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 <[email protected]>
>> Cc: David Hildenbrand <[email protected]>
>> Cc: Zi Yan <[email protected]>
>> Cc: Joshua Hahn <[email protected]>
>> Cc: Rakie Kim <[email protected]>
>> Cc: Byungchul Park <[email protected]>
>> Cc: Gregory Price <[email protected]>
>> Cc: Ying Huang <[email protected]>
>> Cc: Alistair Popple <[email protected]>
>> Cc: Oscar Salvador <[email protected]>
>> Cc: Lorenzo Stoakes <[email protected]>
>> Cc: Baolin Wang <[email protected]>
>> Cc: "Liam R. Howlett" <[email protected]>
>> Cc: Nico Pache <[email protected]>
>> Cc: Ryan Roberts <[email protected]>
>> Cc: Dev Jain <[email protected]>
>> Cc: Barry Song <[email protected]>
>> Cc: Lyude Paul <[email protected]>
>> Cc: Danilo Krummrich <[email protected]>
>> Cc: David Airlie <[email protected]>
>> Cc: Simona Vetter <[email protected]>
>> Cc: Ralph Campbell <[email protected]>
>> Cc: Mika Penttilä <[email protected]>
>> Cc: Matthew Brost <[email protected]>
>> Cc: Francois Dugast <[email protected]>
>> ---
>> 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.
>
>> + 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
> } 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
>> remap_page(folio, 1 << order, remap_flags);
>>
>> /*
>> --
>> 2.50.1
>
>
Thanks for the review
Balbir
