From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id B1FE6EE57CE for ; Wed, 31 Dec 2025 03:03:57 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 222BD6B0088; Tue, 30 Dec 2025 22:03:57 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id 1D03C6B0089; Tue, 30 Dec 2025 22:03:57 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 0A8D86B008A; Tue, 30 Dec 2025 22:03:57 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0016.hostedemail.com [216.40.44.16]) by kanga.kvack.org (Postfix) with ESMTP id ED69E6B0088 for ; Tue, 30 Dec 2025 22:03:56 -0500 (EST) Received: from smtpin21.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay10.hostedemail.com (Postfix) with ESMTP id 80834C1CF7 for ; Wed, 31 Dec 2025 03:03:56 +0000 (UTC) X-FDA: 84278271672.21.5CD43D4 Received: from out-182.mta0.migadu.com (out-182.mta0.migadu.com [91.218.175.182]) by imf26.hostedemail.com (Postfix) with ESMTP id 8B1CA140006 for ; Wed, 31 Dec 2025 03:03:54 +0000 (UTC) Authentication-Results: imf26.hostedemail.com; dkim=pass header.d=linux.dev header.s=key1 header.b=UnQ3nijP; dmarc=pass (policy=none) header.from=linux.dev; spf=pass (imf26.hostedemail.com: domain of lance.yang@linux.dev designates 91.218.175.182 as permitted sender) smtp.mailfrom=lance.yang@linux.dev ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1767150234; a=rsa-sha256; cv=none; b=V+Nxn35TBy53+JaVK5iDYzXyM6rZaMbnJJp26u7Ow6+Ef5JuHfAvPRO5y7ELNqzu5sOYYk NHIzv8E5GXrbAb/2iTCHuBMq9NEI1LNDUFbvKKc9hY1H5qb9fExV+3L39p0m3tyla4VhnG LNN8/fK9WH1HHF0kA2EkZFnO2w/nwZM= ARC-Authentication-Results: i=1; imf26.hostedemail.com; dkim=pass header.d=linux.dev header.s=key1 header.b=UnQ3nijP; dmarc=pass (policy=none) header.from=linux.dev; spf=pass (imf26.hostedemail.com: domain of lance.yang@linux.dev designates 91.218.175.182 as permitted sender) smtp.mailfrom=lance.yang@linux.dev ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1767150234; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=TWAvq8dov7INFoDSveKxp8LJoL97QfNYZbwnUjDG4hM=; b=bVOfNvD+c/jAFOsYIeYh2Jb/WRN7TZ0iAzk/kK555Eir4HpSNbcCh34dUQ0Ws55X1wrYbW psx0CNu3c71G6n+PwbyKOnG9AIrpf/h4CL7Od9iMgjQdhbQ7ZLlMLt0pYyRKrl24lmdGGZ xB2y/XVnEIdT/GeM1/U4jvzPaBNvjoA= Message-ID: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1767150231; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=TWAvq8dov7INFoDSveKxp8LJoL97QfNYZbwnUjDG4hM=; b=UnQ3nijP6X0/b80mQR948EKNaJrCZarFp2fTfHeiSDk84uHzXtZJ2uV6Xetixo917rL93u 1YCs1wkbcEpQskFZ9WFMzZ4LbTB6oBFIPc86SXcd4Q0/KM1eCVW7oZUekXGoOBs3RexWxs 4qdflp3C1Jn82CTaW7XnSF1hlEd/+8E= Date: Wed, 31 Dec 2025 11:03:40 +0800 MIME-Version: 1.0 Subject: Re: [PATCH v2 3/3] mm: embed TLB flush IPI check in tlb_remove_table_sync_one() Content-Language: en-US To: "David Hildenbrand (Red Hat)" , 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, 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 References: <20251229145245.85452-1-lance.yang@linux.dev> <20251229145245.85452-4-lance.yang@linux.dev> X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Lance Yang In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT X-Rspamd-Queue-Id: 8B1CA140006 X-Rspamd-Server: rspam03 X-Stat-Signature: pxpdeha4wgdyow34ojm8hut3hswochdg X-Rspam-User: X-HE-Tag: 1767150234-322096 X-HE-Meta: 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 /LhaAWT8 mHQdD0u95TbJYxFQAiqHXoQ8KiMA9LKw+CIBhKXJ7diJEzMmCRJeWkrpex8JFBRIYF1TKPnFHnKFRnwai+dtpbUPets8QohA6mJ9Pq/NJOql9D4VKBtKyrGPFjkypoZLw4pMhtBIxWWnv5QffxUH8qKGBliHf4RKDa3zQzkb0VsUyni/HdG92TYYROSwdsvvFPsJPM5nI4YimTLQLx45yXeGEEkZ0fTAirMEKkpSN/5Fg7HhOPjGQpMQjnj/pbqZrVJOC+TDqc5w5YYA= 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 2025/12/31 04:33, David Hildenbrand (Red Hat) wrote: > On 12/29/25 15:52, Lance Yang wrote: >> From: Lance Yang >> >> Embed the tlb_table_flush_implies_ipi_broadcast() check directly inside >> tlb_remove_table_sync_one() instead of requiring every caller to check >> it explicitly. This relies on callers to do the right thing: flush with >> freed_tables=true or unshared_tables=true