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

[Sasha Levin <sashal@kernel.org>]

On Fri, Apr 24, 2026 at 04:25:41PM +0200, David Hildenbrand (Arm) wrote:
>> +	/*
>> +	 * Size bitmaps to cover the full PFN range including any holes.
>> +	 * Holes waste a few bits but a flat bitmap keeps the indexing
>> +	 * trivial (pfn - min_pfn) and avoids additional data structures
>> +	 * that would themselves be subject to corruption.  This matches
>> +	 * the approach used by pageblock_flags.
>> +	 */
>> +	pc_state.min_pfn = PHYS_PFN(memblock_start_of_DRAM());
>> +	pc_state.max_pfn = PHYS_PFN(memblock_end_of_DRAM());
>> +	spanned_pfns = pc_state.max_pfn - pc_state.min_pfn;
>> +	if (!spanned_pfns || spanned_pfns > UINT_MAX) {
>> +		pr_err("PFN span %lu cannot be represented by bitmap APIs, feature disabled\n",
>> +		       spanned_pfns);
>> +		return;
>> +	}
>> +
>> +	pc_state.db.nbits = spanned_pfns;
>> +
>> +	bitmap_bytes = BITS_TO_LONGS(pc_state.db.nbits) * sizeof(unsigned long);
>> +
>> +	pr_info("Initializing: PFN range [%lu-%lu), %u bits (%zu KB per bitmap)\n",
>> +		pc_state.min_pfn, pc_state.max_pfn, pc_state.db.nbits,
>> +		bitmap_bytes / 1024);
>> +
>> +	/* Allocate primary bitmap (zeroed by memblock_alloc) */
>> +	pc_state.db.bitmap[DUAL_BITMAP_PRIMARY] =
>> +		memblock_alloc(bitmap_bytes, SMP_CACHE_BYTES);
>> +	if (!pc_state.db.bitmap[DUAL_BITMAP_PRIMARY]) {
>> +		pr_err("Failed to allocate primary bitmap, feature disabled\n");
>> +		return;
>> +	}
>>
>
>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.

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.

-- 
Thanks,
Sasha