mm/huge_memory: implement device-private THP splitting

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

Link: https://lkml.kernel.org/r/20251001065707.920170-6-balbirs@nvidia.com
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>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 57809d02a60a6ef3b4b240e1eec063b2c227abc6..7c0971864ca6b32a3943aa1bc47c2c1112af6555 100644 (file)
--- 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 soft_dirty, uffd_wp = false, young = false, write = false;
        bool anon_exclusive = false, dirty = false;
        unsigned long addr;
        pte_t *pte;
        int i;
+       swp_entry_t 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 +2931,51 @@ 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;
 
+       if (is_pmd_migration_entry(*pmd)) {
                old_pmd = *pmd;
                entry = pmd_to_swp_entry(old_pmd);
                page = pfn_swap_entry_to_page(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(entry);
                if (PageAnon(page))
                        anon_exclusive = is_readable_exclusive_migration_entry(entry);
                young = is_migration_entry_young(entry);
                dirty = is_migration_entry_dirty(entry);
+       } else if (is_pmd_device_private_entry(*pmd)) {
+               old_pmd = *pmd;
+               entry = pmd_to_swp_entry(old_pmd);
+               page = pfn_swap_entry_to_page(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(entry);
+               anon_exclusive = PageAnonExclusive(page);
+
+               /*
+                * Device private THP should be treated the same as regular
+                * folios w.r.t anon exclusive handling. See the comments for
+                * folio handling and anon_exclusive below.
+                */
+               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);
+               }
        } else {
                /*
                 * Up to this point the pmd is present and huge and userland has
@@ -3026,11 +3059,11 @@ 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 (freeze || is_pmd_migration_entry(old_pmd)) {
+               pte_t entry;
+               swp_entry_t swp_entry;
 
+               for (i = 0, addr = haddr; i < HPAGE_PMD_NR; i++, addr += PAGE_SIZE) {
                        if (write)
                                swp_entry = make_writable_migration_entry(
                                                        page_to_pfn(page + i));
@@ -3049,7 +3082,33 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
                                entry = pte_swp_mksoft_dirty(entry);
                        if (uffd_wp)
                                entry = pte_swp_mkuffd_wp(entry);
+                       VM_WARN_ON(!pte_none(ptep_get(pte + i)));
+                       set_pte_at(mm, addr, pte + i, entry);
+               }
+       } else if (is_pmd_device_private_entry(old_pmd)) {
+               pte_t entry;
+               swp_entry_t swp_entry;
 
+               for (i = 0, addr = haddr; i < HPAGE_PMD_NR; i++, addr += PAGE_SIZE) {
+                       /*
+                        * anon_exclusive was already propagated to the relevant
+                        * pages corresponding to the pte entries when freeze
+                        * is false.
+                        */
+                       if (write)
+                               swp_entry = make_writable_device_private_entry(
+                                                       page_to_pfn(page + i));
+                       else
+                               swp_entry = make_readable_device_private_entry(
+                                                       page_to_pfn(page + i));
+                       /*
+                        * Young and dirty bits are not progated via 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);
                        VM_WARN_ON(!pte_none(ptep_get(pte + i)));
                        set_pte_at(mm, addr, pte + i, entry);
                }
@@ -3076,7 +3135,7 @@ 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 +3148,7 @@ 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 +3327,9 @@ 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));
@@ -3883,8 +3945,9 @@ fail:
        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;
+
        remap_page(folio, 1 << order, remap_flags);
 
        /*

diff --git a/mm/migrate.c b/mm/migrate.c
index c0e9f15be2a229798f0492e922b5f1f8f7e5258e..4324fc01bfce4e3b48361a9ede53e430e341cf03 100644 (file)
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -307,6 +307,7 @@ static bool try_to_map_unused_to_zeropage(struct page_vma_mapped_walk *pvmw,
        VM_BUG_ON_PAGE(!PageAnon(page), page);
        VM_BUG_ON_PAGE(!PageLocked(page), page);
        VM_BUG_ON_PAGE(pte_present(old_pte), page);
+       VM_WARN_ON_ONCE_FOLIO(folio_is_device_private(folio), folio);
 
        if (folio_test_mlocked(folio) || (pvmw->vma->vm_flags & VM_LOCKED) ||
            mm_forbids_zeropage(pvmw->vma->vm_mm))