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 43F9D41D644 for ; Wed, 15 Jul 2026 13:16:12 +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=1784121375; cv=none; b=Ynl1Nz3Sbj9phTHBaEagY5Eo9KhB7SRlOa9V88UbyFEjdilhRdiQ4DWunULgKCeaPTZEed6tJ/Chdf2YYUCNRjPsoq757vmZKTIDHQtzdqmWgxsiH6SucuoNsNIPGp92mQsqb0toQ2ZR0QFPHP/7c1upwS2xJOZPq8dkwwViKJs= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1784121375; c=relaxed/simple; bh=ivebf9Of5SKPwWnUyT6oQ2iS8Nn3PTIsZpuRk+R3Whw=; h=Content-Type:Mime-Version:Subject:From:In-Reply-To:Date:Cc: Message-Id:References:To; b=I3x3yYysWkwHBZS/6b++ZZ8K135u8LkmnByxkfHKg1LkMzGnh4pVv3oWgicoymcrRcldkth2hVkCkORiI/UalqvnPp7jaQ6qKboCotb9SyR7b8KfDBYLtNg9N11xsTTkKlQeD1Og+9KfZk9xma9/CX3C+DU2N2+feHbyNP6E+6w= 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=etgzWdvj; 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="etgzWdvj" Content-Type: text/plain; charset=utf-8 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1784121370; 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=srO3k5jRIT7j/Rini580mlp+14o/AyomRtsZhtG4hCI=; b=etgzWdvjNC1TMDtVDX+x9Q5g2sIxuMpIdwS50okxqRoHDutTxG5dTtClX8s+ENM5MZy+PZ qwhf+NGbyHv+Qkh3M07BQIC3pLkxfDd3KElnXMJj7tgajcptqov2RuWkSj0sMRJ3njeP9k MkpMp+Y15IHhczLN3FGWTbSYiqeq7+c= 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 \(3864.400.21\)) Subject: Re: [PATCH 01/17] mm/sparse: drop power-of-2 size requirement for struct mem_section X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Muchun Song In-Reply-To: <20260715103445.31b2f727@pumpkin> Date: Wed, 15 Jul 2026 21:15:45 +0800 Cc: Muchun Song , Andrew Morton , Oscar Salvador , David Hildenbrand , linux-mm@kvack.org, Mike Rapoport , Vlastimil Babka , Lorenzo Stoakes , Michal Hocko , linux-kernel@vger.kernel.org Content-Transfer-Encoding: quoted-printable Message-Id: <71E70E00-98A1-4769-A167-5F4CB59EC96B@linux.dev> References: <20260702093821.2740183-1-songmuchun@bytedance.com> <20260702093821.2740183-2-songmuchun@bytedance.com> <20260715103445.31b2f727@pumpkin> To: David Laight X-Migadu-Flow: FLOW_OUT > On Jul 15, 2026, at 17:34, David Laight = wrote: >=20 > On Thu, 2 Jul 2026 17:38:05 +0800 > Muchun Song wrote: >=20 >> struct mem_section is currently forced to a power-of-2 size so the >> section-to-root lookup can use a mask instead of a modulo. >>=20 >> That requirement adds configuration-dependent padding, especially = with >> CONFIG_PAGE_EXTENSION, just to preserve the lookup scheme. >>=20 >> Drop the constraint and use a plain modulo for the lookup instead. = The >> divisor is constant, so the generated code remains cheap while = avoiding >> the extra padding. It also removes an unnecessary layout constraint >> from the type. >=20 > This has a side effect of changing the size of the 'section' from > PAGE_SIZE to something 'a bit smaller' when CONFIG_PAGE_EXTENSION > is defined. > I don't think it actually matters, the allocation is done by: >=20 > static noinline struct mem_section __ref *sparse_index_alloc(int nid) > { > struct mem_section *section =3D NULL; > unsigned long array_size =3D SECTIONS_PER_ROOT * > sizeof(struct mem_section); >=20 > if (slab_is_available()) { > section =3D kzalloc_node(array_size, GFP_KERNEL, nid); > } else { > section =3D memblock_alloc_node(array_size, = SMP_CACHE_BYTES, > nid); >=20 > so the size might get rounded up to PAGE_SIZE anyway. I'm not sure I really understand what you mean. You might be asking = whether the reduction in the size of the `mem_section` structure does not = actually result in memory savings? If so, please let me explain clearly. As you = mentioned, the size of memory allocated each time here should be PAGE_SIZE. Before the modification, one page could hold 4096/32 =3D 128 `struct mem_section` = instances; with the modified code, the number of `struct mem_section` instances = that can fit is 4096/24 =3D 170. Therefore, the range of memory sections that a = PAGE_SIZE can cover has increased 32%. Please let me know if I didn=E2=80=99t get your point. >=20 > I also suspect that '% 24u' might be enough slower than '% 32u' to > generate a measurable performance drop. > (It doesn't matter whether you do '& 31' or '% 32u'.) David, I agree that % 24u is slower than % 32u =E2=80=94 the latter maps = to =E2=80=98and', while the former requires a multiply-shift sequence. However, since the divisor is = a constant, the compiler should uses the magic multiplier approach at -O2, which is = ~3-5 cycles instead of 1. So I think the per-lookup overhead is real but small. Thanks for your review. Muchun >=20 > David