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 CF40750095D for ; Sun, 18 Jan 2026 05:46:55 +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=1768715218; cv=none; b=Aaf6vHU3ulV/Sav/aZJxUvdh3HUo/VCKcmqmpJLbPXkU08lPCIffcc7SDxPuAP0J7mIfcDd1W6wZ+wIgSQb2Xg+QuQDnP1IxgIDux89NGFAuWf7kddeq4K5Qp2GpnwHZ+QIH9TfPzzReuqgMZPlCRNaCz7c+t6f8gsd33EIqO1A= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1768715218; c=relaxed/simple; bh=cNqAFRumgSnc2UqsEktLRW59YxOp1RYCRK3R29/XCuU=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=pJEixPqG5ZUaidI4BeadcwAmcoqpYCSEyM32rWvj1I1MKyuLgybjN+I+9G8CDiXlF0J3UUNnEwm5Pn0YlpXAw81tEa2oycuA+GOnHayl9Gia6nvDnWqwpYVv+qkUeUu0r0MP0LCNlRVtCWM3rd+uempQTNFITgJq1BP+APdm0kc= 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 C8CE31517; Sat, 17 Jan 2026 21:46:42 -0800 (PST) Received: from [10.164.10.251] (unknown [10.164.10.251]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 5155F3F632; Sat, 17 Jan 2026 21:46:43 -0800 (PST) Message-ID: Date: Sun, 18 Jan 2026 11:16:40 +0530 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 v5 5/5] mm: rmap: support batched unmapping for file large folios To: Barry Song <21cnbao@gmail.com> Cc: Wei Yang , Baolin Wang , akpm@linux-foundation.org, david@kernel.org, catalin.marinas@arm.com, will@kernel.org, lorenzo.stoakes@oracle.com, ryan.roberts@arm.com, Liam.Howlett@oracle.com, vbabka@suse.cz, rppt@kernel.org, surenb@google.com, mhocko@suse.com, riel@surriel.com, harry.yoo@oracle.com, jannh@google.com, willy@infradead.org, linux-mm@kvack.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org References: <142919ac14d3cf70cba370808d85debe089df7b4.1766631066.git.baolin.wang@linux.alibaba.com> <20260106132203.kdxfvootlkxzex2l@master> <20260107014601.dxvq6b7ljgxwg7iu@master> Content-Language: en-US From: Dev Jain In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On 16/01/26 7:58 pm, Barry Song wrote: > On Fri, Jan 16, 2026 at 5:53 PM Dev Jain wrote: >> >> On 07/01/26 7:16 am, Wei Yang wrote: >>> On Wed, Jan 07, 2026 at 10:29:25AM +1300, Barry Song wrote: >>>> On Wed, Jan 7, 2026 at 2:22 AM Wei Yang wrote: >>>>> On Fri, Dec 26, 2025 at 02:07:59PM +0800, Baolin Wang wrote: >>>>>> Similar to folio_referenced_one(), we can apply batched unmapping for file >>>>>> large folios to optimize the performance of file folios reclamation. >>>>>> >>>>>> Barry previously implemented batched unmapping for lazyfree anonymous large >>>>>> folios[1] and did not further optimize anonymous large folios or file-backed >>>>>> large folios at that stage. As for file-backed large folios, the batched >>>>>> unmapping support is relatively straightforward, as we only need to clear >>>>>> the consecutive (present) PTE entries for file-backed large folios. >>>>>> >>>>>> Performance testing: >>>>>> Allocate 10G clean file-backed folios by mmap() in a memory cgroup, and try to >>>>>> reclaim 8G file-backed folios via the memory.reclaim interface. I can observe >>>>>> 75% performance improvement on my Arm64 32-core server (and 50%+ improvement >>>>>> on my X86 machine) with this patch. >>>>>> >>>>>> W/o patch: >>>>>> real 0m1.018s >>>>>> user 0m0.000s >>>>>> sys 0m1.018s >>>>>> >>>>>> W/ patch: >>>>>> real 0m0.249s >>>>>> user 0m0.000s >>>>>> sys 0m0.249s >>>>>> >>>>>> [1] https://lore.kernel.org/all/20250214093015.51024-4-21cnbao@gmail.com/T/#u >>>>>> Reviewed-by: Ryan Roberts >>>>>> Acked-by: Barry Song >>>>>> Signed-off-by: Baolin Wang >>>>>> --- >>>>>> mm/rmap.c | 7 ++++--- >>>>>> 1 file changed, 4 insertions(+), 3 deletions(-) >>>>>> >>>>>> diff --git a/mm/rmap.c b/mm/rmap.c >>>>>> index 985ab0b085ba..e1d16003c514 100644 >>>>>> --- a/mm/rmap.c >>>>>> +++ b/mm/rmap.c >>>>>> @@ -1863,9 +1863,10 @@ static inline unsigned int folio_unmap_pte_batch(struct folio *folio, >>>>>> end_addr = pmd_addr_end(addr, vma->vm_end); >>>>>> max_nr = (end_addr - addr) >> PAGE_SHIFT; >>>>>> >>>>>> - /* We only support lazyfree batching for now ... */ >>>>>> - if (!folio_test_anon(folio) || folio_test_swapbacked(folio)) >>>>>> + /* We only support lazyfree or file folios batching for now ... */ >>>>>> + if (folio_test_anon(folio) && folio_test_swapbacked(folio)) >>>>>> return 