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

[David Hildenbrand (Arm)]

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.

-- 
Cheers,

David