From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id B8604C43334 for ; Mon, 27 Jun 2022 07:35:10 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232978AbiF0HfJ (ORCPT ); Mon, 27 Jun 2022 03:35:09 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:44926 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232866AbiF0HfI (ORCPT ); Mon, 27 Jun 2022 03:35:08 -0400 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by lindbergh.monkeyblade.net (Postfix) with ESMTP id 2B32E5592 for ; Mon, 27 Jun 2022 00:35:06 -0700 (PDT) Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id AFAFF1758; Mon, 27 Jun 2022 00:35:06 -0700 (PDT) Received: from [10.162.42.6] (unknown [10.162.42.6]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id B01E33F5A1; Mon, 27 Jun 2022 00:35:03 -0700 (PDT) Message-ID: <8b4eba5d-545e-c2a4-e4cf-fd5dd6634265@arm.com> Date: Mon, 27 Jun 2022 13:05:00 +0530 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.9.1 Subject: Re: [PATCH] mm: hugetlb: kill set_huge_swap_pte_at() Content-Language: en-US To: Qi Zheng , mike.kravetz@oracle.com, songmuchun@bytedance.com, akpm@linux-foundation.org, catalin.marinas@arm.com, will@kernel.org Cc: linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org References: <20220626145717.53572-1-zhengqi.arch@bytedance.com> <037fc8c3-9b71-cb83-8882-95d5459a494f@bytedance.com> From: Anshuman Khandual In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 6/27/22 12:59, Qi Zheng wrote: > > > On 2022/6/27 15:14, Anshuman Khandual wrote: >> >> >> On 6/27/22 12:25, Qi Zheng wrote: >>> >>> >>> On 2022/6/27 14:18, Anshuman Khandual wrote: >>>> >>>> >>>> On 6/26/22 20:27, Qi Zheng wrote: >>>>> The commit e5251fd43007 ("mm/hugetlb: introduce set_huge_swap_pte_at() >>>>> helper") add set_huge_swap_pte_at() to handle swap entries on >>>>> architectures that support hugepages consisting of contiguous ptes. >>>>> And currently the set_huge_swap_pte_at() is only overridden by arm64. >>>>> >>>>> The set_huge_swap_pte_at() provide a sz parameter to help determine >>>>> the number of entries to be updated. But in fact, all hugetlb swap >>>>> entries contain pfn information, so we can find the corresponding >>>>> folio through the pfn recorded in the swap entry, then the folio_size() >>>>> is the number of entries that need to be updated. >>>>> >>>>> And considering that users will easily cause bugs by ignoring the >>>>> difference between set_huge_swap_pte_at() and set_huge_pte_at(). >>>>> Let's handle swap entries in set_huge_pte_at() and remove the >>>>> set_huge_swap_pte_at(), then we can call set_huge_pte_at() >>>>> anywhere, which simplifies our coding. >>>>> >>>>> Signed-off-by: Qi Zheng >>>>> --- >>>>>    arch/arm64/include/asm/hugetlb.h |  3 --- >>>>>    arch/arm64/mm/hugetlbpage.c      | 34 ++++++++++++++++---------------- >>>>>    include/linux/hugetlb.h          | 13 ------------ >>>>>    mm/hugetlb.c                     |  8 +++----- >>>>>    mm/rmap.c                        | 11 +++-------- >>>>>    5 files changed, 23 insertions(+), 46 deletions(-) >>>>> >>>>> diff --git a/arch/arm64/include/asm/hugetlb.h b/arch/arm64/include/asm/hugetlb.h >>>>> index 1fd2846dbefe..d20f5da2d76f 100644 >>>>> --- a/arch/arm64/include/asm/hugetlb.h >>>>> +++ b/arch/arm64/include/asm/hugetlb.h >>>>> @@ -46,9 +46,6 @@ extern void huge_pte_clear(struct mm_struct *mm, unsigned long addr, >>>>>                   pte_t *ptep, unsigned long sz); >>>>>    #define __HAVE_ARCH_HUGE_PTEP_GET >>>>>    extern pte_t huge_ptep_get(pte_t *ptep); >>>>> -extern void set_huge_swap_pte_at(struct mm_struct *mm, unsigned long addr, >>>>> -                 pte_t *ptep, pte_t pte, unsigned long sz); >>>>> -#define set_huge_swap_pte_at set_huge_swap_pte_at >>>>>      void __init arm64_hugetlb_cma_reserve(void); >>>>>    diff --git a/arch/arm64/mm/hugetlbpage.c b/arch/arm64/mm/hugetlbpage.c >>>>> index c9e076683e5d..58b89b9d13e0 100644 >>>>> --- a/arch/arm64/mm/hugetlbpage.c >>>>> +++ b/arch/arm64/mm/hugetlbpage.c >>>>> @@ -238,6 +238,13 @@ static void clear_flush(struct mm_struct *mm, >>>>>        flush_tlb_range(&vma, saddr, addr); >>>>>    } >>>>>    +static inline struct folio *hugetlb_swap_entry_to_folio(swp_entry_t entry) >>>>> +{ >>>>> +    VM_BUG_ON(!is_migration_entry(entry) && !is_hwpoison_entry(entry)); >>>>> + >>>>> +    return page_folio(pfn_to_page(swp_offset(entry))); >>>>> +} >>>> >>>> Extracting this huge page size from swap entry is an additional operation which >>>> will increase the over all cost for set_huge_swap_pte_at(). At present the size >>> >>> Hmm, I think this cost is very small. And replacing >>> set_huge_swap_pte_at() by transparently handling swap entries helps >>> reduce possible bugs, which is worthwhile. >> >> Possible bugs ? There are just six call sites for this function. > > For the swap entry of hugetlb, we need to remember that we should > call set_huge_swap_pte_at() instead of set_huge_pte_at() every Which is intuitive, so what is the problem ? > time. And the difference between the two is only on arm64, other > architectures should not need to pay attention to this. Hence there is a generic fallback for other platforms. > >> Although this proposed patch is functionally correct, I dont see >> a valid enough reason to increase the overall cost in the path. >