Re: [v6 04/15] mm/huge_memory: implement device-private THP splitting

[Zi Yan <ziy@nvidia.com>]

On 22 Sep 2025, at 21:50, Balbir Singh wrote:

> On 9/23/25 07:09, Zi Yan wrote:
>> On 16 Sep 2025, at 8:21, Balbir Singh wrote:
>>
>>> Add support for splitting device-private THP folios, enabling fallback
>>> to smaller page sizes when large page allocation or migration fails.
>>>
>>> Key changes:
>>> - split_huge_pmd(): Handle device-private PMD entries during splitting
>>> - Preserve RMAP_EXCLUSIVE semantics for anonymous exclusive folios
>>> - Skip RMP_USE_SHARED_ZEROPAGE for device-private entries as they
>>>   don't support shared zero page semantics
>>>
>>> Signed-off-by: Balbir Singh <balbirs@nvidia.com>
>>> Cc: David Hildenbrand <david@redhat.com>
>>> Cc: Zi Yan <ziy@nvidia.com>
>>> Cc: Joshua Hahn <joshua.hahnjy@gmail.com>
>>> Cc: Rakie Kim <rakie.kim@sk.com>
>>> Cc: Byungchul Park <byungchul@sk.com>
>>> Cc: Gregory Price <gourry@gourry.net>
>>> Cc: Ying Huang <ying.huang@linux.alibaba.com>
>>> Cc: Alistair Popple <apopple@nvidia.com>
>>> Cc: Oscar Salvador <osalvador@suse.de>
>>> Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
>>> Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
>>> Cc: "Liam R. Howlett" <Liam.Howlett@oracle.com>
>>> Cc: Nico Pache <npache@redhat.com>
>>> Cc: Ryan Roberts <ryan.roberts@arm.com>
>>> Cc: Dev Jain <dev.jain@arm.com>
>>> Cc: Barry Song <baohua@kernel.org>
>>> Cc: Lyude Paul <lyude@redhat.com>
>>> Cc: Danilo Krummrich <dakr@kernel.org>
>>> Cc: David Airlie <airlied@gmail.com>
>>> Cc: Simona Vetter <simona@ffwll.ch>
>>> Cc: Ralph Campbell <rcampbell@nvidia.com>
>>> Cc: Mika Penttilä <mpenttil@redhat.com>
>>> Cc: Matthew Brost <matthew.brost@intel.com>
>>> Cc: Francois Dugast <francois.dugast@intel.com>
>>> ---
>>>  mm/huge_memory.c | 138 +++++++++++++++++++++++++++++++++--------------
>>>  1 file changed, 98 insertions(+), 40 deletions(-)
>>>
>>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>>> index 78166db72f4d..5291ee155a02 100644
>>> --- a/mm/huge_memory.c
>>> +++ b/mm/huge_memory.c
>>> @@ -2872,16 +2872,18 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
>>>  	struct page *page;
>>>  	pgtable_t pgtable;
>>>  	pmd_t old_pmd, _pmd;
>>> -	bool young, write, soft_dirty, pmd_migration = false, uffd_wp = false;
>>> -	bool anon_exclusive = false, dirty = false;
>>> +	bool soft_dirty, uffd_wp = false, young = false, write = false;
>>> +	bool anon_exclusive = false, dirty = false, present = false;
>>>  	unsigned long addr;
