa4iTP2c2xGHpkLoDJFNy8=; b=BKDk9FRPRXmIOX3ksUhOod0F9/+NnpX8Li8uS0HNo4nUWQTNL7dRgVMgeoK6F8LSFt w2B5NPmCincSDhgv1nq4Vph5dVBXEfAn5uTdvkT5nEw/8dN8VTJ89H2MwMMqos5dQ8J1 YbasppukVQXFkN4DA1glryaIE3DrAPWHdMdxbm53gfGSRCLq4Xf0zIve+EeovcGX2wl8 hco/8MDPPwAdKxTFF8+GHbyttXS4zJktRayWhdventCZL+BgXP5Va2/+2fYXCmN8SpHc oFFtFDZGmD2i0fztDqZVC1rBpIhG/zxvF1ZoucYjpC+HXQ/Z+yAvfn5YIwOuSzlMk6jD dI1g== X-Gm-Message-State: APjAAAVvfI6plbZ8DEq3PUdrU4AzfvqjVLo75EEJqJEiSnMpK6J6obZK xCozp1fBvq8eX0uogfxfae6YbQ== X-Google-Smtp-Source: APXvYqzRjNd30JGdGxDUbfztgm57+tSyCKGF1HpTW7BrtrDfkTqOOUgy+7UM4C9+FtGOReNknateFQ== X-Received: by 2002:ad4:488b:: with SMTP id bv11mr4742945qvb.154.1582812532259; Thu, 27 Feb 2020 06:08:52 -0800 (PST) Received: from localhost ([2620:15c:6:12:9c46:e0da:efbf:69cc]) by smtp.gmail.com with ESMTPSA id 133sm3181880qkh.109.2020.02.27.06.08.51 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 27 Feb 2020 06:08:51 -0800 (PST) Date: Thu, 27 Feb 2020 09:08:51 -0500 From: Joel Fernandes To: Uladzislau Rezki Cc: "Paul E. McKenney" , "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: <20200227140851.GD161459@google.com> References: <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> <20200226150656.GB2935@paulmck-ThinkPad-P72> <20200226155347.GA31097@pc636> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200226155347.GA31097@pc636> User-Agent: Mutt/1.12.2 (2019-09-21) 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 04:53:47PM +0100, Uladzislau Rezki wrote: > On Wed, Feb 26, 2020 at 07:06:56AM -0800, Paul E. McKenney wrote: > > 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? > > > Yes we can do it in run-time, i mean to detect context type, atomic or not. > But only for CONFIG_PREEMPT kernel. In case of !CONFIG_PREEMPT configuration > i do not see a straight forward way how to detect it. For example when caller > holds "spinlock". Therefore for such configuration we can just consider it > as atomic. But in reality it could be not in atomic. > > We need it for emergency/corner case and head-less objects. When we are run > of memory. So in this case we should attach the rcu_head dynamically and > queue the freed object to be processed later on, after GP. > > If atomic context use GFP_ATOMIC flag if not use GFP_KERNEL. It is better > to allocate with GFP_KERNEL flag(if possible) because it has much less > restrictions then GFP_ATOMIC one, i.e. GFP_KERNEL can sleep and wait until > the memory is reclaimed. > > But that is a corner case and i agree that it would be good to avoid of > such passing of extra info by the caller. > > Anyway i just share some extra info :) Hmm, I can't see at the moment how you can use GFP_KERNEL here for !CONFIG_PREEMPT kernels since that sleeps and you can't detect easily if you are in an RCU reader on !CONFIG_PREEMPT unless lockdep is turned on (in which case you could have checked lockdep's map). How about for !PREEMPT using first: GFP_NOWAIT and second GFP_ATOMIC if (NOWAIT fails)? And for PREEMPT, use GFP_KERNEL, then GFP_ATOMIC (if GFP_KERNEL fails). Thoughts? thanks, - Joel