Re: [PATCH 07/13] huge_memory: Allow mappings of PUD sized pages

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On 27.06.24 02:54, Alistair Popple wrote:
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_pud. This will map the entire PUD-sized folio
and take references as it would for a normally mapped page.

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

Do we really have to involve mapcounts/rmap for daxfs pages at this point? Or is this only "to make it look more like other pages" ?

I'm asking this because:

(A) We don't support mixing PUD+PMD mappings yet. I have plans to change
    that in the future, but for now you can only map using a single PUD
    or by PTEs. I suspect that's good enoug for now for dax fs?
(B) As long as we have subpage mapcounts, this prevents vmemmap
    optimizations [1]. Is that only used for device-dax for now and are
    there no plans to make use of that for fs-dax?
(C) We managed without so far :)


Having that said, with folio->_large_mapcount things like folio_mapcount() are no longer terribly slow once we weould PTE-map a PUD-sized folio.

Also, all ZONE_DEVICE pages should currently be marked PG_reserved, translating to "don't touch the memmap". I think we might want to tackle that first.

[1] https://lwn.net/Articles/860218/
[2] https://lkml.kernel.org/r/b0adbb0c-ad59-4bc5-ba0b-0af464b94557@xxxxxxxxxx


Signed-off-by: Alistair Popple <apopple@xxxxxxxxxx>
---
  include/linux/huge_mm.h |   4 ++-
  include/linux/rmap.h    |  14 +++++-
  mm/huge_memory.c        | 108 ++++++++++++++++++++++++++++++++++++++---
  mm/rmap.c               |  48 ++++++++++++++++++-
  4 files changed, 168 insertions(+), 6 deletions(-)

diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
index 2aa986a..b98a3cc 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_pud(struct vm_fault *vmf, pfn_t pfn, bool write);
enum transparent_hugepage_flag {
  	TRANSPARENT_HUGEPAGE_UNSUPPORTED,
@@ -106,6 +107,9 @@ extern struct kobj_attribute shmem_enabled_attr;
  #define HPAGE_PUD_MASK	(~(HPAGE_PUD_SIZE - 1))
  #define HPAGE_PUD_SIZE	((1UL) << HPAGE_PUD_SHIFT)
+#define HPAGE_PUD_ORDER (HPAGE_PUD_SHIFT-PAGE_SHIFT)
+#define HPAGE_PUD_NR (1<<HPAGE_PUD_ORDER)
+
  #ifdef CONFIG_TRANSPARENT_HUGEPAGE
extern unsigned long transparent_hugepage_flags;
diff --git a/include/linux/rmap.h b/include/linux/rmap.h
index 7229b9b..c5a0205 100644
--- a/include/linux/rmap.h
+++ b/include/linux/rmap.h
@@ -192,6 +192,7 @@ typedef int __bitwise rmap_t;
  enum rmap_level {
  	RMAP_LEVEL_PTE = 0,
  	RMAP_LEVEL_PMD,
+	RMAP_LEVEL_PUD,
  };
static inline void __folio_rmap_sanity_checks(struct folio *folio,
@@ -225,6 +226,13 @@ static inline void __folio_rmap_sanity_checks(struct folio *folio,
  		VM_WARN_ON_FOLIO(folio_nr_pages(folio) != HPAGE_PMD_NR, folio);
  		VM_WARN_ON_FOLIO(nr_pages != HPAGE_PMD_NR, folio);
  		break;
+	case RMAP_LEVEL_PUD:
+		/*
+		 * Asume that we are creating * a single "entire" mapping of the folio.
+		 */
+		VM_WARN_ON_FOLIO(folio_nr_pages(folio) != HPAGE_PUD_NR, folio);
+		VM_WARN_ON_FOLIO(nr_pages != HPAGE_PUD_NR, folio);
+		break;
  	default:
  		VM_WARN_ON_ONCE(true);
  	}
@@ -248,12 +256,16 @@ void folio_add_file_rmap_ptes(struct folio *, struct page *, int nr_pages,
  	folio_add_file_rmap_ptes(folio, page, 1, vma)
  void folio_add_file_rmap_pmd(struct folio *, struct page *,
  		struct vm_area_struct *);
+void folio_add_file_rmap_pud(struct folio *, struct page *,
+		struct vm_area_struct *);
  void folio_remove_rmap_ptes(struct folio *, struct page *, int nr_pages,
  		struct vm_area_struct *);
  #define folio_remove_rmap_pte(folio, page, vma) \
  	folio_remove_rmap_ptes(folio, page, 1, vma)
  void folio_remove_rmap_pmd(struct folio *, struct page *,
  		struct vm_area_struct *);
+void folio_remove_rmap_pud(struct folio *, struct page *,
+		struct vm_area_struct *);
void hugetlb_add_anon_rmap(struct folio *, struct vm_area_struct *,
  		unsigned long address, rmap_t flags);
@@ -338,6 +350,7 @@ static __always_inline void __folio_dup_file_rmap(struct folio *folio,
  		atomic_add(orig_nr_pages, &folio->_large_mapcount);
  		break;
  	case RMAP_LEVEL_PMD:
+	case RMAP_LEVEL_PUD:
  		atomic_inc(&folio->_entire_mapcount);
  		atomic_inc(&folio->_large_mapcount);
  		break;
@@ -434,6 +447,7 @@ static __always_inline int __folio_try_dup_anon_rmap(struct folio *folio,
  		atomic_add(orig_nr_pages, &folio->_large_mapcount);
  		break;
  	case RMAP_LEVEL_PMD:
+	case RMAP_LEVEL_PUD:
  		if (PageAnonExclusive(page)) {
  			if (unlikely(maybe_pinned))
  				return -EBUSY;
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index db7946a..e1f053e 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1283,6 +1283,70 @@ vm_fault_t vmf_insert_pfn_pud(struct vm_fault *vmf, pfn_t pfn, bool write)
  	return VM_FAULT_NOPAGE;
  }
  EXPORT_SYMBOL_GPL(vmf_insert_pfn_pud);
+
+/**
+ * dax_insert_pfn_pud - insert a pud size pfn backed by a normal page
+ * @vmf: Structure describing the fault
+ * @pfn: pfn of the page to insert
+ * @write: whether it's a write fault
+ *
+ * Return: vm_fault_t value.
+ */
+vm_fault_t dax_insert_pfn_pud(struct vm_fault *vmf, pfn_t pfn, bool write)
+{
+	struct vm_area_struct *vma = vmf->vma;
+	unsigned long addr = vmf->address & PUD_MASK;
