[PATCH v6 5/9] mm: thp: Extend THP to allocate anonymous large folios

[Ryan Roberts]

Introduce the logic to allow THP to be configured (through the new
anon_orders interface we just added) to allocate large folios to back
anonymous memory, which are smaller than PMD-size (for example order-2,
order-3, order-4, etc).

These THPs continue to be PTE-mapped, but in many cases can still
provide similar benefits to traditional PMD-sized THP: Page faults are
significantly reduced (by a factor of e.g. 4, 8, 16, etc. depending on
the configured order), but latency spikes are much less prominent
because the size of each page isn't as huge as the PMD-sized variant and
there is less memory to clear in each page fault. The number of per-page
operations (e.g. ref counting, rmap management, lru list management) are
also significantly reduced since those ops now become per-folio.

Some architectures also employ TLB compression mechanisms to squeeze
more entries in when a set of PTEs are virtually and physically
contiguous and approporiately aligned. In this case, TLB misses will
occur less often.

The new behaviour is disabled by default because the anon_orders
defaults to only enabling PMD-order, but can be enabled at runtime by
writing to anon_orders (see documentation in previous commit). The long
term aim is to default anon_orders to include suitable lower orders, but
there are some risks around internal fragmentation that need to be
better understood first.

Signed-off-by: Ryan Roberts <ryan.roberts@xxxxxxx>
---
 Documentation/admin-guide/mm/transhuge.rst |   9 +-
 include/linux/huge_mm.h                    |   6 +-
 mm/memory.c                                | 108 +++++++++++++++++++--
 3 files changed, 111 insertions(+), 12 deletions(-)

diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst
index 9f954e73a4ca..732c3b2f4ba8 100644
--- a/Documentation/admin-guide/mm/transhuge.rst
+++ b/Documentation/admin-guide/mm/transhuge.rst
@@ -353,7 +353,9 @@ anonymous transparent huge pages, it is necessary to read
 ``/proc/PID/smaps`` and count the AnonHugePages and AnonHugePteMap
 fields for each mapping. Note that in both cases, AnonHugePages refers
 only to PMD-mapped THPs. AnonHugePteMap refers to THPs that are mapped
-using PTEs.
+using PTEs. This includes all THPs whose order is smaller than
+PMD-order, as well as any PMD-order THPs that happen to be PTE-mapped
+for other reasons.
 
 The number of file transparent huge pages mapped to userspace is available
 by reading ShmemPmdMapped and ShmemHugePages fields in ``/proc/meminfo``.
@@ -367,6 +369,11 @@ frequently will incur overhead.
 There are a number of counters in ``/proc/vmstat`` that may be used to
 monitor how successfully the system is providing huge pages for use.
 
+.. note::
+   Currently the below counters only record events relating to
+   PMD-order THPs. Events relating to smaller order THPs are not
+   included.
+
 thp_fault_alloc
 	is incremented every time a huge page is successfully
 	allocated to handle a page fault.
diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
index 2e7c338229a6..c4860476a1f5 100644
--- a/include/linux/huge_mm.h
+++ b/include/linux/huge_mm.h
@@ -68,9 +68,11 @@ extern struct kobj_attribute shmem_enabled_attr;
 #define HPAGE_PMD_NR (1<<HPAGE_PMD_ORDER)
 
 /*
- * Mask of all large folio orders supported for anonymous THP.
+ * Mask of all large folio orders supported for anonymous THP; all orders up to
+ * and including PMD_ORDER, except order-0 (which is not "huge") and order-1
+ * (which is a limitation of the THP implementation).
  */
-#define THP_ORDERS_ALL_ANON	BIT(PMD_ORDER)
+#define THP_ORDERS_ALL_ANON	((BIT(PMD_ORDER + 1) - 1) & ~(BIT(0) | BIT(1)))
 
 /*
  * Mask of all large folio orders supported for file THP.
diff --git a/mm/memory.c b/mm/memory.c
index b5b82fc8e164..92ed9c782dc9 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -4059,6 +4059,87 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
 	return ret;
 }
 
