From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-188.mta1.migadu.com (out-188.mta1.migadu.com [95.215.58.188]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 01FF835B150 for ; Thu, 15 Jan 2026 12:28:28 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=95.215.58.188 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1768480111; cv=none; b=CBLUfV0r/laJeRicZx5BVGiZ2mderpE1J7u2lTFLhE4mz8G7S6Q6+1i/SMmx/QE0fNdV1kJrVFEzIhbEaHbcxuNQtnWRRXuRr2XVBFjkMBUVqSxPUIGnQ1p1mnht73gEU46oRimnNXmBfaDuEXkeSzcUnD3VmTljNnO8GjiYJPA= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1768480111; c=relaxed/simple; bh=w3JdtQV6pYvOMWlt7tPUlC6TEhXXtpp+k4r8B+ovh58=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=ILfjZ6pgU9KXpnKEphZu+sxcJ67cMSM9OPkZ0f711Oth5pXPNe5ODMecyQU7nZMmDFXqo0GgIAUkB12MiPse3XYgc5Y1G4e69QjtRVfbxlEpXaFLTYFzMth7pz61USRelhkhQZBXF0zdFvFwOYZf7HeEl5ss+PZWXB7mda/D43o= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=JWMIDcJe; arc=none smtp.client-ip=95.215.58.188 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="JWMIDcJe" Message-ID: <8754b56d-b9a8-4b94-b5bc-982384234527@linux.dev> DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1768480106; h=from:from: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; bh=LO/MKTvdUHcs8CszjDJdblzD/1oYrlivkdR1yyi35cU=; b=JWMIDcJecMA3p9Vmm72Ebn8zDuxzyx/GKRb/mD/2zdzp3SqjedZCT51yRxeimkv9dmFtB9 4YurRCRgzy+JZciV4ltY1TfcAg+398L+2iUmGQpQl80peR2wsdTB/QdPCIdFcKj3GLzNtf uU0YDlSauJHJRa2zDifeCus80bCQvk8= Date: Thu, 15 Jan 2026 20:28:13 +0800 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Subject: Re: [PATCH v1 1/1] mm/khugepaged: move tlb_remove_table_sync_one out from under PTL Content-Language: en-US To: Baolin Wang 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 References: <20260115071651.42626-1-lance.yang@linux.dev> <0bc3807e-2c1b-43af-9aab-665ce30d49d4@linux.alibaba.com> X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Lance Yang In-Reply-To: <0bc3807e-2c1b-43af-9aab-665ce30d49d4@linux.alibaba.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT 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. > > 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. Thanks, Lance > >> 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); >