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X-CSE-ConnectionGUID: 0dzj7e2ISe64j3oziRsI1A== X-CSE-MsgGUID: L7AdzeD8RKW6b2iuI1+DGQ== X-IronPort-AV: E=McAfee;i="6800,10657,11663"; a="79717501" X-IronPort-AV: E=Sophos;i="6.21,206,1763452800"; d="scan'208";a="79717501" Received: from fmviesa003.fm.intel.com ([10.60.135.143]) by fmvoesa105.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 06 Jan 2026 08:24:59 -0800 X-CSE-ConnectionGUID: 4r2XnTedSR661g4k+b1XLQ== X-CSE-MsgGUID: GBe/ka8KSW2hj94lXax2Ow== X-ExtLoop1: 1 Received: from cjhill-mobl.amr.corp.intel.com (HELO [10.125.109.89]) ([10.125.109.89]) by fmviesa003-auth.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 06 Jan 2026 08:24:57 -0800 Message-ID: Date: Tue, 6 Jan 2026 08:24:57 -0800 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH RESEND v3 1/2] mm/tlb: skip redundant IPI when TLB flush already synchronized To: Lance Yang , akpm@linux-foundation.org Cc: david@kernel.org, dave.hansen@linux.intel.com, will@kernel.org, aneesh.kumar@kernel.org, npiggin@gmail.com, peterz@infradead.org, tglx@linutronix.de, mingo@redhat.com, bp@alien8.de, 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, shy828301@gmail.com, riel@surriel.com, jannh@google.com, linux-arch@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, ioworker0@gmail.com References: <20260106120303.38124-1-lance.yang@linux.dev> <20260106120303.38124-2-lance.yang@linux.dev> From: Dave Hansen Content-Language: en-US Autocrypt: addr=dave.hansen@intel.com; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 1/6/26 04:03, Lance Yang wrote: > From: Lance Yang > > When unsharing hugetlb PMD page tables, we currently send two IPIs: one > for TLB invalidation, and another to synchronize with concurrent GUP-fast > walkers via tlb_remove_table_sync_one(). > > However, if the TLB flush already sent IPIs to all CPUs (when freed_tables > or unshared_tables is true), the second IPI is redundant. GUP-fast runs > with IRQs disabled, so when the TLB flush IPI completes, any concurrent > GUP-fast must have finished. > > To avoid the redundant IPI, we add a flag to mmu_gather to track whether > the TLB flush sent IPIs. We pass the mmu_gather pointer through the TLB > flush path via flush_tlb_info, so native_flush_tlb_multi() can set the > flag when it sends IPIs for freed_tables. We also set the flag for > local-only flushes, since disabling IRQs provides the same guarantee. The lack of imperative voice is killing me. :) > diff --git a/arch/x86/include/asm/tlb.h b/arch/x86/include/asm/tlb.h > index 866ea78ba156..c5950a92058c 100644 > --- a/arch/x86/include/asm/tlb.h > +++ b/arch/x86/include/asm/tlb.h > @@ -20,7 +20,8 @@ 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, tlb); > } I think this hunk sums up v3 pretty well. Where there was a single boolean, now there are two. To add to that, the structure that contains the booleans is itself being passed in. The boolean is still named 'freed_tables', and is going from: tlb->freed_tables which is pretty obviously correct to: tlb->freed_tables || tlb->unshared_tables which is _far_ from obviously correct. I'm at a loss for why the patch wouldn't just do this: - flush_tlb_mm_range(tlb->mm, start, end, stride_shift, tlb->freed_tables); + flush_tlb_mm_range(tlb->mm, start, end, stride_shift, tlb); I suspect these were sent out in a bit of haste, which isn't the first time I've gotten that feeling with this series. Could we slow down, please? > static inline void invlpg(unsigned long addr) > diff --git a/arch/x86/include/asm/tlbflush.h b/arch/x86/include/asm/tlbflush.h > index 00daedfefc1b..83c260c88b80 100644 > --- a/arch/x86/include/asm/tlbflush.h > +++ b/arch/x86/include/asm/tlbflush.h > @@ -220,6 +220,7 @@ struct flush_tlb_info { > * will be zero. > */ > struct mm_struct *mm; > + struct mmu_gather *tlb; > unsigned long start; > unsigned long end; > u64 new_tlb_gen; This also gives me pause. There is a *lot* of redundant information between 'struct mmu_gather' and 'struct tlb_flush_info'. There needs to at least be a description of what the relationship is and how these relate to each other. I would have naively thought that the right move here would be to pull the mmu_gather data out at one discrete time rather than store a pointer to it. What I see here is, I suspect, the most expedient way to do it. I'd _certainly_ have done this myself if I was just hacking something together to play with as quickly as possible. So, in the end, I don't hate the approach here (yet). But it is almost impossible to evaluate it because the series is taking some rather egregious shortcuts and is lacking any real semblance of a refactoring effort.