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 CA5E5C05027 for ; Mon, 23 Jan 2023 17:17:14 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233549AbjAWRRN (ORCPT ); Mon, 23 Jan 2023 12:17:13 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:51794 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233620AbjAWRQ7 (ORCPT ); Mon, 23 Jan 2023 12:16:59 -0500 Received: from smtp-out1.suse.de (smtp-out1.suse.de [IPv6:2001:67c:2178:6::1c]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 91573302A2 for ; Mon, 23 Jan 2023 09:16:12 -0800 (PST) Received: from imap2.suse-dmz.suse.de (imap2.suse-dmz.suse.de [192.168.254.74]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (P-521) server-digest SHA512) (No client certificate requested) by smtp-out1.suse.de (Postfix) with ESMTPS id C3D803368D; Mon, 23 Jan 2023 17:16:09 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1674494169; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=E3QyA5tV4ij54OKFg/BSxirkicwOIIF6CWUxZDpWF10=; b=AtlDpGDrQS1l2ngKwtZOdy5knRnagGprgIgeJGZtYrGHv/G2od26NO/Jhh/7oIBqPIt+AA XefbkiDdmsnw/lJQV+n5YOC0LH4k7estyyE+NkwdWuEjSwEqN7BvYH1bKZmG+856FRNra+ UPnPGPgvws6jda5Y10QPR9iVNdL1XGI= Received: from imap2.suse-dmz.suse.de (imap2.suse-dmz.suse.de [192.168.254.74]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (P-521) server-digest SHA512) (No client certificate requested) by imap2.suse-dmz.suse.de (Postfix) with ESMTPS id 99BA61357F; Mon, 23 Jan 2023 17:16:09 +0000 (UTC) Received: from dovecot-director2.suse.de ([192.168.254.65]) by imap2.suse-dmz.suse.de with ESMTPSA id pqYEJdnAzmNEFQAAMHmgww (envelope-from ); Mon, 23 Jan 2023 17:16:09 +0000 Date: Mon, 23 Jan 2023 18:16:08 +0100 From: Michal Hocko To: Suren Baghdasaryan Cc: Matthew Wilcox , "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: <20230120170815.yuylbs27r6xcjpq5@revolver> 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 23-01-23 09:07:34, Suren Baghdasaryan wrote: > On Mon, Jan 23, 2023 at 8:55 AM Michal Hocko wrote: > > > > On Mon 23-01-23 08:22:53, Suren Baghdasaryan wrote: > > > On Mon, Jan 23, 2023 at 1:56 AM Michal Hocko wrote: > > > > > > > > On Fri 20-01-23 09:50:01, Suren Baghdasaryan wrote: > > > > > On Fri, Jan 20, 2023 at 9:32 AM Matthew Wilcox wrote: > > > > [...] > > > > > > The page fault handler (or whatever other reader -- ptrace, proc, etc) > > > > > > should have a refcount on the mm_struct, so we can't be in this path > > > > > > trying to free VMAs. Right? > > > > > > > > > > Hmm. That sounds right. I checked process_mrelease() as well, which > > > > > operated on mm with only mmgrab()+mmap_read_lock() but it only unmaps > > > > > VMAs without freeing them, so we are still good. Michal, do you agree > > > > > this is ok? > > > > > > > > Don't we need RCU procetions for the vma life time assurance? Jann has > > > > already shown how rwsem is not safe wrt to unlock and free without RCU. > > > > > > Jann's case requires a thread freeing the VMA to be blocked on vma > > > write lock waiting for the vma real lock to be released by a page > > > fault handler. However exit_mmap() means mm->mm_users==0, which in > > > turn suggests that there are no racing page fault handlers and no new > > > page fault handlers will appear. Is that a correct assumption? If so, > > > then races with page fault handlers can't happen while in exit_mmap(). > > > Any other path (other than page fault handlers), accesses vma->lock > > > under protection of mmap_lock (for read or write, does not matter). > > > One exception is when we operate on an isolated VMA, then we don't > > > need mmap_lock protection, but exit_mmap() does not deal with isolated > > > VMAs, so out of scope here. exit_mmap() frees vm_area_structs under > > > protection of mmap_lock in write mode, so races with anything other > > > than page fault handler should be safe as they are today. > > > > I do not see you talking about #PF (RCU + vma read lock protected) with > > munmap. It is my understanding that the latter will synchronize over per > > vma lock (along with mmap_lock exclusive locking). But then we are back > > to the lifetime guarantees, or do I miss anything. > > munmap() or any VMA-freeing operation other than exit_mmap() will free > using call_rcu(), as implemented today. The suggestion is to free VMAs > directly, without RCU grace period only when done from exit_mmap(). OK, I have clearly missed that. This makes more sense but it also adds some more complexity and assumptions - a harder to maintain code in the end. Whoever wants to touch this scheme in future would have to re-evaluate all of them. So, I would just avoid that special casing if that is feasible. Dealing with the flood of call_rcu during exit_mmap is a trivial thing to deal with as proposed elsewhere (just batch all of them in a single run). This will surely add some more code but at least the locking would consistent. -- Michal Hocko SUSE Labs