From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id AE4451EB9FA for ; Mon, 16 Jun 2025 21:28:05 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=217.140.110.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1750109287; cv=none; b=PoNnFUOmZn3buVbH5GvnWYrA9+LF8Ila8sKu+juo8i6niWPOioLpT+Grp6pUfYDJHRXwXA5pYQOJdEAquO7/4T0tKXleakFOi9Zl2oitSprN1Kmsrn61BYTr/CW02l1+dZ5Odjmm/zr0W85hLxmOq36g00bU1q6P+HylNvsDurY= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1750109287; c=relaxed/simple; bh=lmZMUhJc/RUM1Y2atd9G3IsircftwdnZdNMJDV8MIf4=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=ob/Vt4z2MfV8rTuKAXWsxmthC3mehT3hk+uQ4tvzbSRkO8SVVNVbCL0bT3IdFyjn2K+G0MXKjp4v9K0BLOncDUb0q0xnyTgHfErH8MOFlQSnz+30VFVTpGRy4BvB5vG7En1GcOjFSlwD9ZGuY8mJTTwBV/7QNbvE53mCA/r1bPo= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com; spf=pass smtp.mailfrom=arm.com; arc=none smtp.client-ip=217.140.110.172 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=arm.com 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 5E7E2150C; Mon, 16 Jun 2025 14:27:43 -0700 (PDT) Received: from [10.57.84.117] (unknown [10.57.84.117]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 35D953F58B; Mon, 16 Jun 2025 14:28:00 -0700 (PDT) Message-ID: <355818a3-cee2-4802-ab16-2045da1ca950@arm.com> Date: Mon, 16 Jun 2025 22:27:58 +0100 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4 2/2] mm: Optimize mremap() by PTE batching Content-Language: en-GB To: Barry Song <21cnbao@gmail.com>, Dev Jain Cc: akpm@linux-foundation.org, Liam.Howlett@oracle.com, lorenzo.stoakes@oracle.com, vbabka@suse.cz, jannh@google.com, pfalcato@suse.de, linux-mm@kvack.org, linux-kernel@vger.kernel.org, david@redhat.com, peterx@redhat.com, mingo@kernel.org, libang.li@antgroup.com, maobibo@loongson.cn, zhengqi.arch@bytedance.com, anshuman.khandual@arm.com, willy@infradead.org, ioworker0@gmail.com, yang@os.amperecomputing.com, baolin.wang@linux.alibaba.com, ziy@nvidia.com, hughd@google.com References: <20250610035043.75448-1-dev.jain@arm.com> <20250610035043.75448-3-dev.jain@arm.com> From: Ryan Roberts In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On 10/06/2025 09:11, Barry Song wrote: > On Tue, Jun 10, 2025 at 7:45 PM Dev Jain wrote: >> >> >> On 10/06/25 12:33 pm, Barry Song wrote: >>> Hi Dev, >>> >>> On Tue, Jun 10, 2025 at 3:51 PM Dev Jain wrote: >>>> Use folio_pte_batch() to optimize move_ptes(). On arm64, if the ptes >>>> are painted with the contig bit, then ptep_get() will iterate through all 16 >>>> entries to collect a/d bits. Hence this optimization will result in a 16x >>>> reduction in the number of ptep_get() calls. Next, ptep_get_and_clear() >>>> will eventually call contpte_try_unfold() on every contig block, thus >>>> flushing the TLB for the complete large folio range. Instead, use >>>> get_and_clear_full_ptes() so as to elide TLBIs on each contig block, and only >>>> do them on the starting and ending contig block. >>>> >>>> For split folios, there will be no pte batching; nr_ptes will be 1. For >>>> pagetable splitting, the ptes will still point to the same large folio; >>>> for arm64, this results in the optimization described above, and for other >>>> arches (including the general case), a minor improvement is expected due to >>>> a reduction in the number of function calls. >>>> >>>> Signed-off-by: Dev Jain >>>> --- >>>> mm/mremap.c | 39 ++++++++++++++++++++++++++++++++------- >>>> 1 file changed, 32 insertions(+), 7 deletions(-) >>>> >>>> diff --git a/mm/mremap.c b/mm/mremap.c >>>> index 180b12225368..18b215521ada 100644 >>>> --- a/mm/mremap.c >>>> +++ b/mm/mremap.c >>>> @@ -170,6 +170,23 @@ static pte_t move_soft_dirty_pte(pte_t pte) >>>> return pte; >>>> } >>>> >>>> +static int mremap_folio_pte_batch(struct vm_area_struct *vma, unsigned long addr, >>>> + pte_t *ptep, pte_t pte, int max_nr) >>>> +{ >>>> + const fpb_t flags = FPB_IGNORE_DIRTY | FPB_IGNORE_SOFT_DIRTY; >>>> + struct folio *folio; >>>> + >>>> + if (max_nr == 1) >>>> + return 1; >>>> + >>>> + folio = vm_normal_folio(vma, addr, pte); >>>> + if (!folio || !folio_test_large(folio)) >>> I'm curious about the following case: >>> If the addr/ptep is not the first subpage of the folio—for example, the >>> 14th subpage—will mremap_folio_pte_batch() return 3? >> >> It will return the number of PTEs, starting from the PTE pointing to the 14th >> subpage, that point to consecutive pages of the same large folio, up till max_nr. >> For