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b=GUXRZ7/pbe/AJpkN5e5FcdLKuJgDihokRM9FwbQxVdtvp+2LRSkZRdVJpTE58W4V6 NadEbJJreUG5ZHlSm5lZ/kScR9TQREkuTlYMfmadYLCnnhcsFnoxqr2OiLnEUNZWuq ezVr4PgORcyU4YJMkI9wQr6DJOzeoucauYENm7ogq5W1+ZswY3InNw/cE9J6Qp5m7D cvQCMK5tlmnYwkiHKEmF5ux/U5Wfqstw1sIxE2ErO9W6TbmevR9YrLXjkXZrkcTvuD NW0AbPYQSCmA2pBAFsyhQ7q7ZYL7z/j4tXZa8uou3/iBwv4XtuDPfDM2G1wFOlRTFC ONz6uDRN7u2yA== Date: Wed, 7 Oct 2026 08:11:50 +0200 From: Mike Rapoport To: Chris Bainbridge Cc: Pasha Tatashin , Jason Miu , Pratyush Yadav , Alexander Graf , Ran Xiaokai , regressions@lists.linux.dev, linux-kernel@vger.kernel.org, kexec@lists.infradead.org, linux-mm@kvack.org Subject: Re: [BUG] KHO handover hangs when memmap reserves PMEM Message-ID: References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspam-User: X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: 98550A0007 X-Stat-Signature: z5hfy4bx6n6tjyo8o6fiw8knkbuqyokm X-HE-Tag: 1791353518-498137 X-HE-Meta: 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 EfEgGIYg z+wjp0Iz4MYMVfMJ/ik5lPrvesru6360yPtlbnETNURm2cAlIskE4OIeJy0psTfNnw/qqt0burtLLEBI8KotE5PfLMnl+6NIwASQeTy9lDQTwFQ2jscAVgjqPrQ14edrBeF60HVkVdyWUo0eLojWSFtoKHvunqEqbLyBkkO6VuBf4rspOwaIscbuFGXDvRl7PodOjjiz9T7vaZ5WAet6jhTu5bJoMAxcjU5pcQIcUBEYS6XRbpo/hGoYGgnfnC3UWgJdB Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Tue, Oct 06, 2026 at 07:02:22PM +0100, Chris Bainbridge wrote: > On Tue, Oct 06, 2026 at 12:55:31PM +0200, Mike Rapoport wrote: > > On Sat, Oct 03, 2026 at 06:02:27PM +0100, Chris Bainbridge wrote: > > > 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. > > > > TO make sure I understand this correctly, you enable KHO in the builds of > > the installer kernel and then the installer command line explicitly enables > > KHO. > > Yes. > > > Is there anything you actually preserve with KHO when kexec'ing the second > > kernel? > > No. kho=off is a valid workaround for the mini ISO. I'd even say that setting CONFIG_KEXEC_HANDOVER_ENABLE_DEFAULT=n and even CONFIG_KEXEC_HANDOVER=n is a reasonable choice for a distro kernel. We don't have CONFIG_EXPERIMENTAL anymore, otherwise kho and luo would be gated by it. > I only reported it as a regression here because this was working until > the bisected commit was merged. If this is intended behaviour then it is > not a problem. Thanks for the clarification. This is a bug, but since it happens for a combination of an opt-in option (kho) and a hack (memmap=) I consider it low priority. -- Sincerely yours, Mike.