beforehand. >> >> All existing callers satisfy this requirement: >> >> 1. mm/khugepaged.c:1188 (collapse_huge_page): >> >>     pmdp_collapse_flush(vma, address, pmd) >>     -> flush_tlb_range(vma, address, address + HPAGE_PMD_SIZE) >>        -> flush_tlb_mm_range(mm, ..., freed_tables = true) >>           -> flush_tlb_multi(mm_cpumask(mm), info) >> >>     So freed_tables=true before calling tlb_remove_table_sync_one(). >> >> 2. include/asm-generic/tlb.h:861 (tlb_flush_unshared_tables): >> >>     tlb_flush_mmu_tlbonly(tlb) >>     -> tlb_flush(tlb) >>        -> flush_tlb_mm_range(mm, ..., unshared_tables = true) >>           -> flush_tlb_multi(mm_cpumask(mm), info) >> >>     unshared_tables=true (equivalent to freed_tables for sending IPIs). >> >> 3. mm/mmu_gather.c:341 (__tlb_remove_table_one): >> >>     When we can't allocate a batch page in tlb_remove_table(), we do: >> >>     tlb_table_invalidate(tlb) >>     -> tlb_flush_mmu_tlbonly(tlb) >>        -> flush_tlb_mm_range(mm, ..., freed_tables = true) >>           -> flush_tlb_multi(mm_cpumask(mm), info) >> >>     Then: >>     tlb_remove_table_one(table) >>     -> __tlb_remove_table_one(table) // if !CONFIG_PT_RECLAIM >>        -> tlb_remove_table_sync_one() >> >>     freed_tables=true, and this should work too. >> >>     Why is tlb->freed_tables guaranteed? Because callers like >>     pte_free_tlb() (via free_pte_range) set freed_tables=true before >>     calling __pte_free_tlb(), which then calls tlb_remove_table(). >>     We cannot free page tables without freed_tables=true. >> >>     Note that tlb_remove_table_sync_one() was a NOP on bare metal x86 >>     (CONFIG_MMU_GATHER_RCU_TABLE_FREE=n) before commit a37259732a7d >>     ("x86/mm: Make MMU_GATHER_RCU_TABLE_FREE unconditional"). >> >> 4-5. mm/khugepaged.c:1683,1819 (pmdp_get_lockless_sync macro): >> >>     Same as #1. These also use pmdp_collapse_flush() beforehand. >> >> Suggested-by: David Hildenbrand (Red Hat) >> Signed-off-by: Lance Yang > > LGTM. I think we should document that somewhere. Can we add some Thanks! > kerneldoc for tlb_remove_table_sync_one() where we document that it > doesn't to any sync if a previous TLB flush when removing/unsharing page > tables would have already performed an IPI? Fair point. Would something like this work? ---8<--- diff --git a/mm/mmu_gather.c b/mm/mmu_gather.c index 7b588643cbae..9139f0a6b8bd 100644 --- a/mm/mmu_gather.c +++ b/mm/mmu_gather.c @@ -274,6 +274,20 @@ static void tlb_remove_table_smp_sync(void *arg) /* Simply deliver the interrupt */ } +/** + * tlb_remove_table_sync_one - Send IPI to synchronize page table operations + * + * Sends an IPI to all CPUs to synchronize when freeing or unsharing page + * tables (e.g., to ensure concurrent GUP-fast walkers have completed). + * + * If a previous TLB flush (when removing/unsharing page tables) already + * broadcast IPIs to all CPUs, the redundant IPI is skipped. The optimization + * relies on architectures implementing tlb_table_flush_implies_ipi_broadcast() + * to indicate when their TLB flush provides sufficient synchronization. + * + * Note that callers must ensure that a TLB flush with freed_tables=true or + * unshared_tables=true has been performed before calling. + */ void tlb_remove_table_sync_one(void) { /* Skip the IPI if the TLB flush already synchronized with other CPUs. */ --- Cheers, Lance > >> --- >>   mm/mmu_gather.c | 4 ++++ >>   1 file changed, 4 insertions(+) >> >> diff --git a/mm/mmu_gather.c b/mm/mmu_gather.c >> index 7468ec388455..7b588643cbae 100644 >> --- a/mm/mmu_gather.c >> +++ b/mm/mmu_gather.c >> @@ -276,6 +276,10 @@ static void tlb_remove_table_smp_sync(void *arg) >>   void tlb_remove_table_sync_one(void) >>   { >> +    /* Skip the IPI if the TLB flush already synchronized with other >> CPUs. */ >> +    if (tlb_table_flush_implies_ipi_broadcast()) >> +        return; >> + >>       /* >>        * This isn't an RCU grace period and hence the page-tables >> cannot be >>        * assumed to be actually RCU-freed. > >