1; >>>>>> + >>>>>> if (pte_unused(pte)) >>>>>> return 1; >>>>>> >>>>>> @@ -2231,7 +2232,7 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma, >>>>>> * >>>>>> * See Documentation/mm/mmu_notifier.rst >>>>>> */ >>>>>> - dec_mm_counter(mm, mm_counter_file(folio)); >>>>>> + add_mm_counter(mm, mm_counter_file(folio), -nr_pages); >>>>>> } >>>>>> discard: >>>>>> if (unlikely(folio_test_hugetlb(folio))) { >>>>>> -- >>>>>> 2.47.3 >>>>>> >>>>> Hi, Baolin >>>>> >>>>> When reading your patch, I come up one small question. >>>>> >>>>> Current try_to_unmap_one() has following structure: >>>>> >>>>> try_to_unmap_one() >>>>> while (page_vma_mapped_walk(&pvmw)) { >>>>> nr_pages = folio_unmap_pte_batch() >>>>> >>>>> if (nr_pages = folio_nr_pages(folio)) >>>>> goto walk_done; >>>>> } >>>>> >>>>> I am thinking what if nr_pages > 1 but nr_pages != folio_nr_pages(). >>>>> >>>>> If my understanding is correct, page_vma_mapped_walk() would start from >>>>> (pvmw->address + PAGE_SIZE) in next iteration, but we have already cleared to >>>>> (pvmw->address + nr_pages * PAGE_SIZE), right? >>>>> >>>>> Not sure my understanding is correct, if so do we have some reason not to >>>>> skip the cleared range? >>>> I don’t quite understand your question. For nr_pages > 1 but not equal >>>> to nr_pages, page_vma_mapped_walk will skip the nr_pages - 1 PTEs inside. >>>> >>>> take a look: >>>> >>>> next_pte: >>>> do { >>>> pvmw->address += PAGE_SIZE; >>>> if (pvmw->address >= end) >>>> return not_found(pvmw); >>>> /* Did we cross page table boundary? */ >>>> if ((pvmw->address & (PMD_SIZE - PAGE_SIZE)) == 0) { >>>> if (pvmw->ptl) { >>>> spin_unlock(pvmw->ptl); >>>> pvmw->ptl = NULL; >>>> } >>>> pte_unmap(pvmw->pte); >>>> pvmw->pte = NULL; >>>> pvmw->flags |= PVMW_PGTABLE_CROSSED; >>>> goto restart; >>>> } >>>> pvmw->pte++; >>>> } while (pte_none(ptep_get(pvmw->pte))); >>>> >>> Yes, we do it in page_vma_mapped_walk() now. Since they are pte_none(), they >>> will be skipped. >>> >>> I mean maybe we can skip it in try_to_unmap_one(), for example: >>> >>> diff --git a/mm/rmap.c b/mm/rmap.c >>> index 9e5bd4834481..ea1afec7c802 100644 >>> --- a/mm/rmap.c >>> +++ b/mm/rmap.c >>> @@ -2250,6 +2250,10 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma, >>> */ >>> if (nr_pages == folio_nr_pages(folio)) >>> goto walk_done; >>> + else { >>> + pvmw.address += PAGE_SIZE * (nr_pages - 1); >>> + pvmw.pte += nr_pages - 1; >>> + } >>> continue; >>> walk_abort: >>> ret = false; >> I am of the opinion that we should do something like this. In the internal pvmw code, > I am still not convinced that skipping PTEs in try_to_unmap_one() > is the right place. If we really want to skip certain PTEs early, > should we instead hint page_vma_mapped_walk()? That said, I don't > see much value in doing so, since in most cases nr is either 1 or > folio_nr_pages(folio). > >> we keep skipping ptes till the ptes are none. With my proposed uffd-fix [1], if the old >> ptes were uffd-wp armed, pte_install_uffd_wp_if_needed will convert all ptes from none >> to not none, and we will lose the batching effect. I also plan to extend support to >> anonymous folios (therefore generalizing for all types of memory) which will set a >> batch of ptes as swap, and the internal pvmw code won't be able to skip through the >> batch. > Thanks for catching this, Dev. I already filter out some of the more > complex cases, for example: > if (pte_unused(pte)) > return 1; > > Since the userfaultfd write-protection case is also a corner case, > could we filter it out as well? > > diff --git a/mm/rmap.c b/mm/rmap.c > index c86f1135222b..6bb8ba6f046e 100644 > --- a/mm/rmap.c > +++ b/mm/rmap.c > @@ -1870,6 +1870,9 @@ static inline unsigned int > folio_unmap_pte_batch(struct folio *folio, > if (pte_unused(pte)) > return 1; > > + if (userfaultfd_wp(vma)) > + return 1; > + > return folio_pte_batch(folio, pvmw->pte, pte, max_nr); > } > > Just offering a second option — yours is probably better. No. This is not an edge case. This is a case which gets exposed by your work, and I believe that if you intend to get the file folio batching thingy in, then you need to fix the uffd stuff too. > > Thanks > Barry