From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-179.mta0.migadu.com (out-179.mta0.migadu.com [91.218.175.179]) (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 D1538345CDD for ; Wed, 19 Nov 2025 11:02:21 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.179 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1763550144; cv=none; b=n/X3gseBoLwrkiaNfGifgrmPJT3abK4BgF87zsBbEpAcWZ55k4GgJmiFbbshjVK3wzFyCxKFQjQ+k+7thkiOgiHguV83DylWVIr1/aOZIOqCD4058wu0/7Qfvudkh8J+ZsCrlk6aXJlCXhYBgpshmg8Nb2JVunbF0EW5UUfAxzM= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1763550144; c=relaxed/simple; bh=udimc1P+CCMn+0HXpPW/n5CllgbQhyKx2s+dwi05tCM=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=e5I2UXxWIFjd/A2tsfyXzhkDaSpChdczW0fPdwEpU+h4cWzQP7yGxXwyRkzBLL1zt+MQEzhpwTdPkBkuJV/kmz9RhGOYS4EcqQVTL62k1hmwRQDCO6/j14BMKN/ikzbEblScBDIgIXOjndKSoHZE+Uql7PJjcqxCebt3AtAvV5I= 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=mP/P3lxt; arc=none smtp.client-ip=91.218.175.179 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="mP/P3lxt" Message-ID: <956c7ca1-bce8-4eed-8a86-bc8adfc708b8@linux.dev> DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1763550129; 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=pWCjd1KJePPhJ2kBW/1JQctwuUbeN7emyFaJsPF+13s=; b=mP/P3lxthQRt4RBNI75YZfNBnXZAmkVQUWXJ/rGkrYoFs+cdyTatJFvEAkxHy17h5PcVU7 Lk+c5TNjUFkOX+52zWjraznru5UeZZ0zArHNg+EAlhJsCa8P85HvXHgzZV0dRjfvL/z3qw 3uyQEOwjBq6far+PazqEXVEHQS/Z8sg= Date: Wed, 19 Nov 2025 19:02:01 +0800 Precedence: bulk X-Mailing-List: linux-parisc@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Subject: Re: [PATCH 7/7] mm: make PT_RECLAIM depend on MMU_GATHER_RCU_TABLE_FREE && 64BIT To: "David Hildenbrand (Red Hat)" , will@kernel.org, aneesh.kumar@kernel.org, npiggin@gmail.com, peterz@infradead.org, dev.jain@arm.com, akpm@linux-foundation.org, ioworker0@gmail.com Cc: linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-alpha@vger.kernel.org, linux-snps-arc@lists.infradead.org, loongarch@lists.linux.dev, linux-mips@vger.kernel.org, linux-parisc@vger.kernel.org, linux-um@lists.infradead.org, Qi Zheng References: <0a4d1e6f0bf299cafd1fc624f965bd1ca542cea8.1763117269.git.zhengqi.arch@bytedance.com> <355d3bf3-c6bc-403e-9f19-89259d868611@kernel.org> <195baf7c-1f4e-46a4-a4aa-e68e7d00c0f9@linux.dev> <9386032c-9840-49da-83f9-74b112f3e752@kernel.org> X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Qi Zheng In-Reply-To: <9386032c-9840-49da-83f9-74b112f3e752@kernel.org> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT Hi David, On 11/19/25 6:19 PM, David Hildenbrand (Red Hat) wrote: > On 18.11.25 13:02, Qi Zheng wrote: >> >> >> On 11/18/25 12:57 AM, David Hildenbrand (Red Hat) wrote: >>> On 14.11.25 12:11, Qi Zheng wrote: >>>> From: Qi Zheng >>> >>> Subject: s/&&/&/ >> >> will do. >> >>> >>>> >>>> Make PT_RECLAIM depend on MMU_GATHER_RCU_TABLE_FREE so that PT_RECLAIM >>>> can >>>> be enabled by default on all architectures that support >>>> MMU_GATHER_RCU_TABLE_FREE. >>>> >>>> Considering that a large number of PTE page table pages (such as >>>> 100GB+) >>>> can only be caused on a 64-bit system, let PT_RECLAIM also depend on >>>> 64BIT. >>>> >>>> Signed-off-by: Qi Zheng >>>> --- >>>>    arch/x86/Kconfig | 1 - >>>>    mm/Kconfig       | 6 +----- >>>>    2 files changed, 1 insertion(+), 6 deletions(-) >>>> >>>> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig >>>> index eac2e86056902..96bff81fd4787 100644 >>>> --- a/arch/x86/Kconfig >>>> +++ b/arch/x86/Kconfig >>>> @@ -330,7 +330,6 @@ config X86 >>>>        select FUNCTION_ALIGNMENT_4B >>>>        imply IMA_SECURE_AND_OR_TRUSTED_BOOT    if EFI >>>>        select HAVE_DYNAMIC_FTRACE_NO_PATCHABLE >>>> -    select ARCH_SUPPORTS_PT_RECLAIM        if X86_64 >>>>        