From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from szxga05-in.huawei.com (szxga05-in.huawei.com [45.249.212.191]) (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 54BC66BB2F for ; Thu, 8 Feb 2024 09:44:52 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=45.249.212.191 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1707385496; cv=none; b=G//XuTJBizkYuRFsvuD/C0AW3yYCbafGz93upZwR5voFVBJKZ+e5KaGuSf1UpOtkBCPDqyLX9Kv59yQ7JQ5AzfuSK3neNRO6+27/AAvP9jOpHz25l4U4etVPHaIrRKCSr9ZD16Typyy+WEqbYtUAKXZlVx5jLezPbXMCpWRKEQw= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1707385496; c=relaxed/simple; bh=STtoSs5arH7Z7B6o8Jc5vxJ2oW35KHtTibhQ7eD7N+k=; h=Subject:To:CC:References:From:Message-ID:Date:MIME-Version: In-Reply-To:Content-Type; b=JV+uC4W3Zt3bus7bObf0i9NWDUsI82RU3gwhXgslP+KP0gt97upjt/pT7IzvdGWLbPnf2Uqgjyfau3wWwYVkM6J4iyAbyPyskKLtxfRuZdNYJKVTueXKmXGrD0f13cJJLIoIPJFUEdHh7LYZyONB5cHnaEjsPTU0yMo4ijX6kkg= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=huawei.com; spf=pass smtp.mailfrom=huawei.com; arc=none smtp.client-ip=45.249.212.191 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=huawei.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=huawei.com Received: from mail.maildlp.com (unknown [172.19.88.163]) by szxga05-in.huawei.com (SkyGuard) with ESMTP id 4TVsR03SlZz1FKPw; Thu, 8 Feb 2024 17:40:12 +0800 (CST) Received: from kwepemm600003.china.huawei.com (unknown [7.193.23.202]) by mail.maildlp.com (Postfix) with ESMTPS id 23F7518001A; Thu, 8 Feb 2024 17:44:50 +0800 (CST) Received: from [10.174.179.79] (10.174.179.79) by kwepemm600003.china.huawei.com (7.193.23.202) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2507.35; Thu, 8 Feb 2024 17:44:48 +0800 Subject: Re: [PATCH v3 0/3] A Solution to Re-enable hugetlb vmemmap optimize To: Catalin Marinas , Matthew Wilcox CC: Will Deacon , , , , , , , , References: <20240113094436.2506396-1-sunnanyong@huawei.com> <20240207111252.GA22167@willie-the-truck> From: Nanyong Sun Message-ID: <44075bc2-ac5f-ffcd-0d2f-4093351a6151@huawei.com> Date: Thu, 8 Feb 2024 17:44:48 +0800 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Thunderbird/78.8.1 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 8bit X-ClientProxiedBy: dggems704-chm.china.huawei.com (10.3.19.181) To kwepemm600003.china.huawei.com (7.193.23.202) 在 2024/2/7 20:20, Catalin Marinas 写道: > On Wed, Feb 07, 2024 at 11:21:17AM +0000, Matthew Wilcox wrote: >> On Wed, Feb 07, 2024 at 11:12:52AM +0000, Will Deacon wrote: >>> On Sat, Jan 27, 2024 at 01:04:15PM +0800, Nanyong Sun wrote: >>>> On 2024/1/26 2:06, Catalin Marinas wrote: >>>>> On Sat, Jan 13, 2024 at 05:44:33PM +0800, Nanyong Sun wrote: >>>>>> HVO was previously disabled on arm64 [1] due to the lack of necessary >>>>>> BBM(break-before-make) logic when changing page tables. >>>>>> This set of patches fix this by adding necessary BBM sequence when >>>>>> changing page table, and supporting vmemmap page fault handling to >>>>>> fixup kernel address translation fault if vmemmap is concurrently accessed. >>>>> I'm not keen on this approach. I'm not even sure it's safe. In the >>>>> second patch, you take the init_mm.page_table_lock on the fault path but >>>>> are we sure this is unlocked when the fault was taken? >>>> I think this situation is impossible. In the implementation of the second >>>> patch, when the page table is being corrupted >>>> (the time window when a page fault may occur), vmemmap_update_pte() already >>>> holds the init_mm.page_table_lock, >>>> and unlock it until page table update is done.Another thread could not hold >>>> the init_mm.page_table_lock and >>>> also trigger a page fault at the same time. >>>> If I have missed any points in my thinking, please correct me. Thank you. >>> It still strikes me as incredibly fragile to handle the fault and trying >>> to reason about all the users of 'struct page' is impossible. For example, >>> can the fault happen from irq context? >> The pte lock cannot be taken in irq context (which I think is what >> you're asking?) > With this patchset, I think it can: IRQ -> interrupt handler accesses > vmemmap -> faults -> fault handler in patch 2 takes the > init_mm.page_table_lock to wait for the vmemmap rewriting to complete. > Maybe it works if the hugetlb code disabled the IRQs but, as Will said, > such fault in any kernel context looks fragile. How about take a new lock with irq disabled during BBM, like: +void vmemmap_update_pte(unsigned long addr, pte_t *ptep, pte_t pte) +{ +    spin_lock_irq(NEW_LOCK); +    pte_clear(&init_mm, addr, ptep); +    flush_tlb_kernel_range(addr, addr + PAGE_SIZE); +    set_pte_at(&init_mm, addr, ptep, pte); +    spin_unlock_irq(NEW_LOCK); +}