+	pud_t *pud = vmf->pud;
+	pgprot_t prot = vma->vm_page_prot;
+	struct mm_struct *mm = vma->vm_mm;
+	pud_t entry;
+	spinlock_t *ptl;
+	struct folio *folio;
+	struct page *page;
+
+	if (addr < vma->vm_start || addr >= vma->vm_end)
+		return VM_FAULT_SIGBUS;
+
+	track_pfn_insert(vma, &prot, pfn);
+
+	ptl = pud_lock(mm, pud);
+	if (!pud_none(*pud)) {
+		if (write) {
+			if (pud_pfn(*pud) != pfn_t_to_pfn(pfn)) {
+				WARN_ON_ONCE(!is_huge_zero_pud(*pud));
+				goto out_unlock;
+			}
+			entry = pud_mkyoung(*pud);
+			entry = maybe_pud_mkwrite(pud_mkdirty(entry), vma);
+			if (pudp_set_access_flags(vma, addr, pud, entry, 1))
+				update_mmu_cache_pud(vma, addr, pud);
+		}
+		goto out_unlock;
+	}
+
+	entry = pud_mkhuge(pfn_t_pud(pfn, prot));
+	if (pfn_t_devmap(pfn))
+		entry = pud_mkdevmap(entry);
+	if (write) {
+		entry = pud_mkyoung(pud_mkdirty(entry));
+		entry = maybe_pud_mkwrite(entry, vma);
+	}
+
+	page = pfn_t_to_page(pfn);
+	folio = page_folio(page);
+	folio_get(folio);
+	folio_add_file_rmap_pud(folio, page, vma);
+	add_mm_counter(mm, mm_counter_file(folio), HPAGE_PUD_NR);
+
+	set_pud_at(mm, addr, pud, entry);
+	update_mmu_cache_pud(vma, addr, pud);
+
+out_unlock:
+	spin_unlock(ptl);
+
+	return VM_FAULT_NOPAGE;
+}
+EXPORT_SYMBOL_GPL(dax_insert_pfn_pud);
  #endif /* CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD */
void touch_pmd(struct vm_area_struct *vma, unsigned long addr,
@@ -1836,7 +1900,8 @@ int zap_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
  			zap_deposited_table(tlb->mm, pmd);
  		spin_unlock(ptl);
  	} else if (is_huge_zero_pmd(orig_pmd)) {
-		zap_deposited_table(tlb->mm, pmd);
+		if (!vma_is_dax(vma) || arch_needs_pgtable_deposit())
+			zap_deposited_table(tlb->mm, pmd);
  		spin_unlock(ptl);
  	} else {
  		struct folio *folio = NULL;
@@ -2268,20 +2333,34 @@ spinlock_t *__pud_trans_huge_lock(pud_t *pud, struct vm_area_struct *vma)
  int zap_huge_pud(struct mmu_gather *tlb, struct vm_area_struct *vma,
  		 pud_t *pud, unsigned long addr)
  {
+	pud_t orig_pud;
  	spinlock_t *ptl;
ptl = __pud_trans_huge_lock(pud, vma);
  	if (!ptl)
  		return 0;
- pudp_huge_get_and_clear_full(vma, addr, pud, tlb->fullmm);
+	orig_pud = pudp_huge_get_and_clear_full(vma, addr, pud, tlb->fullmm);
  	tlb_remove_pud_tlb_entry(tlb, pud, addr);
-	if (vma_is_special_huge(vma)) {
+	if (!vma_is_dax(vma) && vma_is_special_huge(vma)) {
  		spin_unlock(ptl);
  		/* No zero page support yet */
  	} else {
-		/* No support for anonymous PUD pages yet */
-		BUG();
+		struct page *page = NULL;
+		struct folio *folio;
+
+		/* No support for anonymous PUD pages or migration yet */
+		BUG_ON(vma_is_anonymous(vma) || !pud_present(orig_pud));
+
+		page = pud_page(orig_pud);
+		folio = page_folio(page);
+		folio_remove_rmap_pud(folio, page, vma);
+		VM_BUG_ON_PAGE(page_mapcount(page) < 0, page);
+		VM_BUG_ON_PAGE(!PageHead(page), page);
+		add_mm_counter(tlb->mm, mm_counter_file(folio), -HPAGE_PUD_NR);
+
+		spin_unlock(ptl);
+		tlb_remove_page_size(tlb, page, HPAGE_PUD_SIZE);
  	}
  	return 1;
  }
@@ -2289,6 +2368,8 @@ int zap_huge_pud(struct mmu_gather *tlb, struct vm_area_struct *vma,
  static void __split_huge_pud_locked(struct vm_area_struct *vma, pud_t *pud,
  		unsigned long haddr)
  {
+	pud_t old_pud;
+
  	VM_BUG_ON(haddr & ~HPAGE_PUD_MASK);
  	VM_BUG_ON_VMA(vma->vm_start > haddr, vma);
  	VM_BUG_ON_VMA(vma->vm_end < haddr + HPAGE_PUD_SIZE, vma);
@@ -2296,7 +2377,22 @@ static void __split_huge_pud_locked(struct vm_area_struct *vma, pud_t *pud,
