Re: [RFC 4/7] mm: add page consistency checker implementation

[Sasha Levin <sashal@kernel.org>]

On Fri, Apr 24, 2026 at 08:28:14PM +0200, David Hildenbrand (Arm) wrote:
>On 4/24/26 17:06, Pasha Tatashin wrote:
>> On 04-24 10:49, Sasha Levin wrote:
>>> On Fri, Apr 24, 2026 at 04:25:41PM +0200, David Hildenbrand (Arm) wrote:
>>>>
>>>> One bitmap that covers all sparse memory available at boot.
>>>>
>>>> Conclusion: Just horrible.
>>>
>>> Depends on who's looking at the code :)
>>>
>>> I picked it for auditability: covering the whole range with two
>>> memblock_alloc'd arrays means the only thing on the lookup path is the bitmap
>>> words themselves, which is what the dual-bitmap invariant already checks.
>>
>> The issue is that we are going back in time to a flat memory,
>> without NUMA or hotplug support. We need an abstraction that avoids
>> allocating this memory in enormous contiguous chunks, as thit approach
>> will not work on modern hardware.
>>
>>>
>>> We could go with per-section bitmaps which will fix the waste but pull
>>> mem_section[] into the trust boundary, so we'd have to start validating it too.
>>
>> Page-ext provides all of these capabilities, but as you described in the
>> cover letter, it does not meet your requirements. Therefore, I believe
>> a new abstraction layer is needed.
>
>If we decided that we want this (and I am not convinced), we definitely want
>something that supports sparsity and, in particular, something that support
>memory hotplug.

Makes sense. Let me take a few days and see if I can find some middle ground
here.

-- 
Thanks,
Sasha