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b=f0IbDoosSzUY1ZyRHNqb732zfUiSXMvzWpxfK/unVQgAuPAwYWbNvqyCsfYWR5fit 7+x6bgUuA3CtH3n0GAiOZL9xk/LUcL45zYnr8neQPOgJmqRyuHRUXmN9KjwkF4/Em/ X5CM7WgFFQ4dCoKXPcyKh4BCKw5LCWWQhw9lRKCo0KAMJ1h2QwhHQq82iaCQMCm0ye wqoEM3v8Sx0pioqN8h1Y0r7hLBMCbHwEstBASeMw6n4X9rlqwiuTd6EnP3fE6I5lXH UlQ48qppsN89wNNrss1UqRXly3H5rtZ6QBQkUOt8ix1v5B1mqVfRl873piLgi80d/a 6+kU8A1On/9mw== Message-ID: <6dcfeb2a-dba6-4de9-ac1b-39312c6bbcb6@kernel.org> Date: Fri, 19 Dec 2025 09:25:38 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH RFC 3/3] mm/khugepaged: skip redundant IPI in collapse_huge_page() To: Lance Yang 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, 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, akpm@linux-foundation.org References: <20251213080038.10917-1-lance.yang@linux.dev> <20251213080038.10917-4-lance.yang@linux.dev> <948d425a-2d6e-4439-a280-0ca9e7521b13@kernel.org> <6fdf89ee-3f6a-420f-b4d6-b03e3e2c8c9b@linux.dev> From: "David Hildenbrand (Red Hat)" Content-Language: en-US In-Reply-To: <6fdf89ee-3f6a-420f-b4d6-b03e3e2c8c9b@linux.dev> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspam-User: X-Rspamd-Server: rspam11 X-Rspamd-Queue-Id: 714EA20011 X-Stat-Signature: onxpqk9etrnsfr8jw3in69dimb7gfd3x X-HE-Tag: 1766132753-724532 X-HE-Meta: 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 yAXyqFWv JGAsX9wJIIu2LxtOEdi4QFTBDhWuF/iyReT6mgRrsCLdQH0W81Jo1FfLeXhCzWat+GNJv6Jv3bZvNSg2n2NeuvnJKMFOSobGIdWJqPAi/SGEvHpeTam6uf4xp6hpWMsvks6BrioPcMhGwUpCovAA5rLirm7m0PKYq49MLhss6WArqZph3u9rgYYEQVch1I42DaaVJd3+yS69cjjvBbc7M/kKNiQf3kmORsy2vopEu8mH5Mc09bhr62soQccRNRVof5TnDPHvua5S347qJ5VLbWSl3lTr+LvTb3GUIzqFbkO2AKDbgKUsl8RPpkXiaXtbRCq/D2CBCm0cbw0L1+ZcvGaj3Mt4kxBD/NPso 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: On 12/18/25 15:35, Lance Yang wrote: > > > On 2025/12/18 21:13, David Hildenbrand (Red Hat) wrote: >> On 12/13/25 09:00, Lance Yang wrote: >>> 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(); >> >> We end up calling >> >>     flush_tlb_range(vma, address, address + HPAGE_PMD_SIZE); >> >>     -> flush_tlb_mm_range(freed_tables = true) >> >>     -> flush_tlb_multi(mm_cpumask(mm), info); >> >> So freed_tables=true and we should be doing the right thing. > > Yep ;) > >> BTW, I was wondering whether we should embed that >> tlb_table_flush_implies_ipi_broadcast() check in >> tlb_remove_table_sync_one() instead. >> It then relies on the caller to do the right thing (flush with >> freed_tables=true or unshared_tables = true). >> >> Thoughts? > > Good point! Let me check the other callers to ensure they > are all preceded by a flush with freed_tables=true (or unshared_tables). > > Will get back to you with what I find :) The use case in tlb_table_flush() is a bit confusing. But I would assume that we have a TLB flush with remove_tables=true beforehand. Otherwise we cannot possibly free the page table. -- Cheers David