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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 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 Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspam-User: X-Stat-Signature: x8bpbktc7jmu6b6x73qqhby6b9ypikji X-Rspamd-Queue-Id: 90FDF1C0002 X-Rspamd-Server: rspam11 X-HE-Tag: 1729093649-778087 X-HE-Meta: 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 4bEsqPL/ GKfbBX5nQkiBDft/A46NW4Sa7GRxhbBfksZNFxtyHjkMjM/6o+S0oh7RdR+Ulu/SFnCJy+H//0Qwmsr39k6LceO/g6cVVgH1vNu7r8++bERZcee8sY7ilUGh06G+M66B71OC7PYZ2WV8giM/No5HWw1mAXxTsKzHpwapVUwWIpVOuGPKZHcU/EQE3ZU6yiTMOvgyi1tjA/6kG6QLltxFSjA54x7crEnu2rrpDaltq/dVK81xkVITzcydnZ7fyZy5pbWtGqK9IoTHK0wNidBaDiDFaDvQkywEh13aikOALwHFDoD5eDVBhMibrJgjbr1H6G0Z+Dokh85ya11z4K5/D0VwPFrpwDhG6oqXSdjTd3+ZeyCoKeuFTk3GpIoVnvE9WDTBTE8YAqu3kG/GsY4U8XKaYpNq8hJGSLF76yBsx0A6Ez9BrkgeBjp0xnDMylZvY6zEJ X-Bogosity: Ham, tests=bogofilter, spamicity=0.000027, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: >> >> 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