From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-173.mta0.migadu.com (out-173.mta0.migadu.com [91.218.175.173]) (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 2F4E12857FA for ; Fri, 10 Apr 2026 03:08:01 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.173 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1775790484; cv=none; b=J/YgjamgJP4A4dDi6xgt4qHsKNHIGmkJG77uDXHWce06qT6W+j6SyhdIBhVZDQMZ+ag2JAarBhvYtpgSHy5lX6yTOwwE3vB1+GxJxwbZQs5phHYBb3nd2Ktig9h9JWFrp1WQsF4OH5TYSv9C7nSz46fQ9hbAMoUCtSK0/Geyugc= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1775790484; c=relaxed/simple; bh=SltmdB87AbP5vW239ksMso4SuJbdO8tKWTbTixszXWg=; h=Content-Type:Mime-Version:Subject:From:In-Reply-To:Date:Cc: Message-Id:References:To; b=TiMMOQqIy56iMlZdPTtAC/IiENISIg6sPlWhEw4zCzKhDyYhoHRMH8fg5MHveVSYkEW4hPnrr5A7scUyF+DsNV4NUO3EnfPsnXwT5icg9iRdzeoncdzMokuU/RnUFn2optUrcFcEnR0+8Ll/U2GMTPsqkcCFllrdAI3ZVk12d/U= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=Hcos8igs; arc=none smtp.client-ip=91.218.175.173 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="Hcos8igs" Content-Type: text/plain; charset=us-ascii DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1775790480; 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=UB14TpvH+wdBdvN5Rz/r4yIdPzKNG5cYRBgIcKHvz00=; b=Hcos8igsEf2YlonhWWBpoVgZxButTf3+1qNZdzSeV/B7LqiLUZgGmViPvJoNBXHwD7jkM1 b4awj4beCI/zjqe9HGCZk0FQSPEA0FEa6c+KnA2hWRUotgaUb4hmPt9kcp6CsLZ+GV/r96 5ZQQyU0fHqVOh7Qy7x33io6fxIETBto= Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 (Mac OS X Mail 16.0 \(3864.500.181\)) Subject: Re: [RFC PATCH] mm/sparse: remove sparse_buffer X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Muchun Song In-Reply-To: Date: Fri, 10 Apr 2026 11:07:21 +0800 Cc: Muchun Song , Andrew Morton , yinghai@kernel.org, Lorenzo Stoakes , "Liam R. Howlett" , Vlastimil Babka , Suren Baghdasaryan , Michal Hocko , linux-mm@kvack.org, linux-kernel@vger.kernel.org Content-Transfer-Encoding: quoted-printable Message-Id: References: <20260407083951.2823915-1-songmuchun@bytedance.com> <70EF8E41-31A2-4B43-BABE-7218FD5F7271@linux.dev> To: Mike Rapoport , "David Hildenbrand (Arm)" X-Migadu-Flow: FLOW_OUT > On Apr 9, 2026, at 23:10, Mike Rapoport wrote: >=20 > Hi, >=20 > On Thu, Apr 09, 2026 at 02:29:38PM +0200, David Hildenbrand (Arm) = wrote: >> On 4/9/26 13:40, Muchun Song wrote: >>>=20 >>>=20 >>>> On Apr 8, 2026, at 21:40, David Hildenbrand (Arm) = wrote: >>>>=20 >>>> On 4/7/26 10:39, Muchun Song wrote: >>>>> The sparse_buffer was originally introduced in commit 9bdac9142407 >>>>> ("sparsemem: Put mem map for one node together.") to allocate a >>>>> contiguous block of memory for all memmaps of a NUMA node. >>>>>=20 >>>>> However, the original commit message did not clearly state the = actual >>>>> benefits or the necessity of keeping all memmap areas strictly >>>>> contiguous for a given node. >>>>=20 >>>> We don't want the memmap to be scattered around, given that it is = one of >>>> the biggest allocations during boot. >>>>=20 >>>> It's related to not turning too many memory blocks/sections >>>> un-offlinable I think. >>>>=20 >>>> I always imagined that memblock would still keep these allocations = close >>>> to each other. Can you verify if that is indeed true? >>>=20 >>> You raised a very interesting point about whether memblock keeps >>> these allocations close to each other. I've done a thorough test >>> on a 16GB VM by printing the actual physical allocations. >=20 > memblock always allocates in order, so if there are no other memblock > allocations between the calls to memmap_alloc(), all these allocations = will > be together and they all will be coalesced to a single region in > memblock.reserved. >=20 >>> I enabled the existing debug logs in arch/x86/mm/init_64.c to >>> trace the vmemmap_set_pmd allocations. Here is what really happens: >>>=20 >>> When using vmemmap_alloc_block without sparse_buffer, the >>> memblock allocator allocates 2MB chunks. Because memblock >>> allocates top-down by default, the physical allocations look >>> like this: >>>=20 >>> [ffe6475cc0000000-ffe6475cc01fffff] PMD -> = [ff3cb082bfc00000-ff3cb082bfdfffff] on node 0 >>> [ffe6475cc0200000-ffe6475cc03fffff] PMD -> = [ff3cb082bfa00000-ff3cb082bfbfffff] on node 0 >>> [ffe6475cc0400000-ffe6475cc05fffff] PMD -> = [ff3cb082bf800000-ff3cb082bf9fffff] on node 0 >=20 > ... >=20 >>> Notice that the physical chunks are strictly adjacent to each >>> other, but in descending order! >>>=20 >>> So, they are NOT "scattered around" the whole node randomly. >>> Instead, they are packed densely back-to-back in a single >>> contiguous physical range (just mapped top-down in 2MB pieces). >>>=20 >>> Because they are packed tightly together within the same >>> contiguous physical memory range, they will at most consume or >>> pollute the exact same number of memory blocks as a single >>> contiguous allocation (like sparse_buffer did). Therefore, this >>> will NOT turn additional memory blocks/sections into an >>> "un-offlinable" state. >>>=20 >>> It seems we can safely remove the sparse buffer preallocation >>> mechanism, don't you think? >>=20 >> Yes, what I suspected. Is there a performance implication when doing >> many individual memmap_alloc(), for example, on a larger system with >> many sections? >=20 > memmap_alloc() will be slower than sparse_buffer_alloc(), allocating = from > memblock is more involved that sparse_buffer_alloc(), but without > measurements it's hard to tell how much it'll affect overall = sparse_init(). I ran a test on a 256GB VM, and the results are as follows: With patch: 741,292 ns Without patch: 199,555 ns The performance is approximately 3.7x slower with the patch applied. Thanks, Muchun >=20 >> --=20 >> Cheers, >>=20 >> David >=20 > --=20 > Sincerely yours, > Mike.