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[2003:cb:c71c:a700:bba7:849a:ecf1:5404]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-434b0dbe40csm49883405e9.10.2024.11.29.03.30.26 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 29 Nov 2024 03:30:27 -0800 (PST) Message-ID: Date: Fri, 29 Nov 2024 12:30:25 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [QUESTION] mmap lock in khugepaged To: Dev Jain , ryan.roberts@arm.com, kirill.shutemov@linux.intel.com, willy@infradead.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, ziy@nvidia.com References: <20241128062641.59096-1-dev.jain@arm.com> <4cb26a06-d982-4ca3-a5f7-7c6a6c63428c@arm.com> <3d4c57dd-0821-4684-9018-db8274c170ec@redhat.com> <66bb7496-a445-4ad7-8e56-4f2863465c54@arm.com> From: David Hildenbrand 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: <66bb7496-a445-4ad7-8e56-4f2863465c54@arm.com> X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: TrqPIMpSmSHvwmmRujjG2xJoeBtBxmjCzOxpxU5mG6I_1732879829 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Stat-Signature: tfnshy4hzpj6e54z6ajojz9mt8r4a6ro X-Rspamd-Queue-Id: 7C252C0020 X-Rspam-User: X-Rspamd-Server: rspam01 X-HE-Tag: 1732879828-377888 X-HE-Meta: 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 XQXNqOqI BICSGKUKODzojvLIIgHW5pkO8c4T49EJxlDsQEkAzdRArg8wEvv202ufzid6/Nr3AdKVlvHrn4MBaQ0Q3p21ZVGQKPIjC+ZK70X3DY0jQ7ZIczF5Iii8B8a3kQm/CK3owx4F51HHZBDRHdUrP8PttVJEtIXMfidIDecQ5V80q1pd8zwedziSwg+mYuQ3ss7ajXSo5UD9AKAtYLYF8OK2Jn203mmaMmFleGMURxBG5ahX3pSS7o1r3GRo7xzWsMNWEVmxeLq87pD+hUZa6r8xaZPGfq9yOTZsLvESFM5UqMG1+s9ZuWMXuBSKSLmSN2SeB7ZheXIhJ6CvmjfiGb0jD76kTVtymSi/jXrfvklSdjyb3mP4sOHEMVF1rEA0+dA4Bg71H98ZPFplksjo= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: >> >> Thinking about it, I am also not sure if most other code can deal with >> temporary pmd_none(). These don't necessarily take the PMD lock, >> because "why would they" right now. >> >> See walk_pmd_range() as one example, if it spots pmd_none() it assumes >> "there really is nothing" without checking the PMD lock. >> >> As a more concrete example, assume someone calls MADV_DONTNEED and we >> end up in zap_pmd_range(). If we assume "pmd_none() == really nothing" >> we'll skip that entry without getting the PMD lock involved. That >> would mean that you would be breaking MADV_DONTNEED if you managed to >> collapse or not -- memory would not get discarded. >> >> This is a real problem with anonymous memory. >> > Yes, I thought of different locking fashions but the problem seems to be > that any pagetable walker will choose an action path according to the value > it sees. > >> >> Unless I am missing something it's all very tricky and there might be >> a lot of such code that assumes "if I hold a mmap lock / VMA lock in >> read mode, pmd_none() means there is nothing even without holding the >> PMD lock when checking". > > Yes, I was betting on the fact that, if the code assumes that pmd_none() > means there is nothing, eventually when it will take the PMD lock to > write to it, it will check whether > the PMD changed, and back off. I wasn't aware of the MADV_DONTNEED > thingy, thanks. Note that this is just the tip of the iceberg. Most page table walkers that deal with anonymous memory have the same requirement. >>>> >>>> >>>> I recall that for shmem that's "easier", because we don't have to >>>> reinstall a PMD immediately, we cna be sure that the page table is >>>> kept empty/unmodified, ... >>>> >>> > > All in all, the general solution seems to be that, if I can take all > pagetable walkers into an invalid state and make them backoff, then I am > safe. > For example, we do not zero out the PMD, we take the pte PTL, we do> stuff, then instead of setting the PTEs to zero, we set it to a universal > invalid state upon which no pagetable walker can take an action; an > instance of that can be to set the PTE to a swap entry such that the fault > handler ends up in do_swap_page() ->print_bad_pte(). So now we can take > the PMD lock (in fact we don't need it since any pagetable walking > is rendered useless) and populate the PMD to resume the new pagetable > walking. Another *ridiculous* idea may be to remember the PGD we > came from and nuke it (but I guess there is an alternate path for that > in walk_pgd_range() and so on?) What might work is introducing a PMD marker (note: we don't have PMD markers yet) for that purpose. Likely, the PMD marker may only be set while the PMD lock is being held, and we must not drop the PMD lock temporarily (otherwise people will spin on the value by repeadetly (un)locking the PMD lock, which is stupid). Then you have to make sure that each and every page table walker handles that properly. Then, there is the problem with holding the PMD lock for too long as I mentioned. Last but not least, there are more issues that I haven't even described before (putting aside the other issue): Our page table walkers can handle the transitions: PMD leaf -> PMD none * concurrent zap / MADV_DONTNEED / !anon PMD split PMD leaf -> PTE table * concurrent anon PMD split * concurrent zap / MADV_DONTNEED / !anon PMD split + PTE table allocation PTE table (empty) -> PMD none * concurrent page table reclaim, part of collapsing file THPs * collapse_pte_mapped_thp() * retract_page_tables() I think they *cannot* tolerate the transition *properly*: PTE table (non-empty) -> PMD leaf * what you want do do ;) -> Observe how we skip to the next PMD in all page table walkers / give up if pte_offset_map_lock() and friends fail! I even think there are more issues hiding there with pte_offset_map_ro_nolock(). Of course, on top of all of this, you should net significantly degrade the ordinary page table walker performance ... I don't want to discourage you, but it's all extremely complicated. -- Cheers, David / dhildenb