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(p200300d82f3ae300c6604ff55bfbf5c5.dip0.t-ipconnect.de. [2003:d8:2f3a:e300:c660:4ff5:5bfb:f5c5]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3a568a72cd3sm12056443f8f.32.2025.06.16.13.41.21 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 16 Jun 2025 13:41:22 -0700 (PDT) Message-ID: Date: Mon, 16 Jun 2025 22:41:20 +0200 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 00/11] mm/mremap: introduce more mergeable mremap via MREMAP_RELOCATE_ANON From: David Hildenbrand To: Lorenzo Stoakes , Andrew Morton Cc: Vlastimil Babka , Jann Horn , "Liam R . Howlett" , Suren Baghdasaryan , Matthew Wilcox , Pedro Falcato , Rik van Riel , Harry Yoo , Zi Yan , Baolin Wang , Nico Pache , Ryan Roberts , Dev Jain , Jakub Matena , Wei Yang , Barry Song , linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <7e51e1e2-7272-48d5-9457-40ab87ad7694@redhat.com> 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: <7e51e1e2-7272-48d5-9457-40ab87ad7694@redhat.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 16.06.25 22:24, David Hildenbrand wrote: > Hi Lorenzo, > > as discussed offline, there is a lot going on an this is rather ... a > lot of code+complexity for something that is more a corner cases. :) > > Corner-case as in: only select user space will benefit from this, which > is really a shame. > > After your presentation at LSF/MM, I thought about this further, and I > was wondering whether: > > (a) We cannot make this semi-automatic, avoiding flags. > > (b) We cannot simplify further by limiting it to the common+easy cases > first. > > I think you already to some degree did b) as part of this non-RFC, which > is great. > > > So before digging into the details, let's discuss the high level problem > briefly. > > I think there are three parts to it: > > (1) Detecting whether it is safe to adjust the folio->index (small > folios) > > (2) Performance implications of doing so > > (3) Detecting whether it is safe to adjust the folio->index (large PTE- > mapped folios) > > > Regarding (1), if we simply track whether a folio was ever used for > COW-sharing, it would be very easy: and not only for present folios, but > for any anon folios that are referenced by swap/migration entries. > Skimming over patch #1, I think you apply a similar logic, which is good. > > Regarding (2), it would apply when we mremap() anon VMAs and they happen > to reside next to other anon VMAs. Which workloads are we concerned > about harming by implementing this optimization? I recall that the most > common use case for mremap() is actually for file mappings, but I might > be wrong. In any case, we could just have a different way to enable this > optimization than for each and every mremap() invocation in a process. > > Regarding (3), if we were to split large folios that cross VMA > boundaries during mremap(), it would be simpler. > > How is it handled in this series if we large folio crosses VMA > boundaries? (a) try splitting or (b) fail (not transparent to the user :( ). > > >> This also creates a difference in behaviour, often surprising to users, >> between mappings which are faulted and those which are not - as for the >> latter we adjust vma->vm_pgoff upon mremap() to aid mergeability. >> >> This is problematic firstly because this proliferates kernel allocations >> that are pure memory pressure - unreclaimable and unmovable - >> i.e. vm_area_struct, anon_vma, anon_vma_chain objects that need not exist. > > > Secondly, mremap() exhibits an implicit uAPI in that it does not permit >> remaps which span multiple VMAs (though it does permit remaps that >> constitute a part of a single VMA). > > If I mremap() to create a hole and mremap() it back, I would assume to > automatically get the hole closed again, without special flags. Well, we > both know this is not the case :) > > > > This means that a user must concern themselves with whether merges > succeed >> or not should they wish to use mremap() in such a way which causes multiple >> mremap() calls to be performed upon mappings. > > Right. > >> >> This series provides users with an option to accept the overhead of >> actually updating the VMA and underlying folios via the >> MREMAP_RELOCATE_ANON flag. > > Okay. I wish we could avoid this flag ... > >> >> If MREMAP_RELOCATE_ANON is specified, but an ordinary merge would result in >> the mremap() succeeding, then no attempt is made at relocation of folios as >> this is not required. > > Makes sense. This is the existing behavior then. > >> >> Even if no merge is possible upon moving of the region, vma->vm_pgoff and >> folio->index fields are appropriately updated in order that subsequent >> mremap() or mprotect() calls will succeed in merging. > > By looking at the surrounding VMAs or simply by trying to always keep > the folio->index to correspond to the address in the VMA? (just if > mremap() never happened, I assume?) > >> >> This flag falls back to the ordinary means of mremap() should the operation >> not be feasible. It also transparently undoes the operation, carefully >> holding rmap locks such that no racing rmap operation encounters incorrect >> or missing VMAs. > > I absolutely dislike this undo operation, really. :( > > I hope we can find a way to just detect early whether this optimization > would work. > > Which are the exact error cases you can run into for un-doing? > > I assume: > > (a) cow-shared anon folio (can detect early) > > (b) large folios crossing VMAs (TBD) > > (c) KSM folios? Probably we could move them, I *think* we would have to > update the ksm_rmap_item. Alternatively, we could indicate if a VMA had > any KSM folios and give up early in the first version. Looking at patch #1, I can see that we treat KSM folios as "success". I would have thought we would have to update the corresponding "ksm_rmap_item" ... somehow, to keep the rmap working. I know that Wei Yang (already on cc) is working on selftests, which I am yet to review, but he doesn't cover mremap() yet. Looking at rmap_walk_ksm(), I am left a bit confused. We walk all entries in the stable tree (ksm_rmap_item), looking in the anon_vma interval tree for the entry that corresponds to ksm_rmap_item->address. addr = rmap_item->address & PAGE_MASK; if (addr < vma->vm_start || addr >= vma->vm_end) continue; So I would assume, already when we mremap() ... we are *already* breaking KSM rmap walkers? :) Or there is somewhere some magic that I am missing. A KSM mremap test case for rmap would be nice ;) -- Cheers, David / dhildenb