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[2003:cb:c71b:2c00:7bfb:29fe:9e6f:2e65]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-386219096adsm14438726f8f.85.2024.12.09.13.25.02 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 09 Dec 2024 13:25:04 -0800 (PST) Message-ID: <4825dc70-bca4-437f-a406-df0936aada85@redhat.com> Date: Mon, 9 Dec 2024 22:25:02 +0100 Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3 0/7] hugetlbfs memory HW error fixes To: William Roche , kvm@vger.kernel.org, qemu-devel@nongnu.org, qemu-arm@nongnu.org Cc: peterx@redhat.com, pbonzini@redhat.com, richard.henderson@linaro.org, philmd@linaro.org, peter.maydell@linaro.org, mtosatti@redhat.com, imammedo@redhat.com, eduardo@habkost.net, marcel.apfelbaum@gmail.com, wangyanan55@huawei.com, zhao1.liu@intel.com, joao.m.martins@oracle.com References: <20241125142718.3373203-1-william.roche@oracle.com> <48b09647-d2ba-43e5-8e73-16fb4ace6da5@oracle.com> <874e2625-b5e7-4247-994a-9b341abbdceb@redhat.com> <753a033c-7341-4a3a-8546-c31a50d35aff@redhat.com> <31693f78-dbef-4465-9c8e-a68a25cb4af2@oracle.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: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 06.12.24 19:26, William Roche wrote: > On 12/3/24 16:00, David Hildenbrand wrote: >> On 03.12.24 15:39, William Roche wrote: >>> [...] >>> Our new Qemu code is testing first the fallocate+MADV_DONTNEED procedure >>> for standard sized pages (in ram_block_discard_range()) and only folds >>> back to the mmap() use if it fails. So maybe my proposal to implement: >>> >>> +                    /* >>> +                     * Fold back to using mmap(), but it should not >>> repair a >>> +                     * shared file memory region. In this case we fail. >>> +                     */ >>> +                    if (block->fd >= 0 && qemu_ram_is_shared(block)) { >>> +                        error_report("Shared memory poison recovery >>> failure addr: " >>> +                                     RAM_ADDR_FMT "@" RAM_ADDR_FMT "", >>> +                                     length, addr); >>> +                        exit(1); >>> +                    } >>> >>> Could be the right choice. >> >> Right. But then, what about a mmap(MAP_PRIVATE, shmem), where the >> pagecache page is poisoned and needs an explicit fallocate? :) >> >> It's all tricky. I wonder if we should just say "if it's backed by a >> file, and we cannot discard, then mmap() can't fix it reliably". >> >> if (block->fd >= 0) { >>     ... >> } >> >> After all, we don't even expect the fallocate/MADV_DONTNEED to ever >> fail :) So I was also wondering if we could get rid of the >> mmap(MAP_FIXED) completely ... but who knows what older Linux kernels do. > Hi, > I agree that we expect the ram_block_discard_range() function to be > working on recent kernels, and to do what's necessary to reuse the > poisoned memory area. > > The case where older kernels could be a problem is where fallocate() > would not work on a given file In that case we cannot possibly handle it correctly in all with mmap(MAP_FIXED), especially not with shmem/hugetlb :/ So failing is correct. , or madvice(MADV_DONTNEED or MADV_REMOVE) > would not work on standard sized pages. I assume both can be expected to work in any reasonable Linux setup today (-hugetlb) :) > As ram_block_discard_range() > currently avoids using madvise on huge pages. Right, because support might not be around for hugetlb. > > In this case, the generic/minimal way to make the memory usable again > (in all cases) would be to: > - fallocate/PUNCH_HOLE the given file (if any) > - and remap the area > even if it's not _mandatory_ in all cases. > > Would you like me to add an fallocate(PUNCH_HOLE) call in the helper > function qemu_ram_remap_mmap() when a file descriptor is provided > (before remapping the area) ? > > This way, we don't need to know if ram_block_discard_range() failed on > the fallocate() or the madvise(); in the worst case scenario, we would > PUNCH twice. If fallocate fails or mmap fails, we exit. > I haven't seen a problem punching a file twice - do you see any problem ? Hm. I'd like to avoid another fallocate(). It would really only make sense if we expect fallocate() to work but madvise() to fail; and I don't think that's our expectation. virtio-balloon has been using ram_block_discard_range() forever, and so far nobody really ever reported any of the errors from the function to the best of my knowledge ... > > Do you find this possibility acceptable ? Or should I just go for the > immediate failure when ram_block_discard_range() fails on a case with a > file descriptor as you suggest ? > > Please let me know if you find any problem with this approach, as it > could help to have this poison recovery scenario to work on more kernels. I'd say, let's keep it simple. Try ram_block_discard_range() if that fails (a) and we have a file, bail out, (b) we don't have a file, do the remap(). If we ever run into issues with that approach, we can investigate why it fails and what to do about it (e.g., fallocate). My best guess it that the whole remap part can be avoided completely. -- Cheers, David / dhildenb