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b=QoiadJJTHbytHLLUgcMMJDU2oFapVhRXRlzl12k8kd8u1xmwsp91V3Vv5KdQJ8/8n SRnfB2A4IIMfYR5JpyY29iU+f88a4kaHKDpHr3HlX+Y/2Kdh/cEUvZFKyDjeQp+9sR fdXwZkKRN5on7IdkIwshDYEf/pgjqQW6kScfNW/Yk3BT4/s0+KDfaEPd8WKRNmZTkn K3NccYEeufnFJV7FH2iw27/6wrnSQPDhBnGg9gmyv2S6+421Jkv8hqxUWvc+lTzBjn w8rCq9In5zLZUrPSnFVq5PLTJPIxBMhg3QqnjJDetBIE2lvyL1LhuWZr5G5+6XRRhl JkK1N+yfcGN7A== From: Pratyush Yadav To: Chris Bainbridge Cc: Pratyush Yadav , Mike Rapoport , Pasha Tatashin , Jason Miu , Alexander Graf , Ran Xiaokai , regressions@lists.linux.dev, linux-kernel@vger.kernel.org, kexec@lists.infradead.org, linux-mm@kvack.org, =?utf-8?B?TWljaGHFgiBDxYJhcGnFhHNraQ==?= Subject: Re: [BUG] KHO handover hangs when memmap reserves PMEM In-Reply-To: (Chris Bainbridge's message of "Sun, 4 Oct 2026 16:09:00 +0100") References: <2vxzcxtplh4i.fsf@kernel.org> Date: Sun, 04 Oct 2026 19:53:32 +0200 Message-ID: <2vxzzewtjqsj.fsf@kernel.org> User-Agent: Gnus/5.13 (Gnus v5.13) MIME-Version: 1.0 Content-Type: text/plain X-Rspamd-Queue-Id: 4D22C40008 X-Rspam-User: X-Rspamd-Server: rspam07 X-Stat-Signature: j3a1fjt57zyxwk98i5g4t53r1i71r6ag X-HE-Tag: 1791136417-671615 X-HE-Meta: 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 js6HXABJ rTlzHxt8pVXma433nPlHTx1H2fQTj8q0Jmzc+Fsv1gEo0os5CjeFHdaoWzA7muU3qLuGBp9fVt9CdlcwOpOrNgHcihapw6nQT8dVRrwV/cqh+37LTmvJi8zkBhV1I4RTmZSXKJDLcHubCcea66Go6Eq85SNc9BjfhCc9NscwT+xTt+ham9ML59NRQ6A48zNfmx0crv9JfylkWmIvlfP0AVUvh9A== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Sun, Oct 04 2026, Chris Bainbridge wrote: > On Sun, Oct 04, 2026 at 03:39:25PM +0200, Pratyush Yadav wrote: >> >> Just to confirm, the first kernel boots without memmap= right? > > Yes. > >> I think the main problem is that the radix tree pages can be anywhere in >> memory. So on the first kernel, they can land in an area that the second >> kernel's memmap= reserves. So kho_memory_init() might end up using >> memory that memmap= reserved. I'm not sure why exactly the next kernel >> doesn't produce any output, but IIRC memmap= memory is not mapped to the >> direct map, so accessing those pages likely causes a page fault. > > That's correct, kho_memory_init() page faults. It happens before the > serial console is available, so there is no output. Yeah, then I think you need an extra kexec with KHO off to enable memmap=. Or I guess hot-unplug the memory you'd like to memmap= early on first boot, which should make sure no radix tree pages land there. Apart from that, I don't have any good ideas to properly solve this problem. -- Regards, Pratyush Yadav