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[2003:cb:c71b:7e00:9339:4017:7111:82d0]) by smtp.gmail.com with ESMTPSA id bk19-20020a0560001d9300b0033e2291fbc0sm2469006wrb.68.2024.03.22.10.08.08 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 22 Mar 2024 10:08:09 -0700 (PDT) Message-ID: Date: Fri, 22 Mar 2024 18:08:08 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v1 2/2] s390/mm: re-enable the shared zeropage for !PV and !skeys KVM guests To: Christian Borntraeger , linux-kernel@vger.kernel.org Cc: linux-mm@kvack.org, Janosch Frank , Claudio Imbrenda , Heiko Carstens , Vasily Gorbik , Andrew Morton , Peter Xu , Alexander Gordeev , Sven Schnelle , Gerald Schaefer , Andrea Arcangeli , kvm@vger.kernel.org, linux-s390@vger.kernel.org References: <20240321215954.177730-1-david@redhat.com> <20240321215954.177730-3-david@redhat.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: X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: D5F32120017 X-Rspam-User: X-Rspamd-Server: rspam05 X-Stat-Signature: yrskfkqzdzrysjz839azuui6dnbqoddw X-HE-Tag: 1711127293-330197 X-HE-Meta: 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 vvvdzyRR 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000030, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 22.03.24 11:22, Christian Borntraeger wrote: > > > Am 21.03.24 um 22:59 schrieb David Hildenbrand: >> commit fa41ba0d08de ("s390/mm: avoid empty zero pages for KVM guests to >> avoid postcopy hangs") introduced an undesired side effect when combined >> with memory ballooning and VM migration: memory part of the inflated >> memory balloon will consume memory. >> >> Assuming we have a 100GiB VM and inflated the balloon to 40GiB. Our VM >> will consume ~60GiB of memory. If we now trigger a VM migration, >> hypervisors like QEMU will read all VM memory. As s390x does not support >> the shared zeropage, we'll end up allocating for all previously-inflated >> memory part of the memory balloon: 50 GiB. So we might easily >> (unexpectedly) crash the VM on the migration source. >> >> Even worse, hypervisors like QEMU optimize for zeropage migration to not >> consume memory on the migration destination: when migrating a >> "page full of zeroes", on the migration destination they check whether the >> target memory is already zero (by reading the destination memory) and avoid >> writing to the memory to not allocate memory: however, s390x will also >> allocate memory here, implying that also on the migration destination, we >> will end up allocating all previously-inflated memory part of the memory >> balloon. >> >> This is especially bad if actual memory overcommit was not desired, when >> memory ballooning is used for dynamic VM memory resizing, setting aside >> some memory during boot that can be added later on demand. Alternatives >> like virtio-mem that would avoid this issue are not yet available on >> s390x. >> >> There could be ways to optimize some cases in user space: before reading >> memory in an anonymous private mapping on the migration source, check via >> /proc/self/pagemap if anything is already populated. Similarly check on >> the migration destination before reading. While that would avoid >> populating tables full of shared zeropages on all architectures, it's >> harder to get right and performant, and requires user space changes. >> >> Further, with posctopy live migration we must place a page, so there, >> "avoid touching memory to avoid allocating memory" is not really >> possible. (Note that a previously we would have falsely inserted >> shared zeropages into processes using UFFDIO_ZEROPAGE where >> mm_forbids_zeropage() would have actually forbidden it) >> >> PV is currently incompatible with memory ballooning, and in the common >> case, KVM guests don't make use of storage keys. Instead of zapping >> zeropages when enabling storage keys / PV, that turned out to be >> problematic in the past, let's do exactly the same we do with KSM pages: >> trigger unsharing faults to replace the shared zeropages by proper >> anonymous folios. >> >> What about added latency when enabling storage kes? Having a lot of >> zeropages in applicable environments (PV, legacy guests, unittests) is >> unexpected. Further, KSM could today already unshare the zeropages >> and unmerging KSM pages when enabling storage kets would unshare the >> KSM-placed zeropages in the same way, resulting in the same latency. >> >> Signed-off-by: David Hildenbrand > > Nice work. Looks good to me and indeed it fixes the memory > over-consumption that you mentioned. Thanks for the very fast review and test! > > Reviewed-by: Christian Borntraeger > Tested-by: Christian Borntraeger > (can also be seen with virsh managedsave; virsh start) > > I guess its too invasive for stable, but I would say it is real fix. Should we add a Fixes: Tag? I refrained from doing so, treating this more like an optimization to restore the intended behavior at least as long as the VM does not use storage keys. -- Cheers, David / dhildenb