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a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1767712055; 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=w6u38ZBlE64XscVQRFf/r7lVzbYrkD7bnkzZn9V4fpM=; b=RqoZaSrtRQVmuz+z2NA5lVA4R7dj1K5Ffl+/0dalc0U8jBZZSiJQMH2JZX+R2pCOCrxZpI 3BsGC2jm3iMufPc49/9krY7PaHOf7Myd0aL/2+3Wkr9YR2E2FWihypWR0ClIUg6n3OcudC 0bgmKNEzh3mFsB4Tj4Dmx+LCTx2MgOM= Received: from smtp.kernel.org (transwarp.subspace.kernel.org [100.75.92.58]) by tor.source.kernel.org (Postfix) with ESMTP id E16276011E; Tue, 6 Jan 2026 15:07:34 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id B4C23C116C6; Tue, 6 Jan 2026 15:07:26 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1767712054; bh=8qG/+V95KgS0mZQXBDQS4rA76GBhdjSGltajL1KP3uM=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=ik9S8YpNULD6Yr7Z5c6FjldYO0LXnp8CcBy8uPubZKpLZMSuOwhQ3g+s7DLfdYf/J uBlp/ogoYVroHmuF23LGkqgRmR0mkAaaAavIEV9HSlg4rvUk8535XYsFEZAFhtdFR3 blFTH1Lth7yBClD5NaKVLgPZ5aCy/lmat8Idqa0T7PZo8BCsdd6a8y5jliRtvvk/ej B0Lmk/KVLLDODqRzmuNr2fJqmIILfq64JNz1JLO2WMo5AqIAo6G4yOJ62eTYbDBzkt Qw8T8Y3PWIAQx1wpQ0okdbSpWDV0pfmgc9dS0+7ibU1W3HjcaEgqvIvp+V+GMsdkWv SwXP8ZhXr8VsQ== Message-ID: <86ab8a1f-f6a3-4523-8ccc-f99edfd30a7e@kernel.org> Date: Tue, 6 Jan 2026 16:07:24 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH RESEND v3 2/2] mm: introduce pmdp_collapse_flush_sync() to skip redundant IPI To: Lance Yang , akpm@linux-foundation.org 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, 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> <20260106120303.38124-3-lance.yang@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: 7bit X-Rspamd-Server: rspam01 X-Rspamd-Queue-Id: A397FC0017 X-Stat-Signature: wr3j6ujg3bh7xgediebes8y1w3ue5k1d X-Rspam-User: X-HE-Tag: 1767712055-21923 X-HE-Meta: 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 06j7jnHY hhwLa/pjPwz7ApAK4z3yqlPL0o6z1fWCFbcWANzHre2DzPOMs75LZJ0VpauOeKY+oxXUhyiP9SkJkyUZ/gbHbP7BlKzqLRr0uHVDIilS4kdpBjFdUz6yOMi2XB0aYWUrVsBuSVI740iOHlnO2w+Liah8pi33RIH3P6fdYBVRBAKKuYU7IkIW3x+dMHZ++pg6wxnIkzkO/+jWwGYSgAguH2SYtxceYsRW2TAaKPiNoKSzoyStS1jwI6n8XGo15YcWWv+TiIWDSmZqJ8zs+RcyJ8FJwcDski9boZFJEqDfqE8ck80KTJgIoOHKI9pMvW1cXzxBNHMrnRTV1GQuLfGspk/rppO+OcouTP/TY3wkBo338+F9vPf3PCrV2+jKzGrXiG7tu7jVoH9YUSFc= 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 1/6/26 13:03, Lance Yang wrote: > From: Lance Yang > > pmdp_collapse_flush() may already send IPIs to flush TLBs, and then > callers send another IPI via tlb_remove_table_sync_one() or > pmdp_get_lockless_sync() to synchronize with concurrent GUP-fast walkers. > > However, since GUP-fast runs with IRQs disabled, the TLB flush IPI already > provides the necessary synchronization. We can avoid the redundant second > IPI. > > Introduce pmdp_collapse_flush_sync() which combines flush and sync: > > - For architectures using the generic pmdp_collapse_flush() implementation > (e.g., x86): Use mmu_gather to track IPI sends. If the TLB flush sent > an IPI, tlb_gather_remove_table_sync_one() will skip the redundant one. > > - For architectures with custom pmdp_collapse_flush() (s390, riscv, > powerpc): Fall back to calling pmdp_collapse_flush() followed by > tlb_remove_table_sync_one(). No behavior change. > > Update khugepaged to use pmdp_collapse_flush_sync() instead of separate > flush and sync calls. Remove the now-unused pmdp_get_lockless_sync() macro. > > Suggested-by: David Hildenbrand (Red Hat) > Signed-off-by: Lance Yang > --- > include/linux/pgtable.h | 13 +++++++++---- > mm/khugepaged.c | 9 +++------ > mm/pgtable-generic.c | 34 ++++++++++++++++++++++++++++++++++ > 3 files changed, 46 insertions(+), 10 deletions(-) > > diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h > index eb8aacba3698..69e290dab450 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 > @@ -1174,6 +1170,8 @@ static inline void pudp_set_wrprotect(struct mm_struct *mm, > #ifdef CONFIG_TRANSPARENT_HUGEPAGE > extern pmd_t pmdp_collapse_flush(struct vm_area_struct *vma, > unsigned long address, pmd_t *pmdp); > +extern pmd_t pmdp_collapse_flush_sync(struct vm_area_struct *vma, > + unsigned long address, pmd_t *pmdp); > #else > static inline pmd_t pmdp_collapse_flush(struct vm_area_struct *vma, > unsigned long address, > @@ -1182,6 +1180,13 @@ static inline pmd_t pmdp_collapse_flush(struct vm_area_struct *vma, > BUILD_BUG(); > return *pmdp; > } > +static inline pmd_t pmdp_collapse_flush_sync(struct vm_area_struct *vma, > + unsigned long address, > + pmd_t *pmdp) > +{ > + BUILD_BUG(); > + return *pmdp; > +} > #define pmdp_collapse_flush pmdp_collapse_flush > #endif /* CONFIG_TRANSPARENT_HUGEPAGE */ > #endif > diff --git a/mm/khugepaged.c b/mm/khugepaged.c > index 9f790ec34400..0a98afc85c50 100644 > --- a/mm/khugepaged.c > +++ b/mm/khugepaged.c > @@ -1177,10 +1177,9 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long a > * Parallel GUP-fast is fine since GUP-fast will back off when > * it detects PMD is changed. > */ > - _pmd = pmdp_collapse_flush(vma, address, pmd); > + _pmd = pmdp_collapse_flush_sync(vma, address, pmd); > spin_unlock(pmd_ptl); > mmu_notifier_invalidate_range_end(&range); > - tlb_remove_table_sync_one(); Now you issue the IPI under PTL. [...] > diff --git a/mm/pgtable-generic.c b/mm/pgtable-generic.c > index d3aec7a9926a..be2ee82e6fc4 100644 > --- a/mm/pgtable-generic.c > +++ b/mm/pgtable-generic.c > @@ -233,6 +233,40 @@ pmd_t pmdp_collapse_flush(struct vm_area_struct *vma, unsigned long address, > flush_tlb_range(vma, address, address + HPAGE_PMD_SIZE); > return pmd; > } > + > +pmd_t pmdp_collapse_flush_sync(struct vm_area_struct *vma, unsigned long address, > + pmd_t *pmdp) > +{ > + struct mmu_gather tlb; > + pmd_t pmd; > + > + VM_BUG_ON(address & ~HPAGE_PMD_MASK); > + VM_BUG_ON(pmd_trans_huge(*pmdp)); > + > + tlb_gather_mmu(&tlb, vma->vm_mm); Should we be using the new tlb_gather_mmu_vma(), and do we have to set the TLB pagesize to PMD? -- Cheers David