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From: Muchun Song In-Reply-To: <29442561-9426-4635-9b7c-cdd138076127@kernel.org> Date: Wed, 1 Apr 2026 10:47:10 +0800 Cc: Andrew Morton , Muchun Song , 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: References: <20260331113023.2068075-1-songmuchun@bytedance.com> <20260331125529.de4deb7b46239fb13e4be517@linux-foundation.org> <29442561-9426-4635-9b7c-cdd138076127@kernel.org> To: "David Hildenbrand (Arm)" X-Migadu-Flow: FLOW_OUT X-Rspam-User: X-Rspamd-Queue-Id: 5851C160003 X-Stat-Signature: s1j6oryj545rmxjq96ahpksb5srbdxje X-Rspamd-Server: rspam06 X-HE-Tag: 1775011669-754747 X-HE-Meta: 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 teJ6D79h oLiJJ+y5K3/IB3nwR0W+mEQYSPEK2H/F5Fh/R1YMQ2Q2WsmHEe3mzRjZWwz3zQKC2yupAF8rDoeqsJz/vwQLaR66ktx92hypq3oSBB4aNFEJ7GdFMgAmLKjVNJ1bA412pW/3iyy5kKN60y3IxCBG7ng/pLImLA+Kbiaiu8CehF9UljRqJIuiGHl6KAjOZTLzXQqawdwToL326XHRWUVneI5EgBIhrf7W5/S6ZjZCrbI6/WjeWtuf7IntbxouJ4ZCgSA3CdKkfI+pEUh2xzrEpRFPA+UcMydAlSWXtqsBja11HbnwBM+meynq1JBlb0dmvxNOMbhVWOr4YaQY= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: > On Apr 1, 2026, at 04:04, David Hildenbrand (Arm) = wrote: >=20 > On 3/31/26 21:55, Andrew Morton wrote: >> 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 comment; such details risk falling out of date over time and may 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 >> Thanks. So this can fix the build on some arch/kconfig combinations? >>=20 >> Do you think we should fix older kernels? >>=20 >>> Fixes: def9b71ee651 ("include/linux/mmzone.h: fix explanation of = lower bits in the SPARSEMEM mem_map pointer") >>=20 >> Eight years ago so I'm going with "no". >=20 > IIUC, there is nothing broken. Could only be revealed by some new > architecture. Or increasing SECTION_MAP_LAST_BIT for powerpc in the future. >=20 > So no stable :) Absolutely right. Thanks. >=20 > --=20 > Cheers, >=20 > David