From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-183.mta0.migadu.com (out-183.mta0.migadu.com [91.218.175.183]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 2489E199949 for ; Wed, 31 Dec 2025 03:04:03 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.183 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1767150248; cv=none; b=EAAbSirZwPgaEYc28czp+mQF7YNBgZwQP8JQphYT5AhHw/cl8lKP00IHonMtvhonVeMNIieJQjkBsDGcrX28I2JcFhHsxdc+CBDR5rbdrj/dTn2Eh6BJ4tmzu/EnusIBZZiLHUzNSpHvzI/4SLhI3bFj5DharvxK2lzQEA0Xr9E= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1767150248; c=relaxed/simple; bh=RJ39g3nQ/9p3fLkTnWC3KA9Fm+fzogIq3ixw9ivEg8M=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=OELUAVoRL/Q3z8ORL3M3yOtQ2RqD+bdFPgH4UxDknszM4f2ZylLTSwTmNxkZRSUI3BkWMVEQeVw0S5rqvjmC1l8ojB/YN1J7Bk4tA/eiX1zIXBlrPYUS5uNHigofaclhLWs/xrT6Z45OFbvU1ud/wKPyqAkuO9vakqDkQryECbk= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=UnQ3nijP; arc=none smtp.client-ip=91.218.175.183 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="UnQ3nijP" 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 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: 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 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. > >