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Fri, 16 Jan 2026 09:03:35 +0800 Message-ID: <26cb8bfa-67a7-43d8-b431-a3307df3e559@linux.alibaba.com> Date: Fri, 16 Jan 2026 09:03:33 +0800 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v1 1/1] mm/khugepaged: move tlb_remove_table_sync_one out from under PTL To: Lance Yang Cc: ioworker0@gmail.com, lorenzo.stoakes@oracle.com, david@kernel.org, akpm@linux-foundation.org, Liam.Howlett@oracle.com, vbabka@suse.cz, rppt@kernel.org, surenb@google.com, mhocko@suse.com, ziy@nvidia.com, npache@redhat.com, ryan.roberts@arm.com, dev.jain@arm.com, baohua@kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, "hughd@google.com" References: <20260115071651.42626-1-lance.yang@linux.dev> <0bc3807e-2c1b-43af-9aab-665ce30d49d4@linux.alibaba.com> <8754b56d-b9a8-4b94-b5bc-982384234527@linux.dev> From: Baolin Wang In-Reply-To: <8754b56d-b9a8-4b94-b5bc-982384234527@linux.dev> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspamd-Queue-Id: 3B20340015 X-Stat-Signature: cj57n5k8wtdi7i6d199p1ooa4qnh5mi1 X-Rspam-User: X-Rspamd-Server: rspam05 X-HE-Tag: 1768525419-889134 X-HE-Meta: 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 nk1gKBtH S8W1Q8wpg8CZ1pKWCgo1AgTMuGf2nf5TF+3Zg5/bkiNyhvSE7gNtfmHHr1T8TSYy+lLoLOt06Lc1t3uicuAwAxI/0AE2llrRQN1Xh9vChjU9civaLN8nrJIeDr0wjhmKGLhiOL5TtFifoEC4O1Ysi2NZ7tHp2ramUbs2LGCfh9dURYISuyw1tep4by/V7R7KZEnjLtkDLjMOxiiwr2uwmRIA1W28EwxZJyP8fMuj6tPuumEBUg3yI2uzxqrjojp/hHqVuaPPtRw9FahZWjAA+k7ysYvC0BVCgCnTS1wkrWLq/athnvrN/mFAWCDgmj1zQo77CQpRNTTwdw3ViG2tUUVWIoCQO9ats7X+QWw/w0BrOQz4Q10mCspzIioWFxjo4V9cHm7706+gG0rE= 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 1/15/26 8:28 PM, Lance Yang wrote: > > > On 2026/1/15 18:00, Baolin Wang wrote: >> Hi Lance, >> >> On 1/15/26 3:16 PM, Lance Yang wrote: >>> From: Lance Yang >>> >>> tlb_remove_table_sync_one() sends IPIs to all CPUs and waits for them, >>> which we really don't want to do while holding PTL. >> >> Could you add more comments to explain why this is safe for the PAE case? > > Yep, IIUC, it is safe because we've already done pmdp_collapse_flush() > which ensures the PMD change is visible. > > pmdp_get_lockless_sync() (which calls tlb_remove_table_sync_one() on PAE) > is just to ensure any ongoing lockless pmd readers (e.g., GUP-fast) > complete > before we proceed. It sends IPIs to all CPUs and waits for responses - a > CPU > can only respond when it's not between local_irq_save() and > local_irq_restore(). > > Moving it out from under PTL doesn't change the synchronization semantics, > since lockless readers don't depend on PTL anyway. Cc Hugh who introduced the pmdp_get_lockless_sync(), to double check. Sounds reasonable to me, please add these comments into the commit message. Thanks. >> For the non-PAE case, you added a new tlb_remove_table_sync_one(), why >> we need this (to solve what problem)? Please also add more comments to >> explain. > > Oops, you're right, the original macro was a no-op for non-PAE. > > I should just move the macro call out from under PTL, rather than > replacing it with direct tlb_remove_table_sync_one() calls. OK. >>> Just move the call to after we release PTL, and drop the macro wrapper >>> while we're at it. >>> >>> Signed-off-by: Lance Yang >>> --- >>>   include/linux/pgtable.h | 4 ---- >>>   mm/khugepaged.c         | 5 +++-- >>>   2 files changed, 3 insertions(+), 6 deletions(-) >>> >>> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h >>> index eb8aacba3698..fb04ed22052c 100644 >>> --- a/include/linux/pgtable.h >>> +++ b/include/linux/pgtable.h >>> @@ -755,7 +755,6 @@ static inline pmd_t pmdp_get_lockless(pmd_t *pmdp) >>>       return pmd; >>>   } >>>   #define pmdp_get_lockless pmdp_get_lockless >>> -#define pmdp_get_lockless_sync() tlb_remove_table_sync_one() >>>   #endif /* CONFIG_PGTABLE_LEVELS > 2 */ >>>   #endif /* CONFIG_GUP_GET_PXX_LOW_HIGH */ >>> @@ -774,9 +773,6 @@ static inline pmd_t pmdp_get_lockless(pmd_t *pmdp) >>>   { >>>       return pmdp_get(pmdp); >>>   } >>> -static inline void pmdp_get_lockless_sync(void) >>> -{ >>> -} >>>   #endif >>>   #ifdef CONFIG_TRANSPARENT_HUGEPAGE >>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c >>> index 9f790ec34400..0a6cebf880e0 100644 >>> --- a/mm/khugepaged.c >>> +++ b/mm/khugepaged.c >>> @@ -1664,10 +1664,10 @@ static enum scan_result >>> try_collapse_pte_mapped_thp(struct mm_struct *mm, unsign >>>           } >>>       } >>>       pgt_pmd = pmdp_collapse_flush(vma, haddr, pmd); >>> -    pmdp_get_lockless_sync(); >>>       pte_unmap_unlock(start_pte, ptl); >>>       if (ptl != pml) >>>           spin_unlock(pml); >>> +    tlb_remove_table_sync_one(); >>>       mmu_notifier_invalidate_range_end(&range); >>> @@ -1818,7 +1818,6 @@ static void retract_page_tables(struct >>> address_space *mapping, pgoff_t pgoff) >>>            */ >>>           if (likely(file_backed_vma_is_retractable(vma))) { >>>               pgt_pmd = pmdp_collapse_flush(vma, addr, pmd); >>> -            pmdp_get_lockless_sync(); >>>               success = true; >>>           } >>> @@ -1826,6 +1825,8 @@ static void retract_page_tables(struct >>> address_space *mapping, pgoff_t pgoff) >>>               spin_unlock(ptl); >>>   drop_pml: >>>           spin_unlock(pml); >>> +        if (success) >>> +            tlb_remove_table_sync_one(); >>>           mmu_notifier_invalidate_range_end(&range); >>