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Sun, 23 Mar 2025 05:53:03 -0700 (PDT) Received: from [172.20.3.205] ([99.209.85.25]) by smtp.gmail.com with ESMTPSA id d75a77b69052e-4771d18d541sm33433681cf.36.2025.03.23.05.53.02 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Sun, 23 Mar 2025 05:53:02 -0700 (PDT) Message-ID: <01803b0f-47ff-429c-a6af-33fee39b957b@redhat.com> Date: Sun, 23 Mar 2025 13:53:02 +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 1/7] mm/mremap: introduce more mergeable mremap via MREMAP_RELOCATE_ANON To: Lorenzo Stoakes Cc: Jann Horn , Andrew Morton , Vlastimil Babka , "Liam R . Howlett" , Suren Baghdasaryan , Matthew Wilcox , linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <182bf1ce-1b67-4243-854b-4d0c26aae563@redhat.com> <21f89b73-aaae-4674-aea2-aefc7a4847d9@redhat.com> <9f81bfe4-4cc7-4754-b92f-db3a4e549f86@lucifer.local> 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: <9f81bfe4-4cc7-4754-b92f-db3a4e549f86@lucifer.local> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 22.03.25 08:21, Lorenzo Stoakes wrote: > On Sat, Mar 22, 2025 at 07:17:05AM +0100, David Hildenbrand wrote: >> On 22.03.25 06:33, David Hildenbrand wrote: >>> On 22.03.25 01:14, Jann Horn wrote: >>>> On Fri, Mar 21, 2025 at 10:54 PM Lorenzo Stoakes >>>> wrote: >>>>> diff --git a/mm/mremap.c b/mm/mremap.c >>>>> index 0865387531ed..bb67562a0114 100644 >>>>> --- a/mm/mremap.c >>>>> +++ b/mm/mremap.c >>>> [...] >>>>> +/* >>>>> + * If the folio mapped at the specified pte entry can have its index and mapping >>>>> + * relocated, then do so. >>>>> + * >>>>> + * Returns the number of pages we have traversed, or 0 if the operation failed. >>>>> + */ >>>>> +static unsigned long relocate_anon(struct pagetable_move_control *pmc, >>>>> + unsigned long old_addr, unsigned long new_addr, pte_t pte, >>>>> + bool undo) >>>>> +{ >>>>> + struct page *page; >>>>> + struct folio *folio; >>>>> + struct vm_area_struct *old, *new; >>>>> + pgoff_t new_index; >>>>> + unsigned long ret = 1; >>>>> + >>>>> + old = pmc->old; >>>>> + new = pmc->new; >>>>> + >>>>> + /* Ensure we have truly got an anon folio. */ >>>>> + page = vm_normal_page(old, old_addr, pte); >>>>> + if (!page) >>>>> + return ret; >>>>> + folio = page_folio(page); >>>>> + folio_lock(folio); >>>>> + >>>>> + /* no-op. */ >>>>> + if (!folio_test_anon(folio) || folio_test_ksm(folio)) >>>>> + goto out; >>>>> + >>>>> + /* >>>>> + * This should not happen as we explicitly disallow this, but check >>>>> + * anyway. >>>>> + */ >>>>> + if (folio_test_large(folio)) { >>>>> + ret = 0; >>>>> + goto out; >>>>> + } >>>> >>>> Do I understand correctly that you assume here that the page is >>>> exclusively mapped? Maybe we could at least >>>> WARN_ON(folio_mapcount(folio) != 1) or something like that? >>>> >>>> (I was also wondering if the PageAnonExclusive bit is somehow >>>> relevant, but we should probably not look at or touch that here, >>>> unless we want to think about cases where we _used to_ have a child >>>> from which the page may have been GUP'd...) >>> >>> UFFDIO_MOVE implements something similar. Right now we keep it simple: >>> >>> if (folio_test_large(src_folio) || >>> folio_maybe_dma_pinned(src_folio) || >>> !PageAnonExclusive(&src_folio->page)) { >>> err = -EBUSY; >>> goto out; >>> } >>> >>> Whereby we >>> >>> a) Make sure we cover all PTEs (-> small folio, single PTE). Large >>> PTE-mapped folios are split. >>> >>> b) Make sure there are no GUP pins (maybe not required here?) >>> >>> c) The folio is exclusive to this process >> >> On additional note as my memory comes back: if PAE is set, there cannot be >> other (inactive) mappings from the swapcache. So whenever we use folio lock >> + mapcount data, the possibility of the swapcache (having inactive mappings >> from other processes etc.) must be considered. > > Ack, do you have a feel for how such a check would work? Likely under folio lock if (folio_test_swapcache(folio) && !folio_free_swap(folio)) { /* unable to move. */ folio_unlock(folio) return -ENOTGOINGTOHAPPEN; } -- Cheers, David / dhildenb