From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-182.mta0.migadu.com (out-182.mta0.migadu.com [91.218.175.182]) (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 8A7B335C180 for ; Wed, 1 Apr 2026 02:47:56 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.182 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1775011687; cv=none; b=P+LIZ79L6zISr0r2rDomJa+MeAN+75FCSKaLwLRxOBIa4HnLS7uhBZ62wF8ZBIxR2HxR+Md7GstxKItPxsjEKCkV+QTDd0CYh4WU28uqH2IlSxGZms498A3JYxKYz5sY+w8TtZvv/Bx3+qhz4E85vaGrEzZFn5zay41tTnBJ9W0= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1775011687; c=relaxed/simple; bh=qkBFEs6Y7KuICJyW8FV4Mx1iV6x4/o3piB1MCoqfIR8=; h=Content-Type:Mime-Version:Subject:From:In-Reply-To:Date:Cc: Message-Id:References:To; b=lQR7J26LeZwyQyJkXAWWvI+sDitPjDv3q6tGerwxGHRUtQodXJ9Oy5e9KCkrtr8eRqOWHZabehFpbI7cYdCLyQvDtaacyRu1M++HhTU01Q7E1rMbjs3T0hwU7ceZuEntJkN3/tl4FylQKR7onqy6s44RqDyc1/Y0SNaVH94w+VU= 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=TAAfoIZf; arc=none smtp.client-ip=91.218.175.182 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="TAAfoIZf" Content-Type: text/plain; charset=us-ascii DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1775011673; 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=487xEtTeyex03qkCiMEHTXJ1TpxhwGZlPZ51KtMUovs=; b=TAAfoIZfWAXt61KDYmsy2oYy6slMzxTI2rGBKdZ0p3214RjmJ7zCmrmA5zddannA97lcRs SfVhKdPn7UnoLrT1Al9nhyelBzYFhYBJ91jRCg4gzzcGbsramRVMdW6NrZp/PxpQD8CRqB 5HaN4sE6bAyStJ0qlCw9KpvCBruYTYU= 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] mm/sparse: fix BUILD_BUG_ON check for section map alignment X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Muchun Song In-Reply-To: <20260331130717.d42b64e5179c4c814bc523ea@linux-foundation.org> Date: Wed, 1 Apr 2026 10:47:16 +0800 Cc: Muchun Song , David Hildenbrand , Lorenzo Stoakes , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Petr Tesarik , linux-mm@kvack.org, linux-kernel@vger.kernel.org Content-Transfer-Encoding: quoted-printable Message-Id: <13BC146A-993F-4105-9BE7-56A8691A2E61@linux.dev> References: <20260331113023.2068075-1-songmuchun@bytedance.com> <20260331130717.d42b64e5179c4c814bc523ea@linux-foundation.org> To: Andrew Morton X-Migadu-Flow: FLOW_OUT > On Apr 1, 2026, at 04:07, Andrew Morton = wrote: >=20 > On Tue, 31 Mar 2026 19:30:23 +0800 Muchun Song = wrote: >=20 >> The comment in mmzone.h states that the alignment requirement >> is the minimum of PAGE_SHIFT and PFN_SECTION_SHIFT. However, the >> pointer arithmetic (mem_map - section_nr_to_pfn()) results in >> a byte offset scaled by sizeof(struct page). Thus, the actual >> alignment provided by the second term is PFN_SECTION_SHIFT + >> __ffs(sizeof(struct page)). >>=20 >> Update the compile-time check and the mmzone.h comment to >> accurately reflect this mathematically guaranteed alignment by >> taking the minimum of PAGE_SHIFT and PFN_SECTION_SHIFT + >> __ffs(sizeof(struct page)). This avoids the issue of the check >> being overly restrictive on architectures like powerpc where >> PFN_SECTION_SHIFT alone is very small (e.g., 6). >>=20 >> Also, remove the exhaustive per-architecture bit-width list from the >> comment; such details risk falling out of date over time and may >> inadvertently be left un-updated, while the existing BUILD_BUG_ON >> provides sufficient compile-time verification