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t=1767967911; bh=CdGScel+GqgAXknTQg/2/WjRhWNPeJQzkNl75YgQGmk=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=FH//Tu3DNmjALm9gWWlfNBdkLe455FVzeLixSksG8giXEOXMPuhYyrfm2Wbx8fzud qE54HHMhjm0yb7vQAwMKbx07D6NX3pFJDjezm1taKkwgnKZaAMCfF4e17RIKnwiac+ DrjxTNlUcwV2tDrXqXydinJyPXU6jm0L/VAOq8KQlS/SAzwsP91VPBk9KyQb1v0Igj ATLVeu5ttie+rfclDmQCnmegLLbaShWERe3T19ODmVhAOHUz1WW2Yr1SThfNa7CysB U67T8zGku2JgOrCv6S4TrMQY8g2SSgAwMQqyIbDEKnBMlnGGoB2H7ULorOVdCJFm+n Ei7WBzkYC52EA== Message-ID: <253140b0-c9b9-4ef0-8b36-af307296519b@kernel.org> Date: Fri, 9 Jan 2026 15:11:40 +0100 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 Cc: dave.hansen@intel.com, 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, akpm@linux-foundation.org, 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> <7472056a-3919-429a-845d-c2076496d537@linux.dev> <9b1cb571-99df-44f8-8c0e-8e9bc3f6e8d5@linux.dev> From: "David Hildenbrand (Red Hat)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam03 X-Rspamd-Queue-Id: 1DE8B40015 X-Stat-Signature: re53fa551g4y53hen835hha87gkmu44t X-Rspam-User: X-HE-Tag: 1767967911-590882 X-HE-Meta: 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 Jqw== 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 1/7/26 07:37, Lance Yang wrote: > Hi David, > > On 2026/1/7 00:10, Lance Yang wrote: > [..] >>> What could work is tracking "tlb_table_flush_sent_ipi" really when we >>> are flushing the TLB for removed/unshared tables, and maybe resetting >>> it ... I don't know when from the top of my head. >> > > Seems like we could fix the issue that the flag lifetime was broken > if the MMU gather gets reused by splitting the flush and reset. This > ensures the flag stays valid between flush and sync. > > Now tlb_flush_unshared_tables() does: > 1) __tlb_flush_mmu_tlbonly() - flush only, keeps flags alive > 2) tlb_gather_remove_table_sync_one() - can check the flag > 3) __tlb_reset_range() - reset everything after sync > > Something like this: > > ---8<--- > diff --git a/include/asm-generic/tlb.h b/include/asm-generic/tlb.h > index 3975f7d11553..a95b054dfcca 100644 > --- a/include/asm-generic/tlb.h > +++ b/include/asm-generic/tlb.h > @@ -415,6 +415,7 @@ static inline void __tlb_reset_range(struct > mmu_gather *tlb) > tlb->cleared_puds = 0; > tlb->cleared_p4ds = 0; > tlb->unshared_tables = 0; > + tlb->tlb_flush_sent_ipi = 0; As raised, the "tlb_flush_sent_ipi" is confusing when we sent to different CPUs based on whether we are removing page tables or not. I think you would really want to track that explicitly "tlb_table_flush_sent_ipi" ? > /* > * Do not reset mmu_gather::vma_* fields here, we do not > * call into tlb_start_vma() again to set them if there is an > @@ -492,7 +493,7 @@ tlb_update_vma_flags(struct mmu_gather *tlb, struct > vm_area_struct *vma) > tlb->vma_pfn |= !!(vma->vm_flags & (VM_PFNMAP|VM_MIXEDMAP)); > } > > -static inline void tlb_flush_mmu_tlbonly(struct mmu_gather *tlb) > +static inline void __tlb_flush_mmu_tlbonly(struct mmu_gather *tlb) > { > /* > * Anything calling __tlb_adjust_range() also sets at least one of > @@ -503,6 +504,11 @@ static inline void tlb_flush_mmu_tlbonly(struct > mmu_gather *tlb) > return; > > tlb_flush(tlb); > +} > + > +static inline void tlb_flush_mmu_tlbonly(struct mmu_gather *tlb) > +{ > + __tlb_flush_mmu_tlbonly(tlb); > __tlb_reset_range(tlb); > } > > @@ -824,7 +830,7 @@ static inline void tlb_flush_unshared_tables(struct > mmu_gather *tlb) > * flush the TLB for the unsharer now. > */ > if (tlb->unshared_tables) > - tlb_flush_mmu_tlbonly(tlb); > + __tlb_flush_mmu_tlbonly(tlb); > > /* > * Similarly, we must make sure that concurrent GUP-fast will not > @@ -834,14 +840,16 @@ static inline void > tlb_flush_unshared_tables(struct mmu_gather *tlb) > * We only perform this when we are the last sharer of a page table, > * as the IPI will reach all CPUs: any GUP-fast. > * > - * Note that on configs where tlb_remove_table_sync_one() is a NOP, > - * the expectation is that the tlb_flush_mmu_tlbonly() would have issued > - * required IPIs already for us. > + * Use tlb_gather_remove_table_sync_one() instead of > + * tlb_remove_table_sync_one() to skip the redundant IPI if the > + * TLB flush above already sent one. > */ > if (tlb->fully_unshared_tables) { > - tlb_remove_table_sync_one(); > + tlb_gather_remove_table_sync_one(tlb); > tlb->fully_unshared_tables = false; > } > + > + __tlb_reset_range(tlb); > } > #endif /* CONFIG_HUGETLB_PMD_PAGE_TABLE_SHARING */ > --- > > For khugepaged, it should be fine - it uses a local mmu_gather that > doesn't get reused. The lifetime is simply: > > tlb_gather_mmu() → flush → sync → tlb_finish_mmu() > > Let me know if this addresses your concern :) I'll probably have to see the full picture. But this lifetime stuff in core-mm ends up getting more complicated than v2 without a clear benefit to me (except maybe handling some x86 oddities better ;) ) -- Cheers David