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 58CBD345CC2 for ; Wed, 1 Apr 2026 03:13:01 +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=1775013181; cv=none; b=rMHVlRT5ekTcdieaV1ZPn9qnnPmAeCsEZSq9h3z4EdhaFPemKo60LErwtA032wj49OwRkHH76cpwCCgTQIuQHE7pp6DgwdnJL4zp3o2mprxtYzemv/y39SF3N4RjlmRDC+g/X8bjW2XVOQRI/E9lNdm5gCvNqHQNLLYPIT5sF/g= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1775013181; c=relaxed/simple; bh=Pavu2T4J6VHgg9ihYg0UF6nuGvcAN0v9cPYRJLHQiQU=; h=Date:To:From:Subject:Message-Id; b=IQENPyZIbyMCzG+z+P2lLc/jRXCnlg7LJuh6wBsT1ntOcXsuNCukGtlGGvKM1USm2IT03sokwrbpwk55SxUvDQPZvpMy6g9UV8SJOyRudCwVNUmBLg/5HmzMMno6QVBk6opNMkVjluZtyVUHhKbK02dymNb+IYL49b83wl5KprA= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux-foundation.org header.i=@linux-foundation.org header.b=jXXlmOo2; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux-foundation.org header.i=@linux-foundation.org header.b="jXXlmOo2" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 789A6C19423; Wed, 1 Apr 2026 03:13:00 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linux-foundation.org; s=korg; t=1775013180; bh=Pavu2T4J6VHgg9ihYg0UF6nuGvcAN0v9cPYRJLHQiQU=; h=Date:To:From:Subject:From; b=jXXlmOo2GQlyDq9kPhi+VAtTBl7QNiVb3qUC+B9TtK3Sf7nFzmdpdxiFdarpzSC92 0DeFdiJAyxIZAO18Hh454VL3EjgS2sPsjnsEjFKD7nmsm5BCJ4TsI9wPdQQxMLnbcw bozKNxHMaXCJ9si281SvU8uJEeIE0FMGLg0e4VrE= Date: Tue, 31 Mar 2026 20:12:59 -0700 To: mm-commits@vger.kernel.org,surenb@google.com,rppt@kernel.org,ptesarik@suse.com,mhocko@suse.com,ljs@kernel.org,liam.howlett@oracle.com,songmuchun@bytedance.com,akpm@linux-foundation.org From: Andrew Morton Subject: + mm-sparse-fix-build_bug_on-check-for-section-map-alignment.patch added to mm-unstable branch Message-Id: <20260401031300.789A6C19423@smtp.kernel.org> Precedence: bulk X-Mailing-List: mm-commits@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: The patch titled Subject: mm/sparse: fix BUILD_BUG_ON check for section map alignment has been added to the -mm mm-unstable branch. Its filename is mm-sparse-fix-build_bug_on-check-for-section-map-alignment.patch This patch will shortly appear at https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-sparse-fix-build_bug_on-check-for-section-map-alignment.patch This patch will later appear in the mm-unstable branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm Before you just go and hit "reply", please: a) Consider who else should be cc'ed b) Prefer to cc a suitable mailing list as well c) Ideally: find the original patch on the mailing list and do a reply-to-all to that, adding suitable additional cc's *** Remember to use Documentation/process/submit-checklist.rst when testing your code *** The -mm tree is included into linux-next via various branches at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm and is updated there most days ------------------------------------------------------ From: Muchun Song Subject: mm/sparse: fix BUILD_BUG_ON check for section map alignment Date: Tue, 31 Mar 2026 19:30:23 +0800 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)). 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). 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. No runtime impact so far: SECTION_MAP_LAST_BIT happens to fit within the smaller limit on all existing architectures. Link: https://lkml.kernel.org/r/20260331113023.2068075-1-songmuchun@bytedance.com Fixes: def9b71ee651 ("include/linux/mmzone.h: fix explanation of lower bits in the SPARSEMEM mem_map pointer") Signed-off-by: Muchun Song Cc: Liam Howlett Cc: Lorenzo Stoakes (Oracle) Cc: Michal Hocko Cc: Mike Rapoport Cc: Petr Tesarik Cc: Suren Baghdasaryan Signed-off-by: Andrew Morton --- include/linux/mmzone.h | 24 +++++++++--------------- mm/internal.h | 3 ++- 2 files changed, 11 insertions(+), 16 deletions(-) --- 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); /* - * 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; - 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)); /* * We encode the start PFN of the section into the mem_map such that _ Patches currently in -mm which might be from songmuchun@bytedance.com are mm-memcontrol-remove-dead-code-of-checking-parent-memory-cgroup.patch mm-workingset-use-folio_lruvec-in-workingset_refault.patch mm-rename-unlock_page_lruvec_irq-and-its-variants.patch mm-vmscan-refactor-move_folios_to_lru.patch mm-memcontrol-allocate-object-cgroup-for-non-kmem-case.patch mm-memcontrol-return-root-object-cgroup-for-root-memory-cgroup.patch mm-memcontrol-prevent-memory-cgroup-release-in-get_mem_cgroup_from_folio.patch buffer-prevent-memory-cgroup-release-in-folio_alloc_buffers.patch writeback-prevent-memory-cgroup-release-in-writeback-module.patch mm-memcontrol-prevent-memory-cgroup-release-in-count_memcg_folio_events.patch mm-page_io-prevent-memory-cgroup-release-in-page_io-module.patch mm-migrate-prevent-memory-cgroup-release-in-folio_migrate_mapping.patch mm-mglru-prevent-memory-cgroup-release-in-mglru.patch mm-memcontrol-prevent-memory-cgroup-release-in-mem_cgroup_swap_full.patch mm-workingset-prevent-memory-cgroup-release-in-lru_gen_eviction.patch mm-workingset-prevent-lruvec-release-in-workingset_refault.patch mm-zswap-prevent-lruvec-release-in-zswap_folio_swapin.patch mm-swap-prevent-lruvec-release-in-lru_gen_clear_refs.patch mm-workingset-prevent-lruvec-release-in-workingset_activation.patch mm-memcontrol-prepare-for-reparenting-lru-pages-for-lruvec-lock.patch mm-memcontrol-eliminate-the-problem-of-dying-memory-cgroup-for-lru-folios.patch mm-lru-add-vm_warn_on_once_folio-to-lru-maintenance-helpers.patch mm-sparse-fix-build_bug_on-check-for-section-map-alignment.patch mm-sparse-fix-preinited-section_mem_map-clobbering-on-failure-path.patch