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[2003:d8:2f22:1000:d72d:fd5f:4118:c70b]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-38bf32151e6sm10322184f8f.20.2025.01.20.05.38.35 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 20 Jan 2025 05:38:37 -0800 (PST) Message-ID: Date: Mon, 20 Jan 2025 14:38:35 +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: [RFC 2/2] mm/mthp: relax anon mTHP PTE Mapping restrictions To: Lance Yang , Barry Song <21cnbao@gmail.com> Cc: akpm@linux-foundation.org, ryan.roberts@arm.com, dev.jain@arm.com, shy828301@gmail.com, ziy@nvidia.com, libang.li@antgroup.com, baolin.wang@linux.alibaba.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org, Mingzhe Yang References: <20250120012230.3343-1-ioworker0@gmail.com> <20250120012230.3343-3-ioworker0@gmail.com> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; keydata= xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzSREYXZpZCBIaWxk ZW5icmFuZCA8ZGF2aWRAcmVkaGF0LmNvbT7CwZgEEwEIAEICGwMGCwkIBwMCBhUIAgkKCwQW AgMBAh4BAheAAhkBFiEEG9nKrXNcTDpGDfzKTd4Q9wD/g1oFAl8Ox4kFCRKpKXgACgkQTd4Q 9wD/g1oHcA//a6Tj7SBNjFNM1iNhWUo1lxAja0lpSodSnB2g4FCZ4R61SBR4l/psBL73xktp rDHrx4aSpwkRP6Epu6mLvhlfjmkRG4OynJ5HG1gfv7RJJfnUdUM1z5kdS8JBrOhMJS2c/gPf wv1TGRq2XdMPnfY2o0CxRqpcLkx4vBODvJGl2mQyJF/gPepdDfcT8/PY9BJ7FL6Hrq1gnAo4 3Iv9qV0JiT2wmZciNyYQhmA1V6dyTRiQ4YAc31zOo2IM+xisPzeSHgw3ONY/XhYvfZ9r7W1l pNQdc2G+o4Di9NPFHQQhDw3YTRR1opJaTlRDzxYxzU6ZnUUBghxt9cwUWTpfCktkMZiPSDGd KgQBjnweV2jw9UOTxjb4LXqDjmSNkjDdQUOU69jGMUXgihvo4zhYcMX8F5gWdRtMR7DzW/YE BgVcyxNkMIXoY1aYj6npHYiNQesQlqjU6azjbH70/SXKM5tNRplgW8TNprMDuntdvV9wNkFs 9TyM02V5aWxFfI42+aivc4KEw69SE9KXwC7FSf5wXzuTot97N9Phj/Z3+jx443jo2NR34XgF 89cct7wJMjOF7bBefo0fPPZQuIma0Zym71cP61OP/i11ahNye6HGKfxGCOcs5wW9kRQEk8P9 M/k2wt3mt/fCQnuP/mWutNPt95w9wSsUyATLmtNrwccz63XOwU0EVcufkQEQAOfX3n0g0fZz Bgm/S2zF/kxQKCEKP8ID+Vz8sy2GpDvveBq4H2Y34XWsT1zLJdvqPI4af4ZSMxuerWjXbVWb T6d4odQIG0fKx4F8NccDqbgHeZRNajXeeJ3R7gAzvWvQNLz4piHrO/B4tf8svmRBL0ZB5P5A 2uhdwLU3NZuK22zpNn4is87BPWF8HhY0L5fafgDMOqnf4guJVJPYNPhUFzXUbPqOKOkL8ojk CXxkOFHAbjstSK5Ca3fKquY3rdX3DNo+EL7FvAiw1mUtS+5GeYE+RMnDCsVFm/C7kY8c2d0G NWkB9pJM5+mnIoFNxy7YBcldYATVeOHoY4LyaUWNnAvFYWp08dHWfZo9WCiJMuTfgtH9tc75 7QanMVdPt6fDK8UUXIBLQ2TWr/sQKE9xtFuEmoQGlE1l6bGaDnnMLcYu+Asp3kDT0w4zYGsx 5r6XQVRH4+5N6eHZiaeYtFOujp5n+pjBaQK7wUUjDilPQ5QMzIuCL4YjVoylWiBNknvQWBXS lQCWmavOT9sttGQXdPCC5ynI+1ymZC1ORZKANLnRAb0NH/UCzcsstw2TAkFnMEbo9Zu9w7Kv AxBQXWeXhJI9XQssfrf4Gusdqx8nPEpfOqCtbbwJMATbHyqLt7/oz/5deGuwxgb65pWIzufa N7eop7uh+6bezi+rugUI+w6DABEBAAHCwXwEGAEIACYCGwwWIQQb2cqtc1xMOkYN/MpN3hD3 AP+DWgUCXw7HsgUJEqkpoQAKCRBN3hD3AP+DWrrpD/4qS3dyVRxDcDHIlmguXjC1Q5tZTwNB boaBTPHSy/Nksu0eY7x6HfQJ3xajVH32Ms6t1trDQmPx2iP5+7iDsb7OKAb5eOS8h+BEBDeq 3ecsQDv0fFJOA9ag5O3LLNk+3x3q7e0uo06XMaY7UHS341ozXUUI7wC7iKfoUTv03iO9El5f XpNMx/YrIMduZ2+nd9Di7o5+KIwlb2mAB9sTNHdMrXesX8eBL6T9b+MZJk+mZuPxKNVfEQMQ a5SxUEADIPQTPNvBewdeI80yeOCrN+Zzwy/Mrx9EPeu59Y5vSJOx/z6OUImD/GhX7Xvkt3kq Er5KTrJz3++B6SH9pum9PuoE/k+nntJkNMmQpR4MCBaV/J9gIOPGodDKnjdng+mXliF3Ptu6 3oxc2RCyGzTlxyMwuc2U5Q7KtUNTdDe8T0uE+9b8BLMVQDDfJjqY0VVqSUwImzTDLX9S4g/8 kC4HRcclk8hpyhY2jKGluZO0awwTIMgVEzmTyBphDg/Gx7dZU1Xf8HFuE+UZ5UDHDTnwgv7E th6RC9+WrhDNspZ9fJjKWRbveQgUFCpe1sa77LAw+XFrKmBHXp9ZVIe90RMe2tRL06BGiRZr jPrnvUsUUsjRoRNJjKKA/REq+sAnhkNPPZ/NNMjaZ5b8Tovi8C0tmxiCHaQYqj7G2rgnT0kt WNyWQQ== Organization: Red Hat In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 20.01.25 14:20, Lance Yang wrote: > On Mon, Jan 20, 2025 at 10:15 AM Barry Song <21cnbao@gmail.com> wrote: >> >> On Mon, Jan 20, 2025 at 2:23 PM Lance Yang wrote: >>> >>> Previously, mTHP could only be mapped to PTEs where all entries were none. >>> With this change, PTEs within the range mapping the