of the constraint. >>=20 >> No runtime impact so far: SECTION_MAP_LAST_BIT happens to fit within >> the smaller limit on all existing architectures. >>=20 >> ... >>=20 >> --- a/mm/sparse.c >> +++ b/mm/sparse.c >> @@ -269,7 +269,8 @@ static unsigned long sparse_encode_mem_map(struct = page *mem_map, unsigned long p >> { >> unsigned long coded_mem_map =3D >> (unsigned long)(mem_map - (section_nr_to_pfn(pnum))); >> - BUILD_BUG_ON(SECTION_MAP_LAST_BIT > PFN_SECTION_SHIFT); >> + BUILD_BUG_ON(SECTION_MAP_LAST_BIT > min(PFN_SECTION_SHIFT + = __ffs(sizeof(struct page)), >> + PAGE_SHIFT)); >> BUG_ON(coded_mem_map & ~SECTION_MAP_MASK); >> return coded_mem_map; >> } >=20 > In mm-stable this was moved into mm/internal.h's new > sparse_init_one_section(). By David's 6a2f8fb8ed2d ("mm/sparse: move > sparse_init_one_section() to internal.h") Got it. I see it. >=20 > I did the obvious thing: Thank you for your work for me. >=20 > include/linux/mmzone.h | 24 +++++++++--------------- > mm/internal.h | 3 ++- > 2 files changed, 11 insertions(+), 16 deletions(-) >=20 > --- = a/include/linux/mmzone.h~mm-sparse-fix-build_bug_on-check-for-section-map-= alignment > +++ a/include/linux/mmzone.h > @@ -2068,21 +2068,15 @@ static inline struct mem_section *__nr_t > extern size_t mem_section_usage_size(void); >=20 > /* > - * We use the lower bits of the mem_map pointer to store > - * a little bit of information. The pointer is calculated > - * as mem_map - section_nr_to_pfn(pnum). The result is > - * aligned to the minimum alignment of the two values: > - * 1. All mem_map arrays are page-aligned. > - * 2. section_nr_to_pfn() always clears PFN_SECTION_SHIFT > - * lowest bits. PFN_SECTION_SHIFT is arch-specific > - * (equal SECTION_SIZE_BITS - PAGE_SHIFT), and the > - * worst combination is powerpc with 256k pages, > - * which results in PFN_SECTION_SHIFT equal 6. > - * To sum it up, at least 6 bits are available on all architectures. > - * However, we can exceed 6 bits on some other architectures except > - * powerpc (e.g. 15 bits are available on x86_64, 13 bits are = available > - * with the worst case of 64K pages on arm64) if we make sure the > - * exceeded bit is not applicable to powerpc. > + * We use the lower bits of the mem_map pointer to store a little bit = of > + * information. The pointer is calculated as mem_map - = section_nr_to_pfn(). > + * The result is aligned to the minimum alignment of the two values: > + * > + * 1. All mem_map arrays are page-aligned. > + * 2. section_nr_to_pfn() always clears PFN_SECTION_SHIFT lowest = bits. Because > + * it is subtracted from a struct page pointer, the offset is = scaled by > + * sizeof(struct page). This provides an alignment of = PFN_SECTION_SHIFT + > + * __ffs(sizeof(struct page)). > */ > enum { > SECTION_MARKED_PRESENT_BIT, > --- = a/mm/internal.h~mm-sparse-fix-build_bug_on-check-for-section-map-alignment= > +++ a/mm/internal.h > @@ -972,7 +972,8 @@ static inline void sparse_init_one_secti > { > unsigned long coded_mem_map; >=20 > - BUILD_BUG_ON(SECTION_MAP_LAST_BIT > PFN_SECTION_SHIFT); > + BUILD_BUG_ON(SECTION_MAP_LAST_BIT > min(PFN_SECTION_SHIFT + = __ffs(sizeof(struct page)), > + PAGE_SHIFT)); >=20 > /* > * We encode the start PFN of the section into the mem_map such that > _ >=20 > (boy that's an eyesore on an 80-col xterm!)