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 2A256FC6174 for ; Sat, 3 Jan 2026 08:39:27 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id AF76C6B00A2; Sat, 3 Jan 2026 03:39:26 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id AA4CE6B00A3; Sat, 3 Jan 2026 03:39:26 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 9A2C26B00A4; Sat, 3 Jan 2026 03:39:26 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0017.hostedemail.com [216.40.44.17]) by kanga.kvack.org (Postfix) with ESMTP id 891B76B00A2 for ; Sat, 3 Jan 2026 03:39:26 -0500 (EST) Received: from smtpin10.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay10.hostedemail.com (Postfix) with ESMTP id E7A9BC03EF for ; Sat, 3 Jan 2026 08:39:25 +0000 (UTC) X-FDA: 84290003490.10.82C2D81 Received: from out-188.mta1.migadu.com (out-188.mta1.migadu.com [95.215.58.188]) by imf12.hostedemail.com (Postfix) with ESMTP id D4D1740002 for ; Sat, 3 Jan 2026 08:39:23 +0000 (UTC) Authentication-Results: imf12.hostedemail.com; dkim=pass header.d=linux.dev header.s=key1 header.b=iEJvQZUc; dmarc=pass (policy=none) header.from=linux.dev; spf=pass (imf12.hostedemail.com: domain of lance.yang@linux.dev designates 95.215.58.188 as permitted sender) smtp.mailfrom=lance.yang@linux.dev ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1767429564; a=rsa-sha256; cv=none; b=6G2Uo6XElNkQsoYuHooaDNwwH+zeqnqB3I+HrqQ25kyAKazs/9Q9TCHj2tLAuYD5ItrgBg MBV3n4u0Ig5wGMTs/GIRVFa+sgxg5K2g+NoGfhcJvI/CHCQ559JdEt+uvS/r/DX8PP9/4D 65Uzao0PAyNr+GlaWn+VvWAIH8Rdmsk= ARC-Authentication-Results: i=1; imf12.hostedemail.com; dkim=pass header.d=linux.dev header.s=key1 header.b=iEJvQZUc; dmarc=pass (policy=none) header.from=linux.dev; spf=pass (imf12.hostedemail.com: domain of lance.yang@linux.dev designates 95.215.58.188 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=1767429564; 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=7OjfT8QygV/9IkypXdM6tQtyzkm4OsqeUOl80eRwm6A=; b=FH+kefO/p1eiEhfxIg8WCffJr9kKeewNp4xnU7+go/lsWW5zYklFEJta+kB582xEWY/Mqb 4j6AaD6tq32w5K5ZC4CPEyHCIW7a/Pr1OkNRjivE5mwPRhVLcS7ZHGzZ7dZbXJzy/7Li5U Y8P//OS4nmCw4JF07eM7iFsgiKvUrg4= Message-ID: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1767429558; 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=7OjfT8QygV/9IkypXdM6tQtyzkm4OsqeUOl80eRwm6A=; b=iEJvQZUcy26Jx3TS6d3XXtQYn84JrNobUoTRlR6pCcevu4gIgJIFSBZjboXngtDewNWvfm q6p9kLkt1xS5lXfjlJQQ2GcFLg9mnUTkyK54pe5uONlgnCmGX11O21bt8AGqeP5g3JpRlH UPrWXG4RvDBbm24m2wekcLwZ/mgJ1s0= Date: Sat, 3 Jan 2026 16:39:06 +0800 MIME-Version: 1.0 Subject: Re: [PATCH v2 0/3] skip redundant TLB sync IPIs Content-Language: en-US To: Dave Hansen , "David Hildenbrand (Red Hat)" 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: <20251229145245.85452-1-lance.yang@linux.dev> <1b27a3fa-359a-43d0-bdeb-c31341749367@kernel.org> 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: 7bit X-Migadu-Flow: FLOW_OUT X-Rspam-User: X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: D4D1740002 X-Stat-Signature: y89mz4cc465gzsuqzucsx3xupec9wprs X-HE-Tag: 1767429563-67366 