count_vm_event(THP_SPLIT_PUD); - pudp_huge_clear_flush(vma, haddr, pud);
+	old_pud = pudp_huge_clear_flush(vma, haddr, pud);
+	if (is_huge_zero_pud(old_pud))
+		return;
+
+	if (vma_is_dax(vma)) {
+		struct page *page = pud_page(old_pud);
+		struct folio *folio = page_folio(page);
+
+		if (!folio_test_dirty(folio) && pud_dirty(old_pud))
+			folio_mark_dirty(folio);
+		if (!folio_test_referenced(folio) && pud_young(old_pud))
+			folio_set_referenced(folio);
+		folio_remove_rmap_pud(folio, page, vma);
+		folio_put(folio);
+		add_mm_counter(vma->vm_mm, mm_counter_file(folio), -HPAGE_PUD_NR);
+	}
  }
void __split_huge_pud(struct vm_area_struct *vma, pud_t *pud,
diff --git a/mm/rmap.c b/mm/rmap.c
index e8fc5ec..e949e4f 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1165,6 +1165,7 @@ static __always_inline unsigned int __folio_add_rmap(struct folio *folio,
  		atomic_add(orig_nr_pages, &folio->_large_mapcount);
  		break;
  	case RMAP_LEVEL_PMD:
+	case RMAP_LEVEL_PUD:
  		first = atomic_inc_and_test(&folio->_entire_mapcount);
  		if (first) {
  			nr = atomic_add_return_relaxed(ENTIRELY_MAPPED, mapped);
@@ -1306,6 +1307,12 @@ static __always_inline void __folio_add_anon_rmap(struct folio *folio,
  		case RMAP_LEVEL_PMD:
  			SetPageAnonExclusive(page);
  			break;
+		case RMAP_LEVEL_PUD:
+			/*
+			 * Keep the compiler happy, we don't support anonymous PUD mappings.
+			 */
+			WARN_ON_ONCE(1);
+			break;
  		}
  	}
  	for (i = 0; i < nr_pages; i++) {
@@ -1489,6 +1496,26 @@ void folio_add_file_rmap_pmd(struct folio *folio, struct page *page,
  #endif
  }
+/**
+ * folio_add_file_rmap_pud - add a PUD mapping to a page range of a folio
+ * @folio:	The folio to add the mapping to
+ * @page:	The first page to add
+ * @vma:	The vm area in which the mapping is added
+ *
+ * The page range of the folio is defined by [page, page + HPAGE_PUD_NR)
+ *
+ * The caller needs to hold the page table lock.
+ */
+void folio_add_file_rmap_pud(struct folio *folio, struct page *page,
+		struct vm_area_struct *vma)
+{
+#ifdef CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD
+	__folio_add_file_rmap(folio, page, HPAGE_PUD_NR, vma, RMAP_LEVEL_PUD);
+#else
+	WARN_ON_ONCE(true);
+#endif
+}
+
  static __always_inline void __folio_remove_rmap(struct folio *folio,
  		struct page *page, int nr_pages, struct vm_area_struct *vma,
  		enum rmap_level level)
@@ -1521,6 +1548,7 @@ static __always_inline void __folio_remove_rmap(struct folio *folio,
  		partially_mapped = nr && atomic_read(mapped);
  		break;
  	case RMAP_LEVEL_PMD:
+	case RMAP_LEVEL_PUD:
  		atomic_dec(&folio->_large_mapcount);
  		last = atomic_add_negative(-1, &folio->_entire_mapcount);
  		if (last) {
@@ -1615,6 +1643,26 @@ void folio_remove_rmap_pmd(struct folio *folio, struct page *page,
  #endif
  }
+/**
+ * folio_remove_rmap_pud - remove a PUD mapping from a page range of a folio
+ * @folio:	The folio to remove the mapping from
+ * @page:	The first page to remove
+ * @vma:	The vm area from which the mapping is removed
+ *
+ * The page range of the folio is defined by [page, page + HPAGE_PUD_NR)
+ *
+ * The caller needs to hold the page table lock.
+ */
+void folio_remove_rmap_pud(struct folio *folio, struct page *page,
+		struct vm_area_struct *vma)
+{
+#ifdef CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD
+	__folio_remove_rmap(folio, page, HPAGE_PUD_NR, vma, RMAP_LEVEL_PUD);
+#else
+	WARN_ON_ONCE(true);
+#endif
+}
+
  /*
   * @arg: enum ttu_flags will be passed to this argument
   */

--
Cheers,

David / dhildenb





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