[PATCH v5 3/5] mm: LARGE_ANON_FOLIO for improved performance

[Ryan Roberts]

Introduce LARGE_ANON_FOLIO feature, which allows anonymous memory to be
allocated in large folios of a determined order. All pages of the large
folio are pte-mapped during the same page fault, significantly reducing
the number of page faults. 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.

The new behaviour is hidden behind the new LARGE_ANON_FOLIO Kconfig,
which defaults to disabled for now; The long term aim is for this to
defaut to enabled, but there are some risks around internal
fragmentation that need to be better understood first.

Large anonymous folio (LAF) allocation is integrated with the existing
(PMD-order) THP and single (S) page allocation according to this policy,
where fallback (>) is performed for various reasons, such as the
proposed folio order not fitting within the bounds of the VMA, etc:

                | prctl=dis | prctl=ena   | prctl=ena     | prctl=ena
                | sysfs=X   | sysfs=never | sysfs=madvise | sysfs=always
----------------|-----------|-------------|---------------|-------------
no hint         | S         | LAF>S       | LAF>S         | THP>LAF>S
MADV_HUGEPAGE   | S         | LAF>S       | THP>LAF>S     | THP>LAF>S
MADV_NOHUGEPAGE | S         | S           | S             | S

This approach ensures that we don't violate existing hints to only
allocate single pages - this is required for QEMU's VM live migration
implementation to work correctly - while allowing us to use LAF
independently of THP (when sysfs=never). This makes wide scale
performance characterization simpler, while avoiding exposing any new
ABI to user space.

When using LAF for allocation, the folio order is determined as follows:
The return value of arch_wants_pte_order() is used. For vmas that have
not explicitly opted-in to use transparent hugepages (e.g. where
sysfs=madvise and the vma does not have MADV_HUGEPAGE or sysfs=never),
then arch_wants_pte_order() is limited to 64K (or PAGE_SIZE, whichever
is bigger). This allows for a performance boost without requiring any
explicit opt-in from the workload while limitting internal
fragmentation.

If the preferred order can't be used (e.g. because the folio would
breach the bounds of the vma, or because ptes in the region are already
mapped) then we fall back to a suitable lower order; first
PAGE_ALLOC_COSTLY_ORDER, then order-0.

arch_wants_pte_order() can be overridden by the architecture if desired.
Some architectures (e.g. arm64) can coalsece TLB entries if a contiguous
set of ptes map physically contigious, naturally aligned memory, so this
mechanism allows the architecture to optimize as required.

Here we add the default implementation of arch_wants_pte_order(), used
when the architecture does not define it, which returns -1, implying
that the HW has no preference. In this case, mm will choose it's own
default order.

Signed-off-by: Ryan Roberts <ryan.roberts@xxxxxxx>
---
 include/linux/pgtable.h |  13 ++++
 mm/Kconfig              |  10 +++
 mm/memory.c             | 144 +++++++++++++++++++++++++++++++++++++---
 3 files changed, 158 insertions(+), 9 deletions(-)

diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
index 222a33b9600d..4b488cc66ddc 100644
--- a/include/linux/pgtable.h
+++ b/include/linux/pgtable.h
@@ -369,6 +369,19 @@ static inline bool arch_has_hw_pte_young(void)
 }
 #endif
 
+#ifndef arch_wants_pte_order
+/*
+ * Returns preferred folio order for pte-mapped memory. Must be in range [0,
+ * PMD_SHIFT-PAGE_SHIFT) and must not be order-1 since THP requires large folios
+ * to be at least order-2. Negative value implies that the HW has no preference
+ * and mm will choose it's own default order.
+ */
+static inline int arch_wants_pte_order(void)
+{
+	return -1;
+}
+#endif
+
 #ifndef __HAVE_ARCH_PTEP_GET_AND_CLEAR
 static inline pte_t ptep_get_and_clear(struct mm_struct *mm,
 				       unsigned long address,
diff --git a/mm/Kconfig b/mm/Kconfig
index 721dc88423c7..a1e28b8ddc24 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -1243,4 +1243,14 @@ config LOCK_MM_AND_FIND_VMA
 
 source "mm/damon/Kconfig"
 