an example, if we are operating on a single large folio of order 4, then max_nr >> will be 16 - 14 + 1 = 3. So in this case we will return 3, since the 14th, 15th and >> 16th PTE point to consec pages of the same large folio. >> >>> If so, get_and_clear_full_ptes() would operate on 3 subpages of the folio. >>> In that case, can unfold still work correctly? >> >> Yes, first we unfold as in, we do a BBM sequence: cont -> clear -> non-cont. >> Then, on this non-contig block, we will clear only the PTEs which were asked >> for us to do. > > While going through the code, > > static inline pte_t get_and_clear_full_ptes(struct mm_struct *mm, > unsigned long addr, pte_t *ptep, > unsigned int nr, int full) > { > pte_t pte; > if (likely(nr == 1)) { > contpte_try_unfold(mm, addr, ptep, __ptep_get(ptep)); > pte = __get_and_clear_full_ptes(mm, addr, ptep, nr, full); > } else { > pte = contpte_get_and_clear_full_ptes(mm, addr, ptep, nr, full); > } > > return pte; > } > > Initially, I thought it only unfolded when nr == 1, but after reading > contpte_get_and_clear_full_ptes more closely, I realized we do > support partial unfolding—that's what I had missed. > > pte_t contpte_get_and_clear_full_ptes(struct mm_struct *mm, > unsigned long addr, pte_t *ptep, > unsigned int nr, int full) > { > contpte_try_unfold_partial(mm, addr, ptep, nr); > return __get_and_clear_full_ptes(mm, addr, ptep, nr, full); > } > > I think you are right. Yes, Dev is correct; this works as intended. And yes, the code looks a bit dumb, but IIRC, this inline special-casing on nr=1 was needed to prevent fork and/or munmap microbenchmarks from regressing for the common small folio case. Thanks, Ryan > >> >>> >>> Similarly, if the addr/ptep points to the first subpage, but max_nr is >>> less than CONT_PTES, what will happen in that case? >>> >>> >>>> + return 1; >>>> + >>>> + return folio_pte_batch(folio, addr, ptep, pte, max_nr, flags, NULL, >>>> + NULL, NULL); >>>> +} >>>> + >>>> static int move_ptes(struct pagetable_move_control *pmc, >>>> unsigned long extent, pmd_t *old_pmd, pmd_t *new_pmd) >>>> { >>>> @@ -177,7 +194,7 @@ static int move_ptes(struct pagetable_move_control *pmc, >>>> bool need_clear_uffd_wp = vma_has_uffd_without_event_remap(vma); >>>> struct mm_struct *mm = vma->vm_mm; >>>> pte_t *old_ptep, *new_ptep; >>>> - pte_t pte; >>>> + pte_t old_pte, pte; >>>> pmd_t dummy_pmdval; >>>> spinlock_t *old_ptl, *new_ptl; >>>> bool force_flush = false; >>>> @@ -185,6 +202,8 @@ static int move_ptes(struct pagetable_move_control *pmc, >>>> unsigned long new_addr = pmc->new_addr; >>>> unsigned long old_end = old_addr + extent; >>>> unsigned long len = old_end - old_addr; >>>> + int max_nr_ptes; >>>> + int nr_ptes; >>>> int err = 0; >>>> >>>> /* >>>> @@ -236,14 +255,16 @@ static int move_ptes(struct pagetable_move_control *pmc, >>>> flush_tlb_batched_pending(vma->vm_mm); >>>> arch_enter_lazy_mmu_mode(); >>>> >>>> - for (; old_addr < old_end; old_ptep++, old_addr += PAGE_SIZE, >>>> - new_ptep++, new_addr += PAGE_SIZE) { >>>> + for (; old_addr < old_end; old_ptep += nr_ptes, old_addr += nr_ptes * PAGE_SIZE, >>>> + new_ptep += nr_ptes, new_addr += nr_ptes * PAGE_SIZE) { >>>> VM_WARN_ON_ONCE(!pte_none(*new_ptep)); >>>> >>>> - if (pte_none(ptep_get(old_ptep))) >>>> + nr_ptes = 1; >>>> + max_nr_ptes = (old_end - old_addr) >> PAGE_SHIFT; >>>> + old_pte = ptep_get(old_ptep); >>>> + if (pte_none(old_pte)) >>>> continue; >>>> >>>> - pte = ptep_get_and_clear(mm, old_addr, old_ptep); >>>> /* >>>> * If we are remapping a valid PTE, make sure >>>> * to flush TLB before we drop the PTL for the >>>> @@ -255,8 +276,12 @@ static int move_ptes(struct pagetable_move_control *pmc, >>>> * the TLB entry for the old mapping has been >>>> * flushed. >>>> */ >>>> - if (pte_present(pte)) >>>> + if (pte_present(old_pte)) { >>>> + nr_ptes = mremap_folio_pte_batch(vma, old_addr, old_ptep, >>>> + old_pte, max_nr_ptes); >>>> force_flush = true; >>>> + } >>>> + pte = get_and_clear_full_ptes(mm, old_addr, old_ptep, nr_ptes, 0); >>>> pte = move_pte(pte, old_addr, new_addr); >>>> pte = move_soft_dirty_pte(pte); >>>> >>>> @@ -269,7 +294,7 @@ static int move_ptes(struct pagetable_move_control *pmc, >>>> else if (is_swap_pte(pte)) >>>> pte = pte_swp_clear_uffd_wp(pte); >>>> } >>>> - set_pte_at(mm, new_addr, new_ptep, pte); >>>> + set_ptes(mm, new_addr, new_ptep, pte, nr_ptes); >>>> } >>>> } >>>> >>>> -- >>>> 2.30.2 > > Thanks > Barry