linux-kernel - Re: Kernel 6.15-rc2 unable to boot on 32bit x86 with PAE

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Message-ID: <ecca749e-9c84-40ef-ae2a-d630a249b764@gmx.com>
Date: Tue, 22 Apr 2025 17:02:55 +0930
From: Qu Wenruo <quwenruo.btrfs@....com>
To: Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...en8.de>
Cc: Linux Memory Management List <linux-mm@...ck.org>,
 LKML <linux-kernel@...r.kernel.org>, Mike Rapoport <rppt@...nel.org>,
 Dave Hansen <dave.hansen@...el.com>, x86-ml <x86@...nel.org>
Subject: Re: Kernel 6.15-rc2 unable to boot on 32bit x86 with PAE



在 2025/4/22 16:30, Ingo Molnar 写道:
> 
> * Borislav Petkov <bp@...en8.de> wrote:
> 
>> On Fri, Apr 18, 2025 at 08:31:23PM +0930, Qu Wenruo wrote:
>>> Hi,
>>>
>>> Recently I'm testing a situation where highmem is involved, thus I'm
>>> building the latest 32bit x86 with HIGHMEM and PAE, and run it inside a qemu
>>> VM.
>>
>> Does that fix it:
>>
>> https://git.kernel.org/tip/1e07b9fad022e0e02215150ca1e20912e78e8ec1
>>
>> ?
> 
> That commit caused other problems - the best fix we have right now is:
> 
> 	https://git.kernel.org/tip/83b2d345e1786fdab96fc2b52942eebde125e7cd

Thanks, I'll take a try again.

But considering how rare 32bit + highmem systems are nowadays, I still 
have one questions/concern:

- How to make sure we really got highmems?
   The kernel docs only mention that highmems need temporary mapping,
   and proper kmap/kunmap() handling.

   But I still didn't have a straightforward idea of how to verify,
   without adding manual ftrace events when a highmem page is hit.

   E.g. free only shows 3GB memory even if I enabled highmem + PAE.

   Since HIGHMEM64G is removed, I'd expect to get some values more like
   4G instead?

Thanks,
Qu


> 
> Thanks,
> 
> 	Ingo