+config LARGE_ANON_FOLIO
+	bool "Allocate large folios for anonymous memory"
+	depends on TRANSPARENT_HUGEPAGE
+	default n
+	help
+	  Use large (bigger than order-0) folios to back anonymous memory where
+	  possible, even for pte-mapped memory. This reduces the number of page
+	  faults, as well as other per-page overheads to improve performance for
+	  many workloads.
+
 endmenu
diff --git a/mm/memory.c b/mm/memory.c
index d003076b218d..bbc7d4ce84f7 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -4073,6 +4073,123 @@ 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_LARGE_ANON_FOLIO
+#define ANON_FOLIO_MAX_ORDER_UNHINTED \
+		(ilog2(max_t(unsigned long, SZ_64K, PAGE_SIZE)) - PAGE_SHIFT)
+
+static int anon_folio_order(struct vm_area_struct *vma)
+{
+	int order;
+
+	/*
+	 * If the vma is eligible for thp, allocate a large folio of the size
+	 * preferred by the arch. Or if the arch requested a very small size or
+	 * didn't request a size, then use PAGE_ALLOC_COSTLY_ORDER, which still
+	 * meets the arch's requirements but means we still take advantage of SW
+	 * optimizations (e.g. fewer page faults).
+	 *
+	 * If the vma isn't eligible for thp, take the arch-preferred size and
+	 * limit it to ANON_FOLIO_MAX_ORDER_UNHINTED. This ensures workloads
+	 * that have not explicitly opted-in take benefit while capping the
+	 * potential for internal fragmentation.
+	 */
+
+	order = max(arch_wants_pte_order(), PAGE_ALLOC_COSTLY_ORDER);
+
+	if (!hugepage_vma_check(vma, vma->vm_flags, false, true, true))
+		order = min(order, ANON_FOLIO_MAX_ORDER_UNHINTED);
+
+	return order;
+}
+
+static struct folio *alloc_anon_folio(struct vm_fault *vmf)
+{
+	int i;
+	gfp_t gfp;
+	pte_t *pte;
+	unsigned long addr;
+	struct folio *folio;
+	struct vm_area_struct *vma = vmf->vma;
+	int prefer = anon_folio_order(vma);
+	int orders[] = {
+		prefer,
+		prefer > PAGE_ALLOC_COSTLY_ORDER ? PAGE_ALLOC_COSTLY_ORDER : 0,
+		0,
+	};
+
+	/*
+	 * If uffd is active for the vma we need per-page fault fidelity to
+	 * maintain the uffd semantics.
+	 */
+	if (userfaultfd_armed(vma))
+		goto fallback;
+
+	/*
+	 * If hugepages are explicitly disabled for the vma (either
+	 * MADV_NOHUGEPAGE or prctl) fallback to order-0. Failure to do this
+	 * breaks correctness for user space. We ignore the sysfs global knob.
+	 */
+	if (!hugepage_vma_check(vma, vma->vm_flags, false, true, false))
+		goto fallback;
+
+	for (i = 0; orders[i]; i++) {
+		addr = ALIGN_DOWN(vmf->address, PAGE_SIZE << orders[i]);
+		if (addr >= vma->vm_start &&
+		    addr + (PAGE_SIZE << orders[i]) <= vma->vm_end)
+			break;
+	}
+
+	if (!orders[i])
+		goto fallback;
+
+	pte = pte_offset_map(vmf->pmd, vmf->address & PMD_MASK);
+	if (!pte)
+		return ERR_PTR(-EAGAIN);
+
+	for (; orders[i]; i++) {
+		addr = ALIGN_DOWN(vmf->address, PAGE_SIZE << orders[i]);
+		vmf->pte = pte + pte_index(addr);
+		if (!vmf_pte_range_changed(vmf, 1 << orders[i]))
+			break;
+	}
+
+	vmf->pte = NULL;
+	pte_unmap(pte);
+
+	gfp = vma_thp_gfp_mask(vma);
+
+	for (; orders[i]; i++) {
+		addr = ALIGN_DOWN(vmf->address, PAGE_SIZE << orders[i]);
+		folio = vma_alloc_folio(gfp, orders[i], vma, addr, true);
+		if (folio) {
+			clear_huge_page(&folio->page, addr, 1 << orders[i]);
+			return folio;
+		}
+	}
+
+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.
@@ -4080,6 +4197,9 @@ 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;
@@ -4124,10 +4244,15 @@ 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);
@@ -4144,12 +4269,12 @@ static vm_fault_t do_anonymous_page(struct vm_fault *vmf)
 	if (vma->vm_flags & VM_WRITE)
 		entry = pte_mkwrite(pte_mkdirty(entry));
 
-	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;
 	}
 
@@ -4164,16 +4289,17 @@ 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