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 lists.ozlabs.org (lists.ozlabs.org [112.213.38.117]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id A2042C636BD for ; Mon, 23 Jan 2023 18:25:11 +0000 (UTC) Received: from boromir.ozlabs.org (localhost [IPv6:::1]) by lists.ozlabs.org (Postfix) with ESMTP id 4P0z6Z00Hbz3cFr for ; Tue, 24 Jan 2023 05:25:09 +1100 (AEDT) Authentication-Results: lists.ozlabs.org; dkim=fail reason="signature verification failed" (2048-bit key; secure) header.d=infradead.org header.i=@infradead.org header.a=rsa-sha256 header.s=casper.20170209 header.b=GTYwYH7n; dkim-atps=neutral Authentication-Results: lists.ozlabs.org; spf=none (no SPF record) smtp.mailfrom=infradead.org (client-ip=2001:8b0:10b:1236::1; helo=casper.infradead.org; envelope-from=willy@infradead.org; receiver=) Authentication-Results: lists.ozlabs.org; dkim=pass (2048-bit key; secure) header.d=infradead.org header.i=@infradead.org header.a=rsa-sha256 header.s=casper.20170209 header.b=GTYwYH7n; dkim-atps=neutral Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id 4P0z5T3T8Gz2ynD for ; Tue, 24 Jan 2023 05:24:12 +1100 (AEDT) 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=B+yzq/Z0e8jumLGKSlhRVExf8hQcAkabvnjcTYI9KiY=; b=GTYwYH7nNq+4sMY3Vk3QgXskdG 2L0ANeqflfK/jWWxB3yA/UDdJse/cePbqp6JZ8LYI+ze4uQ2DiYVBbpxRZqbnNpgx7+PiwTdZAWQA SPQ8VEgtJD+rLax4CdxZofnTxPHlBnCYqKh2woEwBT70dhcwKjgs/lXEeBXNZqJJqrbREEMkZCIOC eUu/2OzjQqN0hU4Otc/g+SXPiM42qqmVRB3hjb21IaWZjIZNTf9WW/47lCPpArGf0Iosgqs1mCKFf 0wABKYADlN5xm08SG1OokKkWTXE5xfSYdcp7g2ktegETsutXNqTGIlbKeQljQR19PgiMGAq4KA1NB eZTb8cWA==; Received: from willy by casper.infradead.org with local (Exim 4.94.2 #2 (Red Hat Linux)) id 1pK1TI-004R6r-TB; Mon, 23 Jan 2023 18:23:09 +0000 Date: Mon, 23 Jan 2023 18:23:08 +0000 From: Matthew Wilcox To: Suren Baghdasaryan 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: X-BeenThere: linuxppc-dev@lists.ozlabs.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: michel@lespinasse.org, joelaf@google.com, songliubraving@fb.com, Michal Hocko , leewalsh@google.com, david@redhat.com, peterz@infradead.org, bigeasy@linutronix.de, peterx@redhat.com, dhowells@redhat.com, linux-mm@kvack.org, edumazet@google.com, jglisse@google.com, punit.agrawal@bytedance.com, arjunroy@google.com, dave@stgolabs.net, minchan@google.com, x86@kernel.org, hughd@google.com, gurua@google.com, laurent.dufour@fr.ibm.com, linux-arm-kernel@lists.infradead.org, rientjes@google.com, axelrasmussen@google.com, kernel-team@android.com, soheil@google.com, paulmck@kernel.org, jannh@google.com, "Liam R. Howlett" , shakeelb@google.com, luto@kernel.org, gthelen@google.com, ldufour@linux.ibm.com, vbabka@suse.cz, posk@google.com, lstoakes@gmail.com, peterjung1337@gmail.com, linuxppc-dev@lists.ozlabs.org, kent.overstreet@linux.dev, hughlynch@google.com, linux-kernel@vger.kernel.org, hannes@cmpxchg.org, akpm@linux-foundation.org, tatashin@google. com, mgorman@techsingularity.net Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" On Mon, Jan 23, 2023 at 09:46:20AM -0800, Suren Baghdasaryan wrote: > On Mon, Jan 23, 2023 at 9:16 AM Michal Hocko wrote: > > > > 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. > > Ok, I understand your point. > > > > > 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. > > 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. Having all this extra code to solve two problems badly is a very poor choice. We have two distinct problems, each of which has a simple, efficient solution.