From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (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 139AC32861E; Tue, 21 Jul 2026 11:06:11 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1784631973; cv=none; b=bKZRMrHxUbwfEuEe1nTspviZYJvn7Y2nqNp6ZgIjgKc/2hKC6timiSrlFlywighfuoHjDmPStN0OrPVCgI+jFmGHDesE+oJ3wZMJSLIuFU0DZuL3PTZvDA0qnDnQinGsDfWXNCHBBHBIW2iQspj2sLSfxj4lX/2cR4bdRtuvnYs= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1784631973; c=relaxed/simple; bh=T1mTMz9P94jNWmYvNRr3q4DbGHNst55KgRnteZzb2JQ=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=lQJvsMzSSyJWZpqzCsW2na39Vld4x80qkj52ytlUEugJmxoQWNoCBKvljI998egyU2I5htoyxiH3UEz7dU2McCZL9UEpwTBsLdr0eHyQ+wOWbRRf7zM+gChNG0Fxbp+wm6UmpzVA88CoESEJjsBKhzHYOFDzjnTDF6urzjfEKo0= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=TM9BOLXU; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="TM9BOLXU" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 0057C1F000E9; Tue, 21 Jul 2026 11:06:05 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1784631971; bh=LaVZvxNKUlS8V5YFdO52ViXqG34PCKTpK+eQer0gTnI=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=TM9BOLXUbZv8axude7fnH0r8XrDLOI8/x4LVPjAPG823eqYIStGZxvrdJkC9sdotq SBHMq+1to+F+pV2rXsEE+y/SL3hR+P5QfvhMGN55BSQHAgoUgGOVASH4J0A4s6PdB6 +sTBRPbfwA8vHdjNpB/t5TNmnlbbLcCJ9iamgnXqqt9OTiNFkPedPs3MFjCSNLFplk SYRByFhF/e1L0rg+ADBPrH3PVf1MIN/HvJX4BZKbzLG9u3b9/cD81fCxJgeXrQQv5H TxCewVYgMAegtT6YCaXeCw7KM3G1Ge64txCm6X7Q2apiTlaMdPv4IYHcFzvjCE73kF jZoH0SKUa8C5g== Message-ID: Date: Tue, 21 Jul 2026 13:06:04 +0200 Precedence: bulk X-Mailing-List: linux-rt-devel@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH slab/for-next v4 6/8] mm/slab: introduce struct kvfree_rcu_head for kvfree_rcu batching Content-Language: en-US To: "Harry Yoo (Oracle)" , Andrew Morton , Hao Li , Christoph Lameter , David Rientjes , Roman Gushchin , Alexei Starovoitov , Andrii Nakryiko , Puranjay Mohan , Amery Hung , Sebastian Andrzej Siewior , Clark Williams , Steven Rostedt , "Paul E. McKenney" , Frederic Weisbecker , Neeraj Upadhyay , Joel Fernandes , Josh Triplett , Boqun Feng , Uladzislau Rezki , Mathieu Desnoyers , Lai Jiangshan , Zqiang , Pedro Falcato , Suren Baghdasaryan , Shengming Hu Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-rt-devel@lists.linux.dev, rcu@vger.kernel.org, bpf@vger.kernel.org References: <20260720-kfree_rcu_nolock-v4-0-964e03c41a4e@kernel.org> <20260720-kfree_rcu_nolock-v4-6-964e03c41a4e@kernel.org> From: "Vlastimil Babka (SUSE)" Autocrypt: addr=vbabka@kernel.org; keydata= xsFNBFZdmxYBEADsw/SiUSjB0dM+vSh95UkgcHjzEVBlby/Fg+g42O7LAEkCYXi/vvq31JTB KxRWDHX0R2tgpFDXHnzZcQywawu8eSq0LxzxFNYMvtB7sV1pxYwej2qx9B75qW2plBs+7+YB 