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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 87F90C05027 for ; Mon, 23 Jan 2023 19:31:48 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231540AbjAWTbr (ORCPT ); Mon, 23 Jan 2023 14:31:47 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:39944 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230346AbjAWTbp (ORCPT ); Mon, 23 Jan 2023 14:31:45 -0500 Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 515B9A254 for ; Mon, 23 Jan 2023 11:31:44 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=m/podwHds7JxQLCbcR8/xZsYC7Zym4+9Li/7t1BsGAU=; b=EzNI5XQFHCKDI4RmxwK5H85qS0 6UAEG1u2YhqEZ87tlBlzCqJnVMUhWcWNWEIKOR/n5FWz2FECNBJSBX0T8LiwDOfVkvDYHlmiaPbxA 4I/NbsDKV69v/QZwp9chVKa6zvOlWvAcU51xz5THZAKzOW/kh/weKydSwnigbjDrrWctKg42H6QPk b1fzz6huv7hyG/nqC1cXlZD4V7JGafLOwoac5ND3ZEtCCpvvv3o0wmwA1NBTWfWCQfT97un1xUw+c wkk8l8Ds+GtkptnTWKsm5oeLn7+gHZ3CXmNzIXOxvw9rV/pPUz82oqXeKmYZVB9H5pLDaFgnSfOVz eCAQhGqg==; Received: from willy by casper.infradead.org with local (Exim 4.94.2 #2 (Red Hat Linux)) id 1pK2Wh-004TK7-Jr; Mon, 23 Jan 2023 19:30:43 +0000 Date: Mon, 23 Jan 2023 19:30:43 +0000 From: Matthew Wilcox To: Michal Hocko Cc: Suren Baghdasaryan , "Liam R. Howlett" , akpm@linux-foundation.org, michel@lespinasse.org, jglisse@google.com, vbabka@suse.cz, hannes@cmpxchg.org, mgorman@techsingularity.net, dave@stgolabs.net, peterz@infradead.org, ldufour@linux.ibm.com, laurent.dufour@fr.ibm.com, paulmck@kernel.org, luto@kernel.org, songliubraving@fb.com, peterx@redhat.com, david@redhat.com, dhowells@redhat.com, hughd@google.com, bigeasy@linutronix.de, kent.overstreet@linux.dev, punit.agrawal@bytedance.com, lstoakes@gmail.com, peterjung1337@gmail.com, rientjes@google.com, axelrasmussen@google.com, joelaf@google.com, minchan@google.com, jannh@google.com, shakeelb@google.com, tatashin@google.com, edumazet@google.com, gthelen@google.com, gurua@google.com, arjunroy@google.com, soheil@google.com, hughlynch@google.com, leewalsh@google.com, posk@google.com, linux-mm@kvack.org, linux-arm-kernel@lists.infradead.org, linuxppc-dev@lists.ozlabs.org, x86@kernel.org, linux-kernel@vger.kernel.org, kernel-team@android.com Subject: Re: [PATCH 39/41] kernel/fork: throttle call_rcu() calls in vm_area_free Message-ID: References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Jan 23, 2023 at 08:18:37PM +0100, Michal Hocko wrote: > On Mon 23-01-23 18:23:08, Matthew Wilcox wrote: > > On Mon, Jan 23, 2023 at 09:46:20AM -0800, Suren Baghdasaryan wrote: > [...] > > > Yes, batching the vmas into a list and draining it in remove_mt() and > > > exit_mmap() as you suggested makes sense to me and is quite simple. > > > Let's do that if nobody has objections. > > > > I object. We *know* nobody has a reference to any of the VMAs because > > you have to have a refcount on the mm before you can get a reference > > to a VMA. If Michal is saying that somebody could do: > > > > mmget(mm); > > vma = find_vma(mm); > > lock_vma(vma); > > mmput(mm); > > vma->a = b; > > unlock_vma(mm, vma); > > > > then that's something we'd catch in review -- you obviously can't use > > the mm after you've dropped your reference to it. > > I am not claiming this is possible now. I do not think we want to have > something like that in the future either but that is really hard to > envision. I am claiming that it is subtle and potentially error prone to > have two different ways of mass vma freeing wrt. locking. Also, don't we > have a very similar situation during last munmaps? We shouldn't have two ways of mass VMA freeing. Nobody's suggesting that. There are two cases; there's munmap(), which typically frees a single VMA (yes, theoretically, you can free hundreds of VMAs with a single call which spans multiple VMAs, but in practice that doesn't happen), and there's exit_mmap() which happens on exec() and exit(). For the munmap() case, just RCU-free each one individually. For the exit_mmap() case, there's no need to use RCU because nobody should still have a VMA pointer after calling mmdrop() [1] [1] Sorry, the above example should have been mmgrab()/mmdrop(), not mmget()/mmput(); you're not allowed to look at the VMA list with an mmget(), you need to have grabbed.