linux-kernel - Re: [PATCH v2 0/1] Accept unaccepted kexec segments' destination addresses

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Message-ID: <4599571f-804b-40d8-b5c8-e19478a3ad18@intel.com>
Date: Fri, 14 Feb 2025 08:20:07 -0800
From: Dave Hansen <dave.hansen@...el.com>
To: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc: "Eric W. Biederman" <ebiederm@...ssion.com>, Baoquan He <bhe@...hat.com>,
 "Kirill A. Shutemov" <kirill@...temov.name>, akpm@...ux-foundation.org,
 kexec@...ts.infradead.org, Yan Zhao <yan.y.zhao@...el.com>,
 linux-kernel@...r.kernel.org, linux-coco@...ts.linux.dev, x86@...nel.org,
 rick.p.edgecombe@...el.com, security@...nel.org
Subject: Re: [PATCH v2 0/1] Accept unaccepted kexec segments' destination
 addresses

On 2/14/25 05:46, Kirill A. Shutemov wrote:
>> It sounds like you're advocating for the "slow guest boot" option.
>> Kirill, can you remind us how fast a guest boots to the shell for
>> modestly-sized (say 256GB) memory with "accept_memory=eager" versus
>> "accept_memory=lazy"? IIRC, it was a pretty remarkable difference.
> I only have 128GB machine readily available and posted some number on
> other thread[1]:
> 
>   On single vCPU it takes about a minute to accept 90GiB of memory.
> 
>   It improves a bit with number of vCPUs. It is 40 seconds with 4 vCPU, but
>   it doesn't scale past that in my setup.
> 
> I've mentioned it before in other thread:
> 
> [1] https://lore.kernel.org/all/ihzvi5pwn5hrn4ky2ehjqztjxoixaiaby4igmeihqfehy2vrii@tsg6j5qvmyrm

Oh, wow, from that other thread, you've been trying to get this crash
fix accepted since November?

>From the looks of it, Eric stopped responding to that thread. I _think_
you gave a reasonable explanation of why memory acceptance is slow. He
then popped back up last month raising security concerns. But I don't
see anyone that shares those concerns.

The unaccepted memory stuff is also _already_ touching the page
allocator. If it's a dumb idea, then we should be gleefully ripping it
out of the page allocator, not rejecting a 2-line kexec patch.

Baoquan has also said this looks good to him.

I'm happy to give Eric another week to respond in case he's on vacation
or something, but I'm honestly not seeing a good reason to hold this bug
fix up.

Andrew, is this the kind of thing you can stick into mm and hold on to
for a bit while we give Eric time to respond?