X-HE-Meta: U2FsdGVkX186VxOE12A/wGpKapWgbj9wFfAWwcnkDzObVQkYpKGEXD59rmSaT9Li5Vu3l+95LRRooUCajWdzMlR5aZrPaOcts+ZxI8yGqtL/2azbb289lb+jd0s6W4GKBkgLtxIbYOis5FtDTrM6unqkTXnB2yNWViK3fusacdFn8tpGM/PNM+SDfSKghskjAnHBlJDgyXoLBfdIpQeQlx85DkHoLr7t2yVAobO93haTk14qu5msCSov3qKJ5NmvhTzxorDTsAIm3GVouvQ3Z51hlPUIfumFFcgAhvqIIJDBp4bspxEhKIc9SY8XcGxbKweLA84q/0xmuAgTs6flmVfa/GmF2MWGxy7t8/0MpApTmH6qAsWsGdbx9iZ0hxM7hauM3O3kpKdOHhs90tSuIi4dYhN4coCkQWh5Xv2Wmwy2Qv8BuWBDlwTo5j4GE0MqhuKCkTK2UTGjGlfXmoaJmRgWXWROCyqA3klDlB7CZgt6mc4y/8y67wo4lvWpjOLbAOBH2fCGkXAwCRMN3co+YEFXdMy5V2vS0L65XvnSEVfYsU8qeqI+U72/mMQ6NRvMAQ5rpdtcNVp0lD/KbCJxp2/itt1kbX2jhyX7wv6krY5PKHOvI2/3FNOW6XpGKziQ8I08J0+QtKqAzGqO8NWDyWCJq6Pn3ePURu8brq8FPNOyS+eexm++X4oGObLYPfHTQOFzbZ7eAjVG5/TuPVtq15N2lVmb1pJiLeNDajP+dfZ5c6BUO3TMrYjO8tEB/1i0MvZz4/H5M6FejkYtgicbDmC/ucgNq6Z/Qzi3BBuZ8LoQT1c4totecg/Dsjnztp8rcyBInp0+YtYnR3XAF8X2rq0XWvP9Z6Dc+PRbNQBhDvrxgHr93JKUBY4BCyFFS0Sq2l8gUVeOkNod4gviuTEr7R8auyGbWjG/FxJyRMKbG5JNEb6b2SyL9tYidB38NljB7yoyYA3XeMxDZWDNXk3 n8UaryeC dx2JJO2cWiWoNOIYrNqGPVOuFQCqi66ueRvm85bamRN1ZB01rsio9alaP6wKnwUPRRgLotyC4FdCs/ZYTimSOp2MJEM5Dz/TiLeesgItpq6o/vJGQr6Er/d3mzRjrhsqBJQ/8tBU+CyA6a1YsZWeSBxG/Dc1I00eWQtrZSk0gDMTLVkqu8XfoWxcyAh7+aF49mWJDP9Qxs64LvOqKtQLQvvwyrJFMBkfRnilCH20CgYNs5Cs= 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 2026/1/3 00:41, Dave Hansen wrote: > On 12/31/25 04:33, David Hildenbrand (Red Hat) wrote: >> On 12/31/25 05:26, Dave Hansen wrote: >>> On 12/29/25 06:52, Lance Yang wrote: >>> ... >>>> This series introduces a way for architectures to indicate their TLB >>>> flush >>>> already provides full synchronization, allowing the redundant IPI to be >>>> skipped. For now, the optimization is implemented for x86 first and >>>> applied >>>> to all page table operations that free or unshare tables. >>> >>> I really don't like all the complexity here. Even on x86, there are >>> three or more ways of deriving this. Having the pv_ops check the value >>> of another pv op is also a bit unsettling. >> >> Right. What I actually meant is that we simply have a property "bool >> flush_tlb_multi_implies_ipi_broadcast" that we set only to true from the >> initialization code. >> >> Without comparing the pv_ops. >> >> That should reduce the complexity quite a bit IMHO. > > Yeah, that sounds promising. Thanks a lot for taking the time to review! Yeah, I simplified things to just a bool property set during init (no pv_ops comparison at runtime) as follows: ---8<--- diff --git a/arch/x86/include/asm/paravirt.h b/arch/x86/include/asm/paravirt.h index 13f9cd31c8f8..a926d459e6f5 100644 --- a/arch/x86/include/asm/paravirt.h +++ b/arch/x86/include/asm/paravirt.h @@ -698,6 +698,7 @@ static __always_inline unsigned long arch_local_irq_save(void) extern void default_banner(void); void native_pv_lock_init(void) __init; +void setup_pv_tlb_flush_ipi_broadcast(void) __init; #else /* __ASSEMBLER__ */ @@ -727,6 +728,10 @@ void native_pv_lock_init(void) __init; static inline void native_pv_lock_init(void) { } + +static inline void setup_pv_tlb_flush_ipi_broadcast(void) +{ +} #endif #endif /* !CONFIG_PARAVIRT */ diff --git a/arch/x86/include/asm/paravirt_types.h b/arch/x86/include/asm/paravirt_types.h index 3502939415ad..7c010d8bee60 100644 --- a/arch/x86/include/asm/paravirt_types.h +++ b/arch/x86/include/asm/paravirt_types.h @@ -133,6 +133,12 @@ struct pv_mmu_ops { void (*flush_tlb_multi)(const struct cpumask *cpus, const struct flush_tlb_info *info); + /* + * Indicates whether flush_tlb_multi IPIs provide sufficient + * synchronization for GUP-fast when freeing or unsharing page tables. + */ + bool flush_tlb_multi_implies_ipi_broadcast; + /* Hook for intercepting the destruction of an mm_struct. */ void (*exit_mmap)(struct mm_struct *mm); void (*notify_page_enc_status_changed)(unsigned long pfn, int npages, bool enc); diff --git a/arch/x86/include/asm/tlb.h b/arch/x86/include/asm/tlb.h index 866ea78ba156..f570c7b2d03e 100644 --- a/arch/x86/include/asm/tlb.h +++ b/arch/x86/include/asm/tlb.h @@ -5,10 +5,23 @@ #define tlb_flush tlb_flush static inline void tlb_flush(struct mmu_gather *tlb); +#define tlb_table_flush_implies_ipi_broadcast tlb_table_flush_implies_ipi_broadcast +static inline bool tlb_table_flush_implies_ipi_broadcast(void); + #include #include #include #include +#include + +static inline bool tlb_table_flush_implies_ipi_broadcast(void) +{ +#ifdef CONFIG_PARAVIRT + return pv_ops.mmu.flush_tlb_multi_implies_ipi_broadcast; +#else + return !cpu_feature_enabled(X86_FEATURE_INVLPGB); +#endif +} static inline void tlb_flush(struct mmu_gather *tlb) { @@ -20,7 +33,8 @@ static inline void tlb_flush(struct mmu_gather *tlb) end = tlb->end; } - flush_tlb_mm_range(tlb->mm, start, end, stride_shift, tlb->freed_tables); + flush_tlb_mm_range(tlb->mm, start, end, stride_shift, + tlb->freed_tables || tlb->unshared_tables); } static inline void invlpg(unsigned long addr) diff --git a/arch/x86/kernel/paravirt.c b/arch/x86/kernel/paravirt.c index ab3e172dcc69..0a49c2d79693 100644 --- a/arch/x86/kernel/paravirt.c +++ b/arch/x86/kernel/paravirt.c @@ -60,6 +60,23 @@ void __init native_pv_lock_init(void) static_branch_enable(&virt_spin_lock_key); } +void __init setup_pv_tlb_flush_ipi_broadcast(void) +{ + /* + * For native TLB flush, if we don't have INVLPGB, we use IPI-based + * flushing which sends real IPIs to all CPUs. This provides sufficient + * synchronization for GUP-fast. + * + * For paravirt (e.g., KVM, Xen, HyperV), hypercalls may not send real + * IPIs, so we keep the default value of false. Only set to true when + * using native flush_tlb_multi without INVLPGB. + */ + if (pv_ops.mmu.flush_tlb_multi == native_flush_tlb_multi && + !cpu_feature_enabled(X86_FEATURE_INVLPGB)) + pv_ops.mmu.flush_tlb_multi_implies_ipi_broadcast = true; +} + + struct static_key paravirt_steal_enabled; struct static_key paravirt_steal_rq_enabled; @@ -173,6 +190,7 @@ struct paravirt_patch_template pv_ops = { .mmu.flush_tlb_kernel = native_flush_tlb_global, .mmu.flush_tlb_one_user = native_flush_tlb_one_user, .mmu.flush_tlb_multi = native_flush_tlb_multi, + .mmu.flush_tlb_multi_implies_ipi_broadcast = false, .mmu.exit_mmap = paravirt_nop, .mmu.notify_page_enc_status_changed = paravirt_nop, diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c index 74aa904be6dc..3f673e686b12 100644 --- a/arch/x86/kernel/setup.c +++ b/arch/x86/kernel/setup.c @@ -51,6 +51,7 @@ #include #include #include +#include #include #include #include @@ -1257,6 +1258,7 @@ void __init setup_arch(char **cmdline_p) io_apic_init_mappings(); x86_init.hyper.guest_late_init(); + setup_pv_tlb_flush_ipi_broadcast(); e820__reserve_resources(); e820__register_nosave_regions(max_pfn); --- > >> But maybe you have an even better way on how to indicate support, in a >> very simple way. > > Rather than having some kind of explicit support enumeration, the other > idea I had would be to actually track the state about what needs to get > flushed somewhere. For instance, even CPUs with enabled INVLPGB support > still use IPIs sometimes. That makes the > tlb_table_flush_implies_ipi_broadcast() check a bit imperfect as is > because it will for the extra sync IPI even when INVLPGB isn't being > used for an mm. > > First, we already save some semblance of support for doing different > flushes when freeing page tables mmu_gather->freed_tables. But, the call > sites in question here are for a single flush and don't use mmu_gathers. > > The other pretty straightforward thing to do would be to add something > to mm->context that indicates that page tables need to be freed but > there might still be wild gup walkers out there that need an IPI. It > would get set when the page tables are modified and cleared at all the > sites where an IPIs are sent. Thanks for the suggestion! The mm->context tracking idea makes a lot of sense - it would handle those mixed INVLPGB/IPI cases much better :) Maybe we could do that as a follow-up. I'd like to keep things simple for now, so we just add a bool property to skip redundant TLB sync IPIs on systems without INVLPGB support. Then we could add the mm->context (or something similar) tracking later to handle things more precisely. Anyway, I'm open to going straight to the mm->context approach as well and happy to do that instead :D Thanks, Lance > > >>> That said, complexity can be worth it with sufficient demonstrated >>> gains. But: >>> >>>> When unsharing hugetlb PMD page tables or collapsing pages in >>>> khugepaged, >>>> we send two IPIs: one for TLB invalidation, and another to synchronize >>>> with concurrent GUP-fast walkers. >>> >>> Those aren't exactly hot paths. khugepaged is fundamentally rate >>> limited. I don't think unsharing hugetlb PMD page tables just is all >>> that common either. >> >> Given that the added IPIs during unsharing broke Oracle DBs rather badly >> [1], I think this is actually a case worth optimizing. > ... >> [1] https://lkml.kernel.org/r/20251223214037.580860-1-david@kernel.org > > Gah, that's good context, thanks. > > Are there any tests out there that might catch these this case better? > It might be something good to have 0day watch for.