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Mon, 03 Aug 2026 20:55:47 -0700 (PDT) From: Muchun Song To: Andrew Morton , Oscar Salvador , David Hildenbrand Cc: Mike Rapoport , Vlastimil Babka , Lorenzo Stoakes , Michal Hocko , David Laight , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Muchun Song , Muchun Song Subject: [PATCH v3 01/17] mm/sparse: relax struct mem_section size constraints Date: Tue, 4 Aug 2026 11:55:19 +0800 Message-ID: <20260804035535.2846016-2-songmuchun@bytedance.com> X-Mailer: git-send-email 2.54.0 In-Reply-To: <20260804035535.2846016-1-songmuchun@bytedance.com> References: <20260804035535.2846016-1-songmuchun@bytedance.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 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. That requirement makes future extensions harder than necessary: adding a small field can require configuration-dependent padding or layout checks just to preserve the lookup scheme. Keep the lookup correct for any struct mem_section size by using a plain modulo instead. Do not leave the layout entirely unconstrained, though. Keep struct mem_section double-word aligned so modest size changes, such as adding another word-sized field on 64-bit systems, still keep a compact and efficient layout. If future fields grow the structure beyond that sweet spot, the lookup remains correct; only the exact layout efficiency changes. Signed-off-by: Muchun Song Acked-by: Mike Rapoport (Microsoft) --- v2: - Align struct mem_section to power-of-2 where possible, but support non-power-of-2 lookup (suggested by David Laight) - Collect Acked-by from Mike Rapoport --- include/linux/mmzone.h | 11 +++-------- mm/sparse.c | 2 -- scripts/gdb/linux/mm.py | 6 ++---- 3 files changed, 5 insertions(+), 14 deletions(-) diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h index a26c8b855222..acf02c87900b 100644 --- a/include/linux/mmzone.h +++ b/include/linux/mmzone.h @@ -2028,13 +2028,9 @@ struct mem_section { * section. (see page_ext.h about this.) */ struct page_ext *page_ext; - unsigned long pad; #endif - /* - * WARNING: mem_section must be a power-of-2 in size for the - * calculation and use of SECTION_ROOT_MASK to make sense. - */ -}; +/* Sacrifice minor padding space for efficient lookup. */ +} __aligned(2 * sizeof(unsigned long)); #ifdef CONFIG_SPARSEMEM_EXTREME #define SECTIONS_PER_ROOT (PAGE_SIZE / sizeof (struct mem_section)) @@ -2044,7 +2040,6 @@ struct mem_section { #define SECTION_NR_TO_ROOT(sec) ((sec) / SECTIONS_PER_ROOT) #define NR_SECTION_ROOTS DIV_ROUND_UP(NR_MEM_SECTIONS, SECTIONS_PER_ROOT) -#define SECTION_ROOT_MASK (SECTIONS_PER_ROOT - 1) #ifdef CONFIG_SPARSEMEM_EXTREME extern struct mem_section **mem_section; @@ -2068,7 +2063,7 @@ static inline struct mem_section *__nr_to_section(unsigned long nr) if (!mem_section || !mem_section[root]) return NULL; #endif - return &mem_section[root][nr & SECTION_ROOT_MASK]; + return &mem_section[root][nr % SECTIONS_PER_ROOT]; } extern size_t mem_section_usage_size(void); diff --git a/mm/sparse.c b/mm/sparse.c index 704a9dec2b9a..ca9875f568d3 100644 --- a/mm/sparse.c +++ b/mm/sparse.c @@ -332,8 +332,6 @@ void __init sparse_init(void) unsigned long pnum_end, pnum_begin, map_count = 1; int nid_begin; - /* see include/linux/mmzone.h 'struct mem_section' definition */ - BUILD_BUG_ON(!is_power_of_2(sizeof(struct mem_section))); memblocks_present(); if (compound_info_has_mask()) { diff --git a/scripts/gdb/linux/mm.py b/scripts/gdb/linux/mm.py index dffadccbb01d..da4e8e9655a6 100644 --- a/scripts/gdb/linux/mm.py +++ b/scripts/gdb/linux/mm.py @@ -70,7 +70,6 @@ class x86_page_ops(): self.SECTIONS_PER_ROOT = 1 self.NR_SECTION_ROOTS = DIV_ROUND_UP(self.NR_MEM_SECTIONS, self.SECTIONS_PER_ROOT) - self.SECTION_ROOT_MASK = self.SECTIONS_PER_ROOT - 1 try: self.SECTION_HAS_MEM_MAP = 1 << int(gdb.parse_and_eval('SECTION_HAS_MEM_MAP_BIT')) @@ -100,7 +99,7 @@ class x86_page_ops(): def __nr_to_section(self, nr): root = self.SECTION_NR_TO_ROOT(nr) mem_section = gdb.parse_and_eval("mem_section") - return mem_section[root][nr & self.SECTION_ROOT_MASK] + return mem_section[root][nr % self.SECTIONS_PER_ROOT] def pfn_to_section_nr(self, pfn): return pfn >> self.PFN_SECTION_SHIFT @@ -249,7 +248,6 @@ class aarch64_page_ops(): self.SECTIONS_PER_ROOT = 1 self.NR_SECTION_ROOTS = DIV_ROUND_UP(self.NR_MEM_SECTIONS, self.SECTIONS_PER_ROOT) - self.SECTION_ROOT_MASK = self.SECTIONS_PER_ROOT - 1 self.SUBSECTION_SHIFT = 21 self.SEBSECTION_SIZE = 1 << self.SUBSECTION_SHIFT self.PFN_SUBSECTION_SHIFT = self.SUBSECTION_SHIFT - self.PAGE_SHIFT @@ -304,7 +302,7 @@ class aarch64_page_ops(): def __nr_to_section(self, nr): root = self.SECTION_NR_TO_ROOT(nr) mem_section = gdb.parse_and_eval("mem_section") - return mem_section[root][nr & self.SECTION_ROOT_MASK] + return mem_section[root][nr % self.SECTIONS_PER_ROOT] def pfn_to_section_nr(self, pfn): return pfn >> self.PFN_SECTION_SHIFT -- 2.54.0