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[2003:cb:c74b:d000:3a9:de5c:9ae6:ccb3]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4315069032asm25144845e9.0.2024.10.16.08.47.40 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 16 Oct 2024 08:47:41 -0700 (PDT) Message-ID: <76f4ed45-5a40-4ac4-af24-a40effe7725c@redhat.com> Date: Wed, 16 Oct 2024 17:47:39 +0200 Precedence: bulk X-Mailing-List: linux-s390@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 1/7] s390/kdump: implement is_kdump_kernel() To: Alexander Egorenkov Cc: agordeev@linux.ibm.com, akpm@linux-foundation.org, borntraeger@linux.ibm.com, cohuck@redhat.com, corbet@lwn.net, eperezma@redhat.com, frankja@linux.ibm.com, gor@linux.ibm.com, hca@linux.ibm.com, imbrenda@linux.ibm.com, jasowang@redhat.com, kvm@vger.kernel.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-s390@vger.kernel.org, mcasquer@redhat.com, mst@redhat.com, svens@linux.ibm.com, thuth@redhat.com, virtualization@lists.linux.dev, xuanzhuo@linux.alibaba.com, zaslonko@linux.ibm.com References: <87ed4g5fwk.fsf@li-0ccc18cc-2c67-11b2-a85c-a193851e4c5d.ibm.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: <87ed4g5fwk.fsf@li-0ccc18cc-2c67-11b2-a85c-a193851e4c5d.ibm.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit >> >> When I wrote that code I was rather convinced that the variant in this patch >> is the right thing to do. > > A short explanation about what a stand-alone kdump is. > > * First, it's not really a _regular_ kdump activated with kexec-tools and > executed by Linux itself but a regular stand-alone dump (SCSI) from the > FW's perspective (one has to use HMC or dumpconf to execute it and not > with kexec-tools like for the _regular_ kdump). Ah, that makes sense. > * One has to reserve crashkernel memory region in the old crashed kernel > even if it remains unused until the dump starts. > * zipl uses regular kdump kernel and initramfs to create stand-alone > dumper images and to write them to a dump disk which is used for > IPLIng the stand-alone dumper. > * The zipl bootloader takes care of transferring the old kernel memory > saved in HSA by the FW to the crashkernel memory region reserved by the old > crashed kernel before it enters the dumper. The HSA memory is released > by the zipl bootloader _before_ the dumper image is entered, > therefore, we cannot use HSA to read old kernel memory, and instead > use memory from crashkernel region, just like the regular kdump. > * is_ipl_type_dump() will be true for a stand-alone kdump because we IPL > the dumper like a regular stand-alone dump (e.g. zfcpdump). > * Summarized, zipl bootloader prepares an environment which is expected by > the regular kdump for a stand-alone kdump dumper before it is entered. Thanks for the details! > > In my opinion, the correct version of is_kdump_kernel() would be > > bool is_kdump_kernel(void) > { > return oldmem_data.start; > } > > because Linux kernel doesn't differentiate between both the regular > and the stand-alone kdump where it matters while performing dumper > operations (e.g. reading saved old kernel memory from crashkernel memory region). > Right, but if we consider "/proc/vmcore is available", a better version would IMHO be: bool is_kdump_kernel(void) { return dump_available(); } Because that is mostly (not completely) how is_kdump_kernel() would have worked right now *after* we had the elfcorehdr_alloc() during the fs_init call. > Furthermore, if i'm not mistaken then the purpose of is_kdump_kernel() > is to tell us whether Linux kernel runs in a kdump like environment and not > whether the current mode is identical to the proper and true kdump, > right ? And if stand-alone kdump swims like a duck, quacks like one, then it > is one, regardless how it was started, by kexecing or IPLing > from a disk. Same thinking here. > > The stand-alone kdump has a very special use case which most users will > never encounter. And usually, one just takes zfcpdump instead which is > more robust and much smaller considering how big kdump initrd can get. > stand-alone kdump dumper images cannot exceed HSA memory limit on a Z machine. Makes sense, so it boils down to either bool is_kdump_kernel(void) { return oldmem_data.start; } Which means is_kdump_kernel() can be "false" even though /proc/vmcore is available or bool is_kdump_kernel(void) { return dump_available(); } Which means is_kdump_kernel() can never be "false" if /proc/vmcore is available. There is the chance of is_kdump_kernel() being "true" if "elfcorehdr_alloc()" fails with -ENODEV. You're call :) Thanks! -- Cheers, David / dhildenb