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[2003:cb:c72a:f600:c577:b537:f52a:b994]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-388c8060566sm8570537f8f.102.2024.12.16.09.06.04 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 16 Dec 2024 09:06:06 -0800 (PST) Message-ID: <2215dd8e-233a-427b-b15c-a2ffbce8f46d@redhat.com> Date: Mon, 16 Dec 2024 18:06:04 +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 PATCH 09/12] khugepaged: Introduce vma_collapse_anon_folio() To: Dev Jain , akpm@linux-foundation.org, willy@infradead.org, kirill.shutemov@linux.intel.com Cc: ryan.roberts@arm.com, anshuman.khandual@arm.com, catalin.marinas@arm.com, cl@gentwo.org, vbabka@suse.cz, mhocko@suse.com, apopple@nvidia.com, dave.hansen@linux.intel.com, will@kernel.org, baohua@kernel.org, jack@suse.cz, srivatsa@csail.mit.edu, haowenchao22@gmail.com, hughd@google.com, aneesh.kumar@kernel.org, yang@os.amperecomputing.com, peterx@redhat.com, ioworker0@gmail.com, wangkefeng.wang@huawei.com, ziy@nvidia.com, jglisse@google.com, surenb@google.com, vishal.moola@gmail.com, zokeefe@google.com, zhengqi.arch@bytedance.com, jhubbard@nvidia.com, 21cnbao@gmail.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20241216165105.56185-1-dev.jain@arm.com> <20241216165105.56185-10-dev.jain@arm.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: <20241216165105.56185-10-dev.jain@arm.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 16.12.24 17:51, Dev Jain wrote: > In contrast to PMD-collapse, we do not need to operate on two levels of pagetable > simultaneously. Therefore, downgrade the mmap lock from write to read mode. Still > take the anon_vma lock in exclusive mode so as to not waste time in the rmap path, > which is anyways going to fail since the PTEs are going to be changed. Under the PTL, > copy page contents, clear the PTEs, remove folio pins, and (try to) unmap the > old folios. Set the PTEs to the new folio using the set_ptes() API. > > Signed-off-by: Dev Jain > --- > Note: I have been trying hard to get rid of the locks in here: we still are > taking the PTL around the page copying; dropping the PTL and taking it after > the copying should lead to a deadlock, for example: > khugepaged madvise(MADV_COLD) > folio_lock() lock(ptl) > lock(ptl) folio_lock() > > We can create a locked folio list, altogether drop both the locks, take the PTL, > do everything which __collapse_huge_page_isolate() does *except* the isolation and > again try locking folios, but then it will reduce efficiency of khugepaged > and almost looks like a forced solution :) > Please note the following discussion if anyone is interested: > https://lore.kernel.org/all/66bb7496-a445-4ad7-8e56-4f2863465c54@arm.com/ > (Apologies for not CCing the mailing list from the start) > > mm/khugepaged.c | 108 ++++++++++++++++++++++++++++++++++++++---------- > 1 file changed, 87 insertions(+), 21 deletions(-) > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c > index 88beebef773e..8040b130e677 100644 > --- a/mm/khugepaged.c > +++ b/mm/khugepaged.c > @@ -714,24 +714,28 @@ static void __collapse_huge_page_copy_succeeded(pte_t *pte, > struct vm_area_struct *vma, > unsigned long address, > spinlock_t *ptl, > - struct list_head *compound_pagelist) > + struct list_head *compound_pagelist, int order) > { > struct folio *src, *tmp; > pte_t *_pte; > pte_t pteval; > > - for (_pte = pte; _pte < pte + HPAGE_PMD_NR; > + for (_pte = pte; _pte < pte + (1UL << order); > _pte++, address += PAGE_SIZE) { > pteval = ptep_get(_pte); > if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) { > add_mm_counter(vma->vm_mm, MM_ANONPAGES, 1); > if (is_zero_pfn(pte_pfn(pteval))) { > - /* > - * ptl mostly unnecessary. > - */ > - spin_lock(ptl); > - ptep_clear(vma->vm_mm, address, _pte); > - spin_unlock(ptl); > + if (order == HPAGE_PMD_ORDER) { > + /* > + * ptl mostly unnecessary. > + */ > + spin_lock(ptl); > + ptep_clear(vma->vm_mm, address, _pte); > + spin_unlock(ptl); > + } else { > + ptep_clear(vma->vm_mm, address, _pte); > + } > ksm_might_unmap_zero_page(vma->vm_mm, pteval); > } > } else { > @@ -740,15 +744,20 @@ static void __collapse_huge_page_copy_succeeded(pte_t *pte, > src = page_folio(src_page); > if (!folio_test_large(src)) > release_pte_folio(src); > - /* > - * ptl mostly unnecessary, but preempt has to > - * be disabled to update the per-cpu stats > - * inside folio_remove_rmap_pte(). > - */ > - spin_lock(ptl); > - ptep_clear(vma->vm_mm, address, _pte); > - folio_remove_rmap_pte(src, src_page, vma); > - spin_unlock(ptl); > + if (order == HPAGE_PMD_ORDER) { > + /* > + * ptl mostly unnecessary, but preempt has to > + * be disabled to update the per-cpu stats > + * inside folio_remove_rmap_pte(). > + */ > + spin_lock(ptl); > + ptep_clear(vma->vm_mm, address, _pte); > + folio_remove_rmap_pte(src, src_page, vma); > + spin_unlock(ptl); > + } else { > + ptep_clear(vma->vm_mm, address, _pte); > + folio_remove_rmap_pte(src, src_page, vma); > + } As I've talked to Nico about this code recently ... :) Are you clearing the PTE after the copy succeeded? If so, where is the TLB flush? How do you sync against concurrent write acess + GUP-fast? The sequence really must be: (1) clear PTE/PMD + flush TLB (2) check if there are unexpected page references (e.g., GUP) if so back off (3) copy page content (4) set updated PTE/PMD. To Nico, I suggested doing it simple initially, and still clear the high-level PMD entry + flush under mmap write lock, then re-map the PTE table after modifying the page table. It's not as efficient, but "harder to get wrong". Maybe that's already happening, but I stumbled over this clearing logic in __collapse_huge_page_copy_succeeded(), so I'm curious. -- Cheers, David / dhildenb