demand-zero page can >>> now be treated as `pte_none` and remapped to a new mTHP, providing more >>> opportunities to take advantage of mTHP. >>> >>> Signed-off-by: Mingzhe Yang >>> Signed-off-by: Lance Yang >>> --- >>> mm/memory.c | 11 +++++++++-- >>> 1 file changed, 9 insertions(+), 2 deletions(-) >>> >>> diff --git a/mm/memory.c b/mm/memory.c >>> index 4e148309b3e0..99ec75c6f0fe 100644 >>> --- a/mm/memory.c >>> +++ b/mm/memory.c >>> @@ -4815,7 +4815,8 @@ static struct folio *alloc_anon_folio(struct vm_fault *vmf) >>> order = highest_order(orders); >>> while (orders) { >>> addr = ALIGN_DOWN(vmf->address, PAGE_SIZE << order); >>> - if (pte_range_none(pte + pte_index(addr), 1 << order)) >>> + if (pte_range_none_or_zeropfn(pte + pte_index(addr), 1 << order, >>> + NULL)) >>> break; >>> order = next_order(&orders, order); >>> } >>> @@ -4867,6 +4868,7 @@ static vm_fault_t do_anonymous_page(struct vm_fault *vmf) >>> { >>> struct vm_area_struct *vma = vmf->vma; >>> unsigned long addr = vmf->address; >>> + bool any_zeropfn = false; >>> struct folio *folio; >>> vm_fault_t ret = 0; >>> int nr_pages = 1; >>> @@ -4939,7 +4941,8 @@ static vm_fault_t do_anonymous_page(struct vm_fault *vmf) >>> if (nr_pages == 1 && vmf_pte_changed(vmf)) { >>> update_mmu_tlb(vma, addr, vmf->pte); >>> goto release; >>> - } else if (nr_pages > 1 && !pte_range_none(vmf->pte, nr_pages)) { >>> + } else if (nr_pages > 1 && !pte_range_none_or_zeropfn( >>> + vmf->pte, nr_pages, &any_zeropfn)) { >>> update_mmu_tlb_range(vma, addr, vmf->pte, nr_pages); >>> goto release; >>> } >>> @@ -4965,6 +4968,10 @@ static vm_fault_t do_anonymous_page(struct vm_fault *vmf) >>> entry = pte_mkuffd_wp(entry); >>> set_ptes(vma->vm_mm, addr, vmf->pte, entry, nr_pages); >>> >>> + /* At least one PTE was mapped to the zero page */ >>> + if (nr_pages > 1 && any_zeropfn) >>> + flush_tlb_range(vma, addr, addr + (nr_pages * PAGE_SIZE)); >> > > Thanks for taking time to review! > >> Do we also need mmu_notifier? >> >> mmu_notifier_range_init(...) >> mmu_notifier_invalidate_range_start(&range); >> >> By the way, this is getting much more complex, but are we seeing any real >> benefits? I’ve tested this before, and it seems that zeropfn-mapped >> anonymous folios are quite rare. > > Hmm... Agreed that it's getting more complex. I don't have any data > showing real benefits yet, so let's put this patch aside for now until I do. The motivation for the huge zeropage case was that we would install the huge zeropage on read fault, to then PTE-map it on write fault and only COW a single PTE. It would require khugepaged to collapse in order to get a THP again. So read-then-immediately-write would not give us a PMD-mapped THP with the huge shared zeropage enabled, but would give us a PMD-mapped THP with the huge shared zeropage disabled, which is rather inconsistent. In case of mTHP, the behavior is at least always the same, and the performance gain is likely not as huge as in the PMD case. Maybe we can just wait until khugepaged will fix it up, where a lot of that complexity will reside either way. So I agree that it might be useful, but also that there must be a good reason to have that complexity here. It might also be a valid question if we should even always use the shared zeropage on read faults with mTHP enabled, because maybe it could be more efficient to avoid the TLB flush when ripping out the shared zeropage etc and just give it an mTHP, if our shrinker works as expected Using the huge zeropage is rather "obviously good" because it also reduces TLB pressure + page table walk and can be zapped fairly easily and efficiently to install a fresh PMD-mapped THP. -- Cheers, David / dhildenb