select ARCH_SUPPORTS_SCHED_SMT        if SMP >>>>        select SCHED_SMT            if SMP >>>>        select ARCH_SUPPORTS_SCHED_CLUSTER    if SMP >>>> diff --git a/mm/Kconfig b/mm/Kconfig >>>> index a5a90b169435d..e795fbd69e50c 100644 >>>> --- a/mm/Kconfig >>>> +++ b/mm/Kconfig >>>> @@ -1440,14 +1440,10 @@ config ARCH_HAS_USER_SHADOW_STACK >>>>          The architecture has hardware support for userspace shadow >>>> call >>>>              stacks (eg, x86 CET, arm64 GCS or RISC-V Zicfiss). >>>> -config ARCH_SUPPORTS_PT_RECLAIM >>>> -    def_bool n >>>> - >>>>    config PT_RECLAIM >>>>        bool "reclaim empty user page table pages" >>>>        default y >>>> -    depends on ARCH_SUPPORTS_PT_RECLAIM && MMU && SMP >>>> -    select MMU_GATHER_RCU_TABLE_FREE >>>> +    depends on MMU_GATHER_RCU_TABLE_FREE && MMU && SMP && 64BIT >>> >>> Who would we have MMU_GATHER_RCU_TABLE_FREE without MMU? (can we drop >>> the MMU part) >> >> OK. >> >>> >>> Why do we care about SMP in the first place? (can we frop SMP) >> >> OK. >> >>> >>> But I also wonder why we need "MMU_GATHER_RCU_TABLE_FREE && 64BIT": >>> >>> Would it be harmful on 32bit (sure, we might not reclaim as much, but >>> still there is memory to be reclaimed?)? >> >> This is also fine on 32bit, but the benefits are not significant, So I >> chose to enable it only on 64-bit. > > Right. Address space is smaller, but also memory is smaller. Not that I > think we strictly *must* to support 32bit, I merely wonder why we > wouldn't just enable it here. > > OTOH, if there is a good reason we cannot enable it, we can definitely > just keep it 64bit only. The only difficulty is this: > >> >> I actually tried enabling MMU_GATHER_RCU_TABLE_FREE on all >> architectures, and apart from sparc32 being a bit troublesome (because >> it uses mm->page_table_lock for synchronization within >> __pte_free_tlb()), the modifications were relatively simple. in sparc32: void pte_free(struct mm_struct *mm, pgtable_t ptep) { struct page *page; page = pfn_to_page(__nocache_pa((unsigned long)ptep) >> PAGE_SHIFT); spin_lock(&mm->page_table_lock); if (page_ref_dec_return(page) == 1) pagetable_dtor(page_ptdesc(page)); spin_unlock(&mm->page_table_lock); srmmu_free_nocache(ptep, SRMMU_PTE_TABLE_SIZE); } #define __pte_free_tlb(tlb, pte, addr) pte_free((tlb)->mm, pte) To enable MMU_GATHER_RCU_TABLE_FREE on sparc32, we need to implement __tlb_remove_table(), and call the pte_free() above in __tlb_remove_table(). However, the __tlb_remove_table() does not have an mm parameter: void __tlb_remove_table(void *_table) so we need to use another lock instead of mm->page_table_lock. I have already sent the v2 [1], and perhaps after that I can enable PT_RECLAIM on all 32-bit architectures as well. [1]. https://lore.kernel.org/all/cover.1763537007.git.zhengqi.arch@bytedance.com/ >> >>> >>> If all 64BIT support MMU_GATHER_RCU_TABLE_FREE (as you previously >>> state), why can't we only check for 64BIT? >> >> OK, will do. > > This was also more of a question for discussion: > > Would it make sense to have > > config PT_RECLAIM >     def_bool y >     depends on MMU_GATHER_RCU_TABLE_FREE make sense. > > (a) Would we want to make it configurable (why?) No, it was just out of caution before. > (b) Do we really care about SMP (why?) No. Simply because the following situation is impossible to occur: pte_offset_map traversing the PTE page table call madvise(MADV_DONTNEED) so there's no need to free PTE page via RCU. > (c) Do we want to limit to 64bit (why?) No, just because the profit is greater at 64-BIT. > (d) Do we really need the MMU check in addition to >     MMU_GATHER_RCU_TABLE_FREE No, I was worried about compilation issues before, but now it seems that my worries were unnecessary. Thanks, Qi > >