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Wed, 03 Jun 2026 08:09:13 -0700 (PDT) Received: from pve-server ([49.205.216.49]) by smtp.gmail.com with ESMTPSA id d9443c01a7336-2c164f6d2bbsm29577905ad.1.2026.06.03.08.09.07 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 03 Jun 2026 08:09:12 -0700 (PDT) From: Ritesh Harjani (IBM) To: Muchun Song , Oscar Salvador , David Hildenbrand , Andrew Morton , Madhavan Srinivasan , Michael Ellerman Cc: Muchun Song , Mike Rapoport , Lorenzo Stoakes , "Liam R. Howlett" , Vlastimil Babka , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Nicholas Piggin , "Christophe Leroy (CS GROUP)" , "Aneesh Kumar K.V" , linuxppc-dev@lists.ozlabs.org, Mike Kravetz , Muchun Song Subject: Re: [PATCH v3 03/19] powerpc/mm: Fix wrong addr_pfn tracking in compound vmemmap population In-Reply-To: <20260602101039.1867613-4-songmuchun@bytedance.com> Date: Wed, 03 Jun 2026 20:06:35 +0530 Message-ID: References: <20260602101039.1867613-1-songmuchun@bytedance.com> <20260602101039.1867613-4-songmuchun@bytedance.com> X-Rspamd-Server: rspam06 X-Rspamd-Queue-Id: 9A4A640004 X-Stat-Signature: 9b43txukd76siuorti5u3cp8xewhxsij X-Rspam-User: X-HE-Tag: 1780499354-385950 X-HE-Meta: U2FsdGVkX1+kk6Ht0fNiyyLN9mO42SKAaK1Qww+97HI+ph4Nu1YdUek2SyIzxuuYiemZh2wLms85Ine0Hm9SlCXWJCNWNSe1OAEXUsk7uI4ZylUQiH4o5iceZw0sE/dBBsvneAyCT18JeR6C/E1x/tLcSeWkLe2T1ys4eC4vFVQhvdV2YL8uNHEV5aX4JeMEn8WyFp1sog85STD5Av54ysX8JF1OItl5nF84Oeq+LVZVGdulNgljkF5Uv81yApKeHDZp8U9wJhpL5zc9wpquXChHh4h3WPzmC6+UFrDBm4rFn7b/yKo4GPwt3B0jm0N5O2J/RNDKB0uR+5DsvfMJRmnb+5tShoPi6clX2oBajB7PbOnA64gUPWFg831tCaXCWLDn+JOxG8k3A8ZscfKprue4xRPPTxmDoRoHGCSXTI8Qe07QVdxBYdcDsIQXVr4s3+PjBUtaXCRrUnJXYyBEi0wXTQX5bbQozVXe6kVGpusEBMalSSZEgDZsqO4JtKNsDsj7iH/FtMOqxAWoYRwMBTmW7cgNksbaoHvpI3CNNOkmlPmnpTyxQMPhTI07EWzx5DHYXBEo0F/mPOlPr41eUfbsFa8vmh6jiqxauKNcRMctuaIIY6ZptEATjpgJIUr0DLIBLMZZ9TOn5/dtSsVzRGanZ0XGaTzFAT1ReUQCahZPSEKOfV5LbUpDOAdlFsYmuaq4zUF4mgpOGtiLEzi7n/Dh1VIpEiZn1b5OHU/BckV5AEe8M8CHHA1GZ+ed2URRdVtaug7x75dKMKJRRKpCh+E2kT2Kj9d47QjMG9qXJoKkv8JGj2iCHLwDOfb5uoB5rzvnGb9gN6wrfBnmakLZL5uSNzXlFvaWseGhaZcYV5mD2H1KUzCVVcHEHsozAK/BMgzGH+LnFVtdWvIdmFbPX4ADnqbS907ZvnggDY/CWKNSo1KGMB/g507nOIQlyoFni7IjNpPLp8m5WZHkRty arws0uuf 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 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: Muchun Song writes: > vmemmap_populate_compound_pages() uses addr_pfn to determine the PFN > offset within a compound page and to decide whether the current > vmemmap slot should be populated as a head page mapping or should reuse > a tail page mapping. > > However, addr_pfn is advanced manually in parallel with addr. The loop > itself progresses in vmemmap address space, so each PAGE_SIZE step in > addr covers PAGE_SIZE / sizeof(struct page) struct page slots. Since > addr_pfn is compared against nr_pages in data-PFN units, it should > advance by the same number of PFNs. The existing manual increments do > not match that and therefore do not reliably track the PFN > corresponding to the current addr. > > As a result, pfn_offset can be computed from the wrong PFN and the code > can make the head/tail decision for the wrong compound-page position. > > Fix this by deriving addr_pfn directly from the current vmemmap address > instead of carrying it as loop state. > > Fixes: f2b79c0d7968 ("powerpc/book3s64/radix: add support for vmemmap optimization for radix") > Signed-off-by: Muchun Song > Acked-by: Oscar Salvador Thanks for fixing it. I guess this was not caught because section size on powerpc is 16MB and with 64K pagesize we have 256 pfns to map. The vmemmap size required for this is 256*sizeof(struct page) = 16KB which is < 64K (pagesize). So basically we never loop in vmemmap_populate_compound_page(), because next = addr+PAGE_SIZE will be > end after the 1st iteration itself. But I agree this is a bug which needs fixing and it can be easily caught with 4K pagesize, where we have 4096 pfns to map within a 16MB section. The change looks good to me. Can we please add stable tag too? Cc: stable@kernel.org Also, feel free to add: Reviewed-by: Ritesh Harjani (IBM)