87tMFA+u+L4Z5xAzIimfLD5EKC56kJ1CsXlM8S/LHcmdD9Ctkn3trYDNnat0eoAcfPIP2OZ+ 9oe9IF/R28zmh0ifLXyJQQz5ofdj4bPf8ecEW0rhcqHfTD8k4yK0xxt3xW+6Exqp9n9bydiy tcSAw/TahjW6yrA+6JhSBv1v2tIm+itQc073zjSX8OFL51qQVzRFr7H2UQG33lw2QrvHRXqD Ot7ViKam7v0Ho9wEWiQOOZlHItOOXFphWb2yq3nzrKe45oWoSgkxKb97MVsQ+q2SYjJRBBH4 8qKhphADYxkIP6yut/eaj9ImvRUZZRi0DTc8xfnvHGTjKbJzC2xpFcY0DQbZzuwsIZ8OPJCc LM4S7mT25NE5kUTG/TKQCk922vRdGVMoLA7dIQrgXnRXtyT61sg8PG4wcfOnuWf8577aXP1x 6mzw3/jh3F+oSBHb/GcLC7mvWreJifUL2gEdssGfXhGWBo6zLS3qhgtwjay0Jl+kza1lo+Cv BB2T79D4WGdDuVa4eOrQ02TxqGN7G0Biz5ZLRSFzQSQwLn8fbwARAQABzSNWbGFzdGltaWwg QmFia2EgPHZiYWJrYUBrZXJuZWwub3JnPsLBsAQTAQoAWhYhBKlA1DSZLC6OmRA9UCJPp+fM gqZkBQJqFFy6GxSAAAAAAAQADm1hbnUyLDIuNSsxLjEyLDIsMgIbAwUJGtCBUAULCQgHAwUV CgkICwUWAgMBAAIeBQIXgAAKCRAiT6fnzIKmZJIUEADFx/tREzUImHrEwVHeSvDFmA7tJysI UVrlvrM09E7GIuzphzv7jYmo8n3ANpCczLEVr4G0syYQdTigaZgv3+FQDIIzhKih1IHhu1Ei XHlywNWKnQxxQEUNi5Mwx43wQz5XVw9F1A7gtKBKNtfogO511hAbrzagrYajyQacEJ/+sfhZ 9Da8ltHIXD8pcYaHUfQgEusCgmEd9+KrUwrTbckFKmYq5chuE6yJ4J0EmWknL096jIE6CnzF FRslQ3B1UKDjxVsm1ZHfir5NeWszLkTvGFsddFaWTgh8UycESG6VQzKXjjewXu2pG7YQYRpj QKm1W5X2TkwWkXRBZTmfmbhxIUMh3+zf5wQ463rSmDN/8v81tdqBtAW6rH/kzg1GvkaTHXn0 507yEHFzBksk2viAuIxxr7km8+/KARYLIdGtx30EG8cKzAUZOK6WqxtNCsXUJNrVE8CWrCaD icoNu7Fs1c5hmPHdSTnU48ce67449DdnO4neLSNhRiGlMHJgfJUmgrxu/hcYeOZ3haWmEQ2w uW1Mh01OHi8QZHCEyAbABrPs9GUgccc/4eYXX9hIgxfSkYzn8f+8NuIFPWl/0uTvjgqU29FQ SbzOLxHq9439Ox40G5mS5eZXRGxITYR+6TXvRGI6P/264jvflnr/pDGUttaikU+0W+1uxgKH cmYbEc7ATQRbGTU1AQgAn0H6UrFiWcovkh6EXVcl+SeqyO6JHOPm+e9Wu0Vw+VIUvXZVUVVQ La1PQDUi6j00ChlcR66g9/V0sPIcSutacPKfdKYOBvzd4rlhL8rfrdEsQw5ApZxrA8kYZVMh FmBRKAa6wos25moTlMKpCWzTH84+WO5+ziCTsTUZASAToz3RdunTD+vQcHj0GqNTPAHK63sf bAB2I0BslZkXkY1RLb/YhuA6E7JyEd2pilZOrIuBGl/5q2qSakgnAVFWFBR/DO27JuAksYnq +aH8vI0xGvwn75KqSk4UzAkDzWSmO4ZHuahKtQgZNsMYV+PGayRBX9b9zbldzopoLBdqHc4n jQARAQABwsF8BBgBCgAmAhsMFiEEqUDUNJksLo6ZED1QIk+n58yCpmQFAmfIHFQFCRYU6J8A CgkQIk+n58yCpmS2PA//bqN1LfcotmArgElsa+0EGZSQlYgK48pm8WAeTXTngudP9IJ4SuKY HR5RNjHcBeqN+Me0zxRqYzRb8nGanHEkDyf4Im8DQM8d6vbyU+FcPmG4skud4kgS1zMHnlVd SXfSIwKC/hKgdHG8aBV7545Lz9X6Iohea+94wneD0aw/hqF+QWewGZhWJriWAZtvEkzNjQOi 4U9F/trLten/x7bpphDSnDMKJtITbtzATT1Dq7o7VpIUK1nCTQALMuMjKCdi8OdU/+V+R3O4 0PXWvX8qrvqYapVbZ+9KqT74FsuB0Ya9uXwgBF2Q6cRuETZk5vqaqKxzqoQZCO8AOz/58j6O 2RHNy/mZEN+7tJ5Tsq42zVJ4jxsT8b9YplavCMsnBgDeRWhcbYhCyttoL7nYISyWg4kQYZ/P wIV3OuNv2f8iKYsxNsRuClOAF82+gvqOy1/1pprFjy8uo2pkoOrb63aOP3vO5VHnRKgra6dq NcaZ+c6J4H+nEJGi2SkHAUJz5oBzuThvPudLvPA/SK8sKoM01IRxSihev/S/5WLazXB1PGem OCbvzC1IjWJJraxiDJ5IygokapUa2RP7+WBR22skQ3SSl6G107QgWKSyTOGWEaRmV53vxQLV jXuCmzSSasTL60zq5yGrT4/DYQVSNEUiUbG4pYekxJujNeEDkUlky0Y= In-Reply-To: <20260720-kfree_rcu_nolock-v4-6-964e03c41a4e@kernel.org> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 