+static bool vmf_pte_range_changed(struct vm_fault *vmf, int nr_pages)
+{
+	int i;
+
+	if (nr_pages == 1)
+		return vmf_pte_changed(vmf);
+
+	for (i = 0; i < nr_pages; i++) {
+		if (!pte_none(ptep_get_lockless(vmf->pte + i)))
+			return true;
+	}
+
+	return false;
+}
+
+#ifdef CONFIG_TRANSPARENT_HUGEPAGE
+static struct folio *alloc_anon_folio(struct vm_fault *vmf)
+{
+	gfp_t gfp;
+	pte_t *pte;
+	unsigned long addr;
+	struct folio *folio;
+	struct vm_area_struct *vma = vmf->vma;
+	unsigned int orders;
+	int order;
+
+	/*
+	 * If uffd is active for the vma we need per-page fault fidelity to
+	 * maintain the uffd semantics.
+	 */
+	if (userfaultfd_armed(vma))
+		goto fallback;
+
+	/*
+	 * Get a list of all the (large) orders below PMD_ORDER that are enabled
+	 * for this vma. Then filter out the orders that can't be allocated over
+	 * the faulting address and still be fully contained in the vma.
+	 */
+	orders = hugepage_vma_check(vma, vma->vm_flags, false, true, true,
+				    BIT(PMD_ORDER) - 1);
+	orders = transhuge_vma_suitable(vma, vmf->address, orders);
+
+	if (!orders)
+		goto fallback;
+
+	pte = pte_offset_map(vmf->pmd, vmf->address & PMD_MASK);
+	if (!pte)
+		return ERR_PTR(-EAGAIN);
+
+	order = first_order(orders);
+	while (orders) {
+		addr = ALIGN_DOWN(vmf->address, PAGE_SIZE << order);
+		vmf->pte = pte + pte_index(addr);
+		if (!vmf_pte_range_changed(vmf, 1 << order))
+			break;
+		order = next_order(&orders, order);
+	}
+
+	vmf->pte = NULL;
+	pte_unmap(pte);
+
+	gfp = vma_thp_gfp_mask(vma);
+
+	while (orders) {
+		addr = ALIGN_DOWN(vmf->address, PAGE_SIZE << order);
+		folio = vma_alloc_folio(gfp, order, vma, addr, true);
+		if (folio) {
+			clear_huge_page(&folio->page, addr, 1 << order);
+			return folio;
+		}
+		order = next_order(&orders, order);
+	}
+
+fallback:
+	return vma_alloc_zeroed_movable_folio(vma, vmf->address);
+}
+#else
+#define alloc_anon_folio(vmf) \
+		vma_alloc_zeroed_movable_folio((vmf)->vma, (vmf)->address)
+#endif
+
 /*
  * We enter with non-exclusive mmap_lock (to exclude vma changes,
  * but allow concurrent faults), and pte mapped but not yet locked.
@@ -4066,6 +4147,9 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
  */
 static vm_fault_t do_anonymous_page(struct vm_fault *vmf)
 {
+	int i;
+	int nr_pages = 1;
+	unsigned long addr = vmf->address;
 	bool uffd_wp = vmf_orig_pte_uffd_wp(vmf);
 	struct vm_area_struct *vma = vmf->vma;
 	struct folio *folio;
@@ -4110,10 +4194,15 @@ static vm_fault_t do_anonymous_page(struct vm_fault *vmf)
 	/* Allocate our own private page. */
 	if (unlikely(anon_vma_prepare(vma)))
 		goto oom;
-	folio = vma_alloc_zeroed_movable_folio(vma, vmf->address);
+	folio = alloc_anon_folio(vmf);
+	if (IS_ERR(folio))
+		return 0;
 	if (!folio)
 		goto oom;
 
+	nr_pages = folio_nr_pages(folio);
+	addr = ALIGN_DOWN(vmf->address, nr_pages * PAGE_SIZE);
+
 	if (mem_cgroup_charge(folio, vma->vm_mm, GFP_KERNEL))
 		goto oom_free_page;
 	folio_throttle_swaprate(folio, GFP_KERNEL);
@@ -4130,12 +4219,12 @@ static vm_fault_t do_anonymous_page(struct vm_fault *vmf)
 	if (vma->vm_flags & VM_WRITE)
 		entry = pte_mkwrite(pte_mkdirty(entry), vma);
 
-	vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd, vmf->address,
-			&vmf->ptl);
+	vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd, addr, &vmf->ptl);
 	if (!vmf->pte)
 		goto release;
-	if (vmf_pte_changed(vmf)) {
-		update_mmu_tlb(vma, vmf->address, vmf->pte);
+	if (vmf_pte_range_changed(vmf, nr_pages)) {
+		for (i = 0; i < nr_pages; i++)
+			update_mmu_tlb(vma, addr + PAGE_SIZE * i, vmf->pte + i);
 		goto release;
 	}
 
@@ -4150,16 +4239,17 @@ static vm_fault_t do_anonymous_page(struct vm_fault *vmf)
 		return handle_userfault(vmf, VM_UFFD_MISSING);
 	}
 
-	inc_mm_counter(vma->vm_mm, MM_ANONPAGES);
-	folio_add_new_anon_rmap(folio, vma, vmf->address);
+	folio_ref_add(folio, nr_pages - 1);
+	add_mm_counter(vma->vm_mm, MM_ANONPAGES, nr_pages);
+	folio_add_new_anon_rmap(folio, vma, addr);
 	folio_add_lru_vma(folio, vma);
 setpte:
 	if (uffd_wp)
 		entry = pte_mkuffd_wp(entry);
-	set_pte_at(vma->vm_mm, vmf->address, vmf->pte, entry);
+	set_ptes(vma->vm_mm, addr, vmf->pte, entry, nr_pages);
 
 	/* No need to invalidate - it was non-present before */
-	update_mmu_cache_range(vmf, vma, vmf->address, vmf->pte, 1);
+	update_mmu_cache_range(vmf, vma, addr, vmf->pte, nr_pages);
 unlock:
 	if (vmf->pte)
 		pte_unmap_unlock(vmf->pte, vmf->ptl);
-- 
2.25.1