Re: [PATCH v3 2/6] mm/sparse-vmemmap: support device DAX in common vmemmap path

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

> On Sep 30, 2026, at 16:53, Lance Yang <lance.yang@linux.dev> wrote:
> 
> 
> On Tue, Sep 29, 2026 at 01:32:27PM +0800, Muchun Song wrote:
>> The common vmemmap population path cannot yet handle optimized Device DAX
>> mappings on its own. It uses pfn_to_zone() to find the shared tail page,
>> but Device DAX populates its vmemmap at runtime before the ZONE_DEVICE span
>> is initialized.
>> 
>> Teach the common path to use device_zone() for runtime optimized vmemmap
>> population while retaining pfn_to_zone() for early boot. This allows the
>> same path to support both early boot mappings and Device DAX.
>> 
>> The backing PFN supplied by the Device DAX-specific population path is no
>> longer used, allowing the redundant lookup and population code to be
>> removed later.
>> 
>> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
>> Acked-by: Qi Zheng <qi.zheng@linux.dev>
>> ---
>> v3:
>> - Collect Acked-by from Qi Zheng
>> 
>> v2:
>> - Expand comments around slab initialization to explain zone lookup and
>> page refcounting (suggested by Qi Zheng)
>> ---
>> mm/sparse-vmemmap.c | 64 +++++++++++++++++++++++++--------------------
>> 1 file changed, 35 insertions(+), 29 deletions(-)
>> 
>> diff --git a/mm/sparse-vmemmap.c b/mm/sparse-vmemmap.c
>> index 8219abc6c3e5..ee4c113ca938 100644
>> --- a/mm/sparse-vmemmap.c
>> +++ b/mm/sparse-vmemmap.c
>> @@ -237,18 +237,43 @@ static __meminit void *vmemmap_alloc_pte(unsigned long pfn, int node,
>> struct page *page;
>> const unsigned int order = pfn_to_section_compound_order(pfn);
>> 
>> - /*
>> -  * Device DAX still relies on vmemmap_populate_compound_pages() for
>> -  * head/first-tail allocation and tail-page reuse.
>> -  */
>> if (!vmemmap_optimizable_pfn(pfn))
>> return vmemmap_alloc_block_buf(PAGE_SIZE, node, altmap);
>> 
>> - zone = pfn_to_zone(pfn, node);
>> + /*
>> +  * Before slab is available, vmemmap optimization is used for early
>> +  * system RAM, whose zone can be determined from the PFN.
>> +  *
>> +  * Once slab is available, only ZONE_DEVICE memory reaches this
>> +  * optimized population path. Its zone span has not been initialized
>> +  * while its vmemmap is being populated, so pfn_to_zone() cannot be
>> +  * used. Obtain ZONE_DEVICE directly from the node instead.
>> +  */
>> + zone = slab_is_available() ? device_zone(node) : pfn_to_zone(pfn, node);
>> page = vmemmap_shared_tail_page(order, zone);
>> if (!page)
>> return NULL;
>> 
>> + /*
>> +  * During early vmemmap population, the shared tail vmemmap backing
>> +  * page is allocated from memblock before its struct page can safely
>> +  * participate in page refcounting. Therefore, no reference can be
>> +  * held for each shared PTE mapping, and the mappings must be unshared
>> +  * before the vmemmap is depopulated.
>> +  *
>> +  * Once slab is available, the shared backing page is allocated from
>> +  * the buddy allocator and can be refcounted. Hold one reference for
>> +  * each shared PTE mapping. The architecture vmemmap teardown drops
>> +  * the reference through __free_pages() when removing the mapping,
>> +  * preventing the backing page from being freed while it is shared.
>> +  *
>> +  * The backing page may be shared by enough PTE mappings to exhaust
>> +  * the positive range of its reference count. Stop populating the
>> +  * vmemmap if another reference cannot be acquired.
>> +  */
>> + if (slab_is_available() && !try_get_page(page))
>> + return NULL;
> 
> BTW, shouldn't __add_pages() undo the earlier sections on a population
> failure? Say the first section is added successfully, but populating the
> next one fails, e.g. due to an allocation failure:
> 
> void *memremap_pages(struct dev_pagemap *pgmap, int nid)
> {
> 	...
> 	const int nr_range = pgmap->nr_range;
> 	int error, i;
> 	...
> 	pgmap->nr_range = 0;
> 	error = 0;
> 	for (i = 0; i < nr_range; i++) {
> 		error = pagemap_range(pgmap, &params, i, nid);
> 		if (error)
> 			break;
> 		pgmap->nr_range++;
> 	}
> 
> 	if (i < nr_range) {
> 		memunmap_pages(pgmap);
> 		pgmap->nr_range = nr_range;
> 		return ERR_PTR(error);
> 	}
> ...
> }
> 
> We still need to undo the sections already added in the failed range,
> though ... memunmap_pages() won't touch those, since it only removes
> completed ranges.
> 
> If the range starts at a section boundary, we'd hit -EEXIST in
> fill_subsection_map() on retry while those subsection bits are still set.
> 
> The old DAX path had this issue too. Could we roll back [start_pfn, pfn)
> in __add_pages() as a separate fix? Something like this:
> 
> ---8<---
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index 796af1028ee2..16a0a2c885bc 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -380,6 +380,7 @@ EXPORT_SYMBOL_GPL(pfn_to_online_page);
> int __add_pages(int nid, unsigned long pfn, unsigned long nr_pages,
> struct mhp_params *params)
> {
> + 	const unsigned long start_pfn = pfn;
> 	const unsigned long end_pfn = pfn + nr_pages;
> 	unsigned long cur_nr_pages;
> 	int err;
> @@ -417,6 +418,10 @@ int __add_pages(int nid, unsigned long pfn, unsigned long nr_pages,
> 			break;
> 		cond_resched();
> }
> +
> + 	/* Roll back the sections added before the failure. */
> + 	if (err && pfn != start_pfn)
> + 		__remove_pages(start_pfn, pfn - start_pfn, altmap, params->pgmap);
> 	vmemmap_populate_print_last();
> 	return err;
> }
> --
> 
> Hope I haven't missed anything :)

Good catch. This is indeed a pre-existing issue, and the old DAX path was
affected as well.

Your proposed fix looks correct to me. I would slightly prefer keeping the
rollback close to the failure:

           err = sparse_add_section(nid, pfn, cur_nr_pages, altmap,
                                    params->pgmap);
           if (err) {
                   __remove_pages(start_pfn, pfn - start_pfn, altmap,
                                  params->pgmap);
                   break;
           }

If the first section fails, this simply calls __remove_pages() with an
empty range, which is a harmless no-op.

Would you mind sending this as a separate bug fix? I will ACK it.

Thanks,
Muchun