>>>  	pte_t *pte;
>>>  	int i;
>>> +	swp_entry_t swp_entry;
>>>
>>>  	VM_BUG_ON(haddr & ~HPAGE_PMD_MASK);
>>>  	VM_BUG_ON_VMA(vma->vm_start > haddr, vma);
>>>  	VM_BUG_ON_VMA(vma->vm_end < haddr + HPAGE_PMD_SIZE, vma);
>>> -	VM_BUG_ON(!is_pmd_migration_entry(*pmd) && !pmd_trans_huge(*pmd));
>>> +
>>> +	VM_WARN_ON(!is_pmd_non_present_folio_entry(*pmd) && !pmd_trans_huge(*pmd));
>>>
>>>  	count_vm_event(THP_SPLIT_PMD);
>>>
>>> @@ -2929,20 +2931,47 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
>>>  		return __split_huge_zero_page_pmd(vma, haddr, pmd);
>>>  	}
>>>
>>> -	pmd_migration = is_pmd_migration_entry(*pmd);
>>> -	if (unlikely(pmd_migration)) {
>>> -		swp_entry_t entry;
>>>
>>> +	present = pmd_present(*pmd);
>>> +	if (is_pmd_migration_entry(*pmd)) {
>>>  		old_pmd = *pmd;
>>> -		entry = pmd_to_swp_entry(old_pmd);
>>> -		page = pfn_swap_entry_to_page(entry);
>>> -		write = is_writable_migration_entry(entry);
>>> +		swp_entry = pmd_to_swp_entry(old_pmd);
>>> +		page = pfn_swap_entry_to_page(swp_entry);
>>> +		folio = page_folio(page);
>>> +
>>> +		soft_dirty = pmd_swp_soft_dirty(old_pmd);
>>> +		uffd_wp = pmd_swp_uffd_wp(old_pmd);
>>> +
>>> +		write = is_writable_migration_entry(swp_entry);
>>>  		if (PageAnon(page))
>>> -			anon_exclusive = is_readable_exclusive_migration_entry(entry);
>>> -		young = is_migration_entry_young(entry);
>>> -		dirty = is_migration_entry_dirty(entry);
>>> +			anon_exclusive = is_readable_exclusive_migration_entry(swp_entry);
>>> +		young = is_migration_entry_young(swp_entry);
>>> +		dirty = is_migration_entry_dirty(swp_entry);
>>> +	} else if (is_pmd_device_private_entry(*pmd)) {
>>> +		old_pmd = *pmd;
>>> +		swp_entry = pmd_to_swp_entry(old_pmd);
>>> +		page = pfn_swap_entry_to_page(swp_entry);
>>> +		folio = page_folio(page);
>>> +
>>>  		soft_dirty = pmd_swp_soft_dirty(old_pmd);
>>>  		uffd_wp = pmd_swp_uffd_wp(old_pmd);
>>> +
>>> +		write = is_writable_device_private_entry(swp_entry);
>>> +		anon_exclusive = PageAnonExclusive(page);
>>> +
>>> +		if (freeze && anon_exclusive &&
>>> +		    folio_try_share_anon_rmap_pmd(folio, page))
>>> +			freeze = false;
>>
>> Why is it OK to change the freeze request? OK, it is replicating
>> the code for present PMD folios. Either add a comment to point
>> to the explanation in the comment below, or move
>> “if (is_pmd_device_private_entry(*pmd))“ branch in the else below
>> to deduplicate this code.
>
> Similar to the code for present pages, ideally folio_try_share_anon_rmap_pmd()
> should never fail.

