From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 335ED157A41; Mon, 17 Jun 2024 17:04:45 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1718643886; cv=none; b=qkcsgBQyW0rexLKdZ78TNvWDCma4os597paEArZokglqfi33DymwRx6wfBOI9+7hvgisLvysTBdedq0DzoQFBldOg72vuP/kDK2qUgPXN7A4psdTAQ3+sPrP/fO+LlECCDMcKd9cw0an5bE60LcjeuRegk6Il52uFLHOMIzYM5k= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1718643886; c=relaxed/simple; bh=aTo9WYNfxMDXZ7XIaowP99DTt7UkkCN5iUpRoKtIZ2g=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=Dsl3/YOPNyS6QHfnbNS/r18sXYQod2vZ2/kpzY4mOIvF7ZL4rdhZIT7/xPmHTz7oyckn9poapuPIsDChPy6C3A6WgDxGy6nLvgMV8anFIgBlBMJwawmm6kZFdO+J1xhAPTk23bBVWsv16bMCf5NqDPb+XZrqN3Rb5xz9G7JvvR8= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=zx2c4.com header.i=@zx2c4.com header.b=EB2EkGMp; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=zx2c4.com header.i=@zx2c4.com header.b="EB2EkGMp" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 552BDC2BD10; Mon, 17 Jun 2024 17:04:43 +0000 (UTC) Authentication-Results: smtp.kernel.org; dkim=pass (1024-bit key) header.d=zx2c4.com header.i=@zx2c4.com header.b="EB2EkGMp" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=zx2c4.com; s=20210105; t=1718643881; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=ARICZd145/mpU/6/C4ChaIGt7llEKLdE0hg6cGdHWdo=; b=EB2EkGMp1iSSR/ZoBjTvLsRipbJ/3GyF1pF67bPbTJpZpZViXHWNSdGe3Wp0tZUf6UwcS3 PPQY3tLkcp6pOnHOdpKJJIurtRSa2uJJxmso4eaZw7W4rybzLkYu5iwhPsWnqpDdk5ApWJ +Lmz+dK5Qx+05M2/zNiFzoH33PNzOFM= Received: by mail.zx2c4.com (ZX2C4 Mail Server) with ESMTPSA id 81f95149 (TLSv1.3:TLS_AES_256_GCM_SHA384:256:NO); Mon, 17 Jun 2024 17:04:41 +0000 (UTC) Date: Mon, 17 Jun 2024 19:04:34 +0200 From: "Jason A. Donenfeld" To: Vlastimil Babka Cc: Uladzislau Rezki , "Paul E. McKenney" , Jakub Kicinski , Julia Lawall , linux-block@vger.kernel.org, kernel-janitors@vger.kernel.org, bridge@lists.linux.dev, linux-trace-kernel@vger.kernel.org, Mathieu Desnoyers , kvm@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, "Naveen N. Rao" , Christophe Leroy , Nicholas Piggin , netdev@vger.kernel.org, wireguard@lists.zx2c4.com, linux-kernel@vger.kernel.org, ecryptfs@vger.kernel.org, Neil Brown , Olga Kornievskaia , Dai Ngo , Tom Talpey , linux-nfs@vger.kernel.org, linux-can@vger.kernel.org, Lai Jiangshan , netfilter-devel@vger.kernel.org, coreteam@netfilter.org Subject: Re: [PATCH 00/14] replace call_rcu by kfree_rcu for simple kmem_cache_free callback Message-ID: References: Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: On Mon, Jun 17, 2024 at 06:38:52PM +0200, Vlastimil Babka wrote: > On 6/17/24 6:33 PM, Jason A. Donenfeld wrote: > > On Mon, Jun 17, 2024 at 6:30 PM Uladzislau Rezki wrote: > >> Here if an "err" is less then "0" means there are still objects > >> whereas "is_destroyed" is set to "true" which is not correlated > >> with a comment: > >> > >> "Destruction happens when no objects" > > > > The comment is just poorly written. But the logic of the code is right. > > > >> > >> > out_unlock: > >> > mutex_unlock(&slab_mutex); > >> > cpus_read_unlock(); > >> > diff --git a/mm/slub.c b/mm/slub.c > >> > index 1373ac365a46..7db8fe90a323 100644 > >> > --- a/mm/slub.c > >> > +++ b/mm/slub.c > >> > @@ -4510,6 +4510,8 @@ void kmem_cache_free(struct kmem_cache *s, void *x) > >> > return; > >> > trace_kmem_cache_free(_RET_IP_, x, s); > >> > slab_free(s, virt_to_slab(x), x, _RET_IP_); > >> > + if (s->is_destroyed) > >> > + kmem_cache_destroy(s); > >> > } > >> > EXPORT_SYMBOL(kmem_cache_free); > >> > > >> > @@ -5342,9 +5344,6 @@ static void free_partial(struct kmem_cache *s, struct kmem_cache_node *n) > >> > if (!slab->inuse) { > >> > remove_partial(n, slab); > >> > list_add(&slab->slab_list, &discard); > >> > - } else { > >> > - list_slab_objects(s, slab, > >> > - "Objects remaining in %s on __kmem_cache_shutdown()"); > >> > } > >> > } > >> > spin_unlock_irq(&n->list_lock); > >> > > >> Anyway it looks like it was not welcome to do it in the kmem_cache_free() > >> function due to performance reason. > > > > "was not welcome" - Vlastimil mentioned *potential* performance > > concerns before I posted this. I suspect he might have a different > > view now, maybe? > > > > Vlastimil, this is just checking a boolean (which could be > > unlikely()'d), which should have pretty minimal overhead. Is that > > alright with you? > > Well I doubt we can just set and check it without any barriers? The > completion of the last pending kfree_rcu() might race with > kmem_cache_destroy() in a way that will leave the cache there forever, no? > And once we add barriers it becomes a perf issue? Hm, yea you might be right about barriers being required. But actually, might this point toward a larger problem with no matter what approach, polling or event, is chosen? If the current rule is that kmem_cache_free() must never race with kmem_cache_destroy(), because users have always made diligent use of call_rcu()/rcu_barrier() and such, but now we're going to let those race with each other - either by my thing above or by polling - so we're potentially going to get in trouble and need some barriers anyway. I think? Jason