Re: [PATCH 2/3] mm/page_ext: support early allocation for SPARSEMEM

[Muchun Song <muchun.song@linux.dev>]

> On Nov 15, 2026, at 14:23, Chris Down <chris@chrisdown.name> wrote:
> 
> Hi Muchun,

Hi,

> 
> This looks good to me in general, I only have a few small things.

Thanks for taking a look.

> 
> Muchun Song writes:
>> -	/*
>> -	 * page_owner must be initialized after buddy is ready, and also after
>> -	 * slab is ready so that stack_depot_init() works properly
>> -	 */
>> -	page_ext_init_flatmem_late();
> 
> Not a big deal, but this means that on FLATMEM the init callbacks now run
> after vmalloc_init() rather than straight after kmem_cache_init(). I can't
> see anything that depends on the old place, but FLATMEM isn't what the
> patch is about, so it'd be good to say in the changelog that this moves.

Sure. The new behavior is aligned with the non-FLATMEM case. It should be fine.

> 
>> +	if (!slab_is_available()) {
>> +		addr = memblock_alloc_try_nid(size, PAGE_SIZE, 0,
>> +					      MEMBLOCK_ALLOC_ACCESSIBLE, nid);
> 
> Maybe this should pass __pa(MAX_DMA_ADDRESS) as min_addr, like
> alloc_node_page_ext() does, so that the tables stay out of ZONE_DMA when
> there's room elsewhere?

Make sense.

> 
>> +	page = virt_to_page(addr);
>> +	if (PageReserved(page)) {
>> +		memmap_boot_pages_add(-nr_pages);
>> +		kmemleak_free_part_phys(__pa(addr), table_size);
>> +		free_reserved_area(addr, addr + table_size, -1, "page_ext");
>> +		return;
> 
> free_reserved_area() prints "Freeing page_ext memory" when it's given a
> name, and this runs once for every section, so offlining a few gigabytes
> would put dozens of those in the log. I think it should pass NULL instead.

Good point. I'll pass NULL in v2.

> 
> Did you manage to get this branch to run, by the way? KMSAN is the only

No. I only verified that the KMSCN kernel boots successfully.

> user of need_early, and from a quick check offlining a memory block on a
> KMSAN kernel fails with EBUSY, presumably because the metadata pages can't
> be migrated, so I couldn't exercise it myself.

I'll exercise this code on this path and report back.

> 
> Once the changelog and the log message are fixed, please feel free to add:
> 
> Reviewed-by: Chris Down <chris@chrisdown.name>

Thanks for your review.

Muchun

> 
> Thanks!
> 
> Chris