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Sat, 13 Dec 2025 00:01:33 -0800 (PST) Received: from localhost.localdomain ([45.142.165.134]) by smtp.gmail.com with ESMTPSA id d9443c01a7336-29f05287fa3sm53149155ad.5.2025.12.13.00.01.23 (version=TLS1_3 cipher=TLS_CHACHA20_POLY1305_SHA256 bits=256/256); Sat, 13 Dec 2025 00:01:32 -0800 (PST) From: Lance Yang To: akpm@linux-foundation.org Cc: will@kernel.org, aneesh.kumar@kernel.org, npiggin@gmail.com, peterz@infradead.org, tglx@linutronix.de, mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com, x86@kernel.org, hpa@zytor.com, arnd@arndb.de, david@kernel.org, lorenzo.stoakes@oracle.com, ziy@nvidia.com, baolin.wang@linux.alibaba.com, Liam.Howlett@oracle.com, npache@redhat.com, ryan.roberts@arm.com, dev.jain@arm.com, baohua@kernel.org, ioworker0@gmail.com, shy828301@gmail.com, riel@surriel.com, jannh@google.com, linux-arch@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Lance Yang Subject: [PATCH RFC 3/3] mm/khugepaged: skip redundant IPI in collapse_huge_page() Date: Sat, 13 Dec 2025 16:00:25 +0800 Message-ID: <20251213080038.10917-4-lance.yang@linux.dev> X-Mailer: git-send-email 2.49.0 In-Reply-To: <20251213080038.10917-1-lance.yang@linux.dev> References: <20251213080038.10917-1-lance.yang@linux.dev> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Rspam-User: X-Rspamd-Server: rspam11 X-Rspamd-Queue-Id: 4AB401C0011 X-Stat-Signature: p88n6xstdf4fb16t4tsdikmfnym34xk1 X-HE-Tag: 1765612894-845252 X-HE-Meta: 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 PASP0OMk bV9yD82VK/tIdAhoV8OecvKVFjj7R5rF4M7xhuKLb/3Eiqw8UmQEQcJavxCzFkcwCD23j7FHEeWMklueJO1qPbYXF/5b/gXzE5QynGGk7eUDs4quMUeJsYcLnkBg8LsHY5fh5mZMazcMwQTDVz6h8P/EtoiQ7hFjW7E22nHTwpaoLZo9cxRdHntCaHcQjIujVfHz1ITu+VklScrm0Px3wLRQLiVRjSHx7e7ZQeEBDMKamyV3Yw/2Fc/QQGmhH/F/1dFs7pPaE26RiV1I+aBhiToDiB0D3hHOvhUOyNcjXyUeOSraFo62dThW4bbQ62+u34T02/IoXSXE4yr9ggcza8UY66DEdw38FhKq329dEc4DUeBWA9HTyGvlTh0XbwT3C099Mu1UGvgyDJmmg1hO+tcHjYcmUt0NqO4YgDPCgIG+z15A+jFyEyE0n4THdZkHxukhvNMBJPsdpS+O6HFPdENT0bqUGOmVUrZjF X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: From: Lance Yang Similar to the hugetlb PMD unsharing optimization, skip the second IPI in collapse_huge_page() when the TLB flush already provides necessary synchronization. Before commit a37259732a7d ("x86/mm: Make MMU_GATHER_RCU_TABLE_FREE unconditional"), bare metal x86 didn't enable MMU_GATHER_RCU_TABLE_FREE. In that configuration, tlb_remove_table_sync_one() was a NOP. GUP-fast synchronization relied on IRQ disabling, which blocks TLB flush IPIs. When Rik made MMU_GATHER_RCU_TABLE_FREE unconditional to support AMD's INVLPGB, all x86 systems started sending the second IPI. However, on native x86 this is redundant: - pmdp_collapse_flush() calls flush_tlb_range(), sending IPIs to all CPUs to invalidate TLB entries - GUP-fast runs with IRQs disabled, so when the flush IPI completes, any concurrent GUP-fast must have finished - tlb_remove_table_sync_one() provides no additional synchronization On x86, skip the second IPI when running native (without paravirt) and without INVLPGB. For paravirt with non-native flush_tlb_multi and for INVLPGB, conservatively keep both IPIs. Use tlb_table_flush_implies_ipi_broadcast(), consistent with the hugetlb optimization. Suggested-by: David Hildenbrand (Red Hat) Signed-off-by: Lance Yang --- mm/khugepaged.c | 7 ++++++- 1 file changed, 6 insertions(+), 1 deletion(-) diff --git a/mm/khugepaged.c b/mm/khugepaged.c index 97d1b2824386..06ea793a8190 100644 --- a/mm/khugepaged.c +++ b/mm/khugepaged.c @@ -1178,7 +1178,12 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address, _pmd = pmdp_collapse_flush(vma, address, pmd); spin_unlock(pmd_ptl); mmu_notifier_invalidate_range_end(&range); - tlb_remove_table_sync_one(); + /* + * Skip the second IPI if the TLB flush above already synchronized + * with concurrent GUP-fast via broadcast IPIs. + */ + if (!tlb_table_flush_implies_ipi_broadcast()) + tlb_remove_table_sync_one(); pte = pte_offset_map_lock(mm, &_pmd, address, &pte_ptl); if (pte) { -- 2.49.0