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Sat, 03 Oct 2026 10:02:30 -0700 (PDT) X-Received: by 2002:a05:600c:3e19:b0:49b:8f5e:51fb with SMTP id 5b1f17b1804b1-4a027534b98mr106170055e9.3.1791046949625; Sat, 03 Oct 2026 10:02:29 -0700 (PDT) Received: from px13 ([2a06:5902:822:4600:184c:5422:d333:cf3f]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4a02752aeb3sm192763715e9.0.2026.10.03.10.02.28 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sat, 03 Oct 2026 10:02:29 -0700 (PDT) Date: Sat, 3 Oct 2026 18:02:27 +0100 From: Chris Bainbridge To: Mike Rapoport , Pasha Tatashin , Jason Miu , Pratyush Yadav Cc: Alexander Graf , Ran Xiaokai , regressions@lists.linux.dev, linux-kernel@vger.kernel.org, kexec@lists.infradead.org, linux-mm@kvack.org Subject: [BUG] KHO handover hangs when memmap reserves PMEM Message-ID: Precedence: bulk X-Mailing-List: regressions@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Hello, I am reporting a regression in KHO handover. In the most recent Ubuntu 26.10 beta mini-ISO releases (2026-09-18 onwards), the first kernel reserves RAM for the downloaded ISO with a memmap directive such as memmap=!4G, then kexecs a second kernel. On affected kernels, the first kernel reaches "kexec_core: Starting new kernel", but the second kernel produces no further output and QEMU spins at 100% CPU. On unaffected kernels, the second kernel starts and finds /dev/pmem0 with the expected "Persistent Memory (legacy)" range in /proc/iomem. A source bisect identified 3f2ad90060f6 ("kho: adopt radix tree for preserved memory tracking") as the first bad commit; reverting it at the bisected revision restored the handover. The regression appears related to KHO metadata being accessed after the receiving kernel's memmap directive changes how the metadata's physical range is mapped. #regzbot introduced: 3f2ad90060f6 I have an AI-generated candidate fix that changes 22 files. I have not attached or published it: although the AI did extensive verification on both x64 and arm64, I cannot review or explain the complete change, so I am not submitting it as a fix. If one of you thinks an unreviewed candidate would be useful for further analysis, please let me know.