anon_exclusive = PageAnonExclusive(page);
if (freeze && anon_exclusive &&
    folio_try_share_anon_rmap_pmd(folio, page))
        freeze = false;
if (!freeze) {
        rmap_t rmap_flags = RMAP_NONE;

        folio_ref_add(folio, HPAGE_PMD_NR - 1);
        if (anon_exclusive)
                rmap_flags |= RMAP_EXCLUSIVE;
        folio_add_anon_rmap_ptes(folio, page, HPAGE_PMD_NR,
                                    vma, haddr, rmap_flags);
}

are the same for both device private and present. Can it be deduplicated
by doing below?

if (is_pmd_migration_entry(*pmd)) {
...
} else {
	if (is_pmd_device_private_entry(*pmd)) {
		...
	} else if (pmd_present()) {
		...
	}

	/* the above code */
}

If not, at least adding a comment in the device private copy of the code
pointing to the present copy's comment.

>
>>
>>> +		if (!freeze) {
>>> +			rmap_t rmap_flags = RMAP_NONE;
>>> +
>>> +			folio_ref_add(folio, HPAGE_PMD_NR - 1);
>>> +			if (anon_exclusive)
>>> +				rmap_flags |= RMAP_EXCLUSIVE;
>>> +
>>> +			folio_add_anon_rmap_ptes(folio, page, HPAGE_PMD_NR,
>>> +						 vma, haddr, rmap_flags);
>>> +		}
>>>  	} else {
>>>  		/*
>>>  		 * Up to this point the pmd is present and huge and userland has
>>> @@ -3026,32 +3055,57 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
>>>  	 * Note that NUMA hinting access restrictions are not transferred to
>>>  	 * avoid any possibility of altering permissions across VMAs.
>>>  	 */
>>> -	if (freeze || pmd_migration) {
>>> -		for (i = 0, addr = haddr; i < HPAGE_PMD_NR; i++, addr += PAGE_SIZE) {
>>> -			pte_t entry;
>>> -			swp_entry_t swp_entry;
>>> -
>>> -			if (write)
>>> -				swp_entry = make_writable_migration_entry(
>>> -							page_to_pfn(page + i));
>>> -			else if (anon_exclusive)
>>> -				swp_entry = make_readable_exclusive_migration_entry(
>>> -							page_to_pfn(page + i));
>>> -			else
>>> -				swp_entry = make_readable_migration_entry(
>>> -							page_to_pfn(page + i));
>>> -			if (young)
>>> -				swp_entry = make_migration_entry_young(swp_entry);
>>> -			if (dirty)
>>> -				swp_entry = make_migration_entry_dirty(swp_entry);
>>> -			entry = swp_entry_to_pte(swp_entry);
>>> -			if (soft_dirty)
>>> -				entry = pte_swp_mksoft_dirty(entry);
>>> -			if (uffd_wp)
>>> -				entry = pte_swp_mkuffd_wp(entry);
>>> +	if (freeze || !present) {
>>> +		pte_t entry;
>>>
>>> -			VM_WARN_ON(!pte_none(ptep_get(pte + i)));
>>> -			set_pte_at(mm, addr, pte + i, entry);
>>> +		if (freeze || is_migration_entry(swp_entry)) {
>>>
>> <snip>
>>> +		} else {
>> <snip>
>>>  		}
>>>  	} else {
>>>  		pte_t entry;
>>
>> David already pointed this out in v5. It can be done such as:
>>
>> if (freeze || pmd_migration) {
>> ...
>> } else if (is_pmd_device_private_entry(old_pmd)) {
>> ...
>
> No.. freeze can be true for device private entries as well

When freeze is true, migration entry is installed in place of
device private entry, since the "if (freeze || pmd_migration)"
branch is taken. This proposal is same as your code. What is
the difference?

>
>> } else {
>> /* for present, non freeze case */
>> }
>>
>>> @@ -3076,7 +3130,7 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
>>>  	}
>>>  	pte_unmap(pte);
>>>
>>> -	if (!pmd_migration)
>>> +	if (!is_pmd_migration_entry(*pmd))
>>>  		folio_remove_rmap_pmd(folio, page, vma);
>>>  	if (freeze)
>>>  		put_page(page);
>>> @@ -3089,7 +3143,7 @@ void split_huge_pmd_locked(struct vm_area_struct *vma, unsigned long address,
>>>  			   pmd_t *pmd, bool freeze)
>>>  {
>>>  	VM_WARN_ON_ONCE(!IS_ALIGNED(address, HPAGE_PMD_SIZE));
>>> -	if (pmd_trans_huge(*pmd) || is_pmd_migration_entry(*pmd))
>>> +	if (pmd_trans_huge(*pmd) || is_pmd_non_present_folio_entry(*pmd))
>>>  		__split_huge_pmd_locked(vma, pmd, address, freeze);
>>>  }
>>>
>>> @@ -3268,6 +3322,9 @@ static void lru_add_split_folio(struct folio *folio, struct folio *new_folio,
>>>  	VM_BUG_ON_FOLIO(folio_test_lru(new_folio), folio);
>>>  	lockdep_assert_held(&lruvec->lru_lock);
>>>
>>> +	if (folio_is_device_private(folio))
>>> +		return;
>>> +
>>>  	if (list) {
>>>  		/* page reclaim is reclaiming a huge page */
>>>  		VM_WARN_ON(folio_test_lru(folio));
>>> @@ -3885,8 +3942,9 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
>>>  	if (nr_shmem_dropped)
>>>  		shmem_uncharge(mapping->host, nr_shmem_dropped);
>>>
>>> -	if (!ret && is_anon)
>>> +	if (!ret && is_anon && !folio_is_device_private(folio))
>>>  		remap_flags = RMP_USE_SHARED_ZEROPAGE;
>>> +
>>
>> You should remove this and add
>>
>> if (folio_is_device_private(folio))
>> 	return false;
>>
>> in try_to_map_unused_to_zeropage(). Otherwise, no one would know
>> device private folios need to be excluded from mapping unused to
>> zero page.
>>
>
> I had that upto v2 and then David asked me to remove it. FYI, this
> is the only call site for RMP_USE_SHARED_ZEROPAGE

Can you provide a link?

Even if this is the only call site, there is no guarantee that
there will be none in the future. I am not sure why we want caller
to handle this special case. Who is going to tell the next user
of RMP_USE_SHARED_ZEROPAGE or caller to try_to_map_unused_to_zeropage()
that device private is incompatible with them?

>
>>>  	remap_page(folio, 1 << order, remap_flags);
>>>
>>>  	/*
>>> -- 
>>> 2.50.1
>>
>>
>
> Thanks for the review
> Balbir


--
Best Regards,
Yan, Zi