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b=PrAmyVvEnpZ4yRI0wUa+185Y7q0FM/Tw3glVvKO5aACCa44jDc+P3QXzXw4+OfTfo gOr9uPnI9cTfK7MH1w1F81H84Ygc0l7FXK2cLSASh6Q2r8VSoz07W59zj3BUPoCN2D BPalls68LBODQR68nQJuK+fPOz9D9ZAuJKLlPwctvqVEefMmhTueXozTyHCpOMRbQz NqXfiidlc2SnQsUfOKDlVwSYUVdhxWXlrSkUjpKnT/GMUcIPbYL/lbgpXZ6188Gs8G nMoCTTrrsk7d+STX9wdSJ7qliax3KSJGjlULX2X/L8zszSXrBY+P4cg+HUh1eMVQzC k4LOD1usLY82A== Message-ID: Date: Tue, 30 Dec 2025 21:33:52 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 3/3] mm: embed TLB flush IPI check in tlb_remove_table_sync_one() To: Lance Yang , 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> From: "David Hildenbrand (Red Hat)" Content-Language: en-US In-Reply-To: <20251229145245.85452-4-lance.yang@linux.dev> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspam-User: X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: 6C069A0012 X-Stat-Signature: ugsd1ru3erczt6hdfk3kyaa39p8316jo X-HE-Tag: 1767126843-999716 X-HE-Meta: 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 EKHblqwm k/6LOgHhDHMSrFhNvznTaGE+nVtlZBLerGipnTiqH4F0dEhHhR9LQJoRvkovryn/zSMEuxa61J+DKDEuGgoYuRn1H7PjPOVJ6+vW6u7hqw37Fa+OnYCdgH1tVdwPewEQhYBtdAq8EYRIuemx0WzzbsPl9cXJBsGY/NsB4YX5BparfdBG+g5ebIXv76m/E32ng+rADKu3OWDQGEZt+YrHm6y1okrgdt6m4UAxR227mlUQx/DW05B0q7rYgiPYru0Xgv1/EFEisquUmDJ8z1vXj1fJ+s0Z6PAE8AvLB 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 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 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? > --- > 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. -- Cheers David