7/20/26 14:44, Harry Yoo (Oracle) wrote: > rcu_head is overkill for kvfree_rcu() because the callback > function is always either kfree(), vfree(), or free_large_kmalloc(), > and thus there is no need for a function pointer. > > kvfree_rcu batching reuses the field to store the start address > of an object, however, this is not strictly needed because we can > calculate the start address in the slowpath. For the purpose of > kvfree_rcu batching, it is sufficient to implement a linked list using > a single pointer. > > Introduce a new struct called kvfree_rcu_head (the name was suggested > by Vlastimil Babka), which is similar to rcu_head but is only a single > pointer to build a linked list, without a function pointer, when > CONFIG_KVFREE_RCU_BATCHED=y. > > When kvfree_rcu is not batched, kvfree_rcu_head is the same size > as rcu_head. Note that shrinking struct kvfree_rcu_head on > CONFIG_KVFREE_RCU_BATCHED=n kernels would inevitably require additional > complexity and also some sort of batching (which defeats the purpose of > the config option) because it cannot fall back to call_rcu(). > > For now there are no user-visible changes to the API. k[v]free_rcu() > simply casts rcu_head to kvfree_rcu_head. While this does not affect > the API, it allows kfree_rcu_nolock() to reuse kvfree_rcu batching > as a fallback when trylock or sheaf allocation fails. > > Stop storing the object pointer in rcu_head.func and instead calculate > the object's start address in kvfree_rcu_list(). Factor out the existing > logic to calculate the start address from kvfree_rcu_cb() to > kvmalloc_obj_start_addr(). > > Signed-off-by: Harry Yoo (Oracle) Nice. Reviewed-by: Vlastimil Babka (SUSE) Nit: > --- a/mm/slab.h > +++ b/mm/slab.h > @@ -351,6 +351,33 @@ static inline int objs_per_slab(const struct kmem_cache *cache, > return slab->objects; > } > > +/* kvfree_rcu_head offset can be only less than page size */ > +static inline void *kvmalloc_obj_start_addr(void *head) > +{ > + void *obj = head; > + > + if (unlikely(is_vmalloc_addr(obj))) { > + obj = (void *) PAGE_ALIGN_DOWN((unsigned long)obj); > + } else { > + struct page *page = virt_to_page(obj); > + struct slab *slab = page_slab(page); Can use virt_to_slab(). > + > + if (!slab) { > + obj = (void *) PAGE_ALIGN_DOWN((unsigned long)obj); > + } else if (is_kfence_address(obj)) { > + obj = kfence_object_start(obj); > + } else { > + struct kmem_cache *s = slab->slab_cache; > + unsigned int idx = __obj_to_index(s, slab_address(slab), obj); > + > + obj = slab_address(slab) + s->size * idx; > + obj = fixup_red_left(s, obj); > + } > + } > + > + return obj; > +} > + > /* > * State of the slab allocator. > *