[PATCH 08/13] huge_memory: Allow mappings of PMD sized pages

Alistair Popple
2024-06-27T00:55:17Z

Currently DAX folio/page reference counts are managed differently to
normal pages. To allow these to be managed the same as normal pages
introduce dax_insert_pfn_pmd. This will map the entire PMD-sized folio
and take references as it would for a normally mapped page.

This is distinct from the current mechanism, vmf_insert_pfn_pmd, which
simply inserts a special devmap PMD entry into the page table without
holding a reference to the page for the mapping.

Signed-off-by: Alistair Popple <apopple@nvidia.com>
---
include/linux/huge_mm.h | 1 +-
mm/huge_memory.c | 70 ++++++++++++++++++++++++++++++++++++++++++-
2 files changed, 71 insertions(+)

diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
index b98a3cc..9207d8e 100644
--- a/include/linux/huge_mm.h
+++ b/include/linux/huge_mm.h
@@ -39,6 +39,7 @@ int change_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,

vm_fault_t vmf_insert_pfn_pmd(struct vm_fault *vmf, pfn_t pfn, bool write);
vm_fault_t vmf_insert_pfn_pud(struct vm_fault *vmf, pfn_t pfn, bool write);
+vm_fault_t dax_insert_pfn_pmd(struct vm_fault *vmf, pfn_t pfn, bool write);
vm_fault_t dax_insert_pfn_pud(struct vm_fault *vmf, pfn_t pfn, bool write);

enum transparent_hugepage_flag {
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index e1f053e..a9874ac 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1202,6 +1202,76 @@ vm_fault_t vmf_insert_pfn_pmd(struct vm_fault *vmf, pfn_t pfn, bool write)
}
EXPORT_SYMBOL_GPL(vmf_insert_pfn_pmd);

+vm_fault_t dax_insert_pfn_pmd(struct vm_fault *vmf, pfn_t pfn, bool write)
+{
+ struct vm_area_struct *vma = vmf->vma;
+ unsigned long addr = vmf->address & PMD_MASK;
+ pmd_t *pmd = vmf->pmd;
+ struct mm_struct *mm = vma->vm_mm;
+ pmd_t entry;
+ spinlock_t *ptl;
+ pgtable_t pgtable = NULL;
+ struct folio *folio;
+ struct page *page;
+
+ if (addr < vma->vm_start || addr >= vma->vm_end)
+ return VM_FAULT_SIGBUS;
+
+ if (arch_needs_pgtable_deposit()) {
+ pgtable = pte_alloc_one(vma->vm_mm);
+ if (!pgtable)
+ return VM_FAULT_OOM;
+ }
+
+ track_pfn_insert(vma, &vma->vm_page_prot, pfn);
+
+ ptl = pmd_lock(mm, pmd);
+ if (!pmd_none(*pmd)) {
+ if (write) {
+ if (pmd_pfn(*pmd) != pfn_t_to_pfn(pfn)) {
+ WARN_ON_ONCE(!is_huge_zero_pmd(*pmd));
+ goto out_unlock;
+ }
+ entry = pmd_mkyoung(*pmd);
+ entry = maybe_pmd_mkwrite(pmd_mkdirty(entry), vma);
+ if (pmdp_set_access_flags(vma, addr, pmd, entry, 1))
+ update_mmu_cache_pmd(vma, addr, pmd);
+ }
+
+ goto out_unlock;
+ }
+
+ entry = pmd_mkhuge(pfn_t_pmd(pfn, vma->vm_page_prot));
+ if (pfn_t_devmap(pfn))
+ entry = pmd_mkdevmap(entry);
+ if (write) {
+ entry = pmd_mkyoung(pmd_mkdirty(entry));
+ entry = maybe_pmd_mkwrite(entry, vma);
+ }
+
+ if (pgtable) {
+ pgtable_trans_huge_deposit(mm, pmd, pgtable);
+ mm_inc_nr_ptes(mm);
+ pgtable = NULL;
+ }
+
+ page = pfn_t_to_page(pfn);
+ folio = page_folio(page);
+ folio_get(folio);
+ folio_add_file_rmap_pmd(folio, page, vma);
+ add_mm_counter(mm, mm_counter_file(folio), HPAGE_PMD_NR);
+ set_pmd_at(mm, addr, pmd, entry);
+ update_mmu_cache_pmd(vma, addr, pmd);
+
+out_unlock:
+ spin_unlock(ptl);
+ if (pgtable)
+ pte_free(mm, pgtable);
+
+ return VM_FAULT_NOPAGE;
+}
+EXPORT_SYMBOL_GPL(dax_insert_pfn_pmd);
+
#ifdef CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD
static pud_t maybe_pud_mkwrite(pud_t pud, struct vm_area_struct *vma)
{
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