arm64/mm: wire up PTE_CONT for user mappings

With the ptep API sufficiently refactored, we can now introduce a new
"contpte" API layer, which transparently manages the PTE_CONT bit for user
mappings.

In this initial implementation, only suitable batches of PTEs, set via
set_ptes(), are mapped with the PTE_CONT bit.  Any subsequent modification
of individual PTEs will cause an "unfold" operation to repaint the contpte
block as individual PTEs before performing the requested operation. 
While, a modification of a single PTE could cause the block of PTEs to
which it belongs to become eligible for "folding" into a contpte entry,
"folding" is not performed in this initial implementation due to the costs
of checking the requirements are met.  Due to this, contpte mappings will
degrade back to normal pte mappings over time if/when protections are
changed.  This will be solved in a future patch.

Since a contpte block only has a single access and dirty bit, the semantic
here changes slightly; when getting a pte (e.g.  ptep_get()) that is part
of a contpte mapping, the access and dirty information are pulled from the
block (so all ptes in the block return the same access/dirty info).  When
changing the access/dirty info on a pte (e.g.  ptep_set_access_flags())
that is part of a contpte mapping, this change will affect the whole
contpte block.  This is works fine in practice since we guarantee that
only a single folio is mapped by a contpte block, and the core-mm tracks
access/dirty information per folio.

In order for the public functions, which used to be pure inline, to
continue to be callable by modules, export all the contpte_* symbols that
are now called by those public inline functions.

The feature is enabled/disabled with the ARM64_CONTPTE Kconfig parameter
at build time.  It defaults to enabled as long as its dependency,
TRANSPARENT_HUGEPAGE is also enabled.  The core-mm depends upon
TRANSPARENT_HUGEPAGE to be able to allocate large folios, so if its not
enabled, then there is no chance of meeting the physical contiguity
requirement for contpte mappings.

Link: https://lkml.kernel.org/r/20240215103205.2607016-13-ryan.roberts@arm.com
Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Tested-by: John Hubbard <jhubbard@nvidia.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Andrey Ryabinin <ryabinin.a.a@gmail.com>
Cc: Barry Song <21cnbao@gmail.com>
Cc: Borislav Petkov (AMD) <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: James Morse <james.morse@arm.com>
Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Will Deacon <will@kernel.org>
Cc: Yang Shi <shy828301@gmail.com>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

2 files changed, 286 insertions, 0 deletions

diff --git a/arch/arm64/mm/Makefile b/arch/arm64/mm/Makefile
index dbd1bc95967d..60454256945b 100644
--- a/arch/arm64/mm/Makefile
+++ b/arch/arm64/mm/Makefile
@@ -3,6 +3,7 @@ obj-y				:= dma-mapping.o extable.o fault.o init.o \
 				   cache.o copypage.o flush.o \
 				   ioremap.o mmap.o pgd.o mmu.o \
 				   context.o proc.o pageattr.o fixmap.o
+obj-$(CONFIG_ARM64_CONTPTE)	+= contpte.o
 obj-$(CONFIG_HUGETLB_PAGE)	+= hugetlbpage.o
 obj-$(CONFIG_PTDUMP_CORE)	+= ptdump.o
 obj-$(CONFIG_PTDUMP_DEBUGFS)	+= ptdump_debugfs.o
diff --git a/arch/arm64/mm/contpte.c b/arch/arm64/mm/contpte.c
new file mode 100644
index 000000000000..6d7f40667fa2
--- /dev/null
+++ b/arch/arm64/mm/contpte.c
@@ -0,0 +1,285 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Copyright (C) 2023 ARM Ltd.
+ */
+
+#include <linux/mm.h>
+#include <linux/efi.h>
+#include <linux/export.h>
+#include <asm/tlbflush.h>
+
+static inline bool mm_is_user(struct mm_struct *mm)
+{
+	/*
+	 * Don't attempt to apply the contig bit to kernel mappings, because
+	 * dynamically adding/removing the contig bit can cause page faults.
+	 * These racing faults are ok for user space, since they get serialized
+	 * on the PTL. But kernel mappings can't tolerate faults.
+	 */
+	if (unlikely(mm_is_efi(mm)))
+		return false;
+	return mm != &init_mm;
+}
+
+static inline pte_t *contpte_align_down(pte_t *ptep)
+{
+	return PTR_ALIGN_DOWN(ptep, sizeof(*ptep) * CONT_PTES);
+}
+
+static void contpte_convert(struct mm_struct *mm, unsigned long addr,
+			    pte_t *ptep, pte_t pte)
+{
+	struct vm_area_struct vma = TLB_FLUSH_VMA(mm, 0);
+	unsigned long start_addr;
+	pte_t *start_ptep;
+	int i;
+
+	start_ptep = ptep = contpte_align_down(ptep);
+	start_addr = addr = ALIGN_DOWN(addr, CONT_PTE_SIZE);
+	pte = pfn_pte(ALIGN_DOWN(pte_pfn(pte), CONT_PTES), pte_pgprot(pte));
+
+	for (i = 0; i < CONT_PTES; i++, ptep++, addr += PAGE_SIZE) {
+		pte_t ptent = __ptep_get_and_clear(mm, addr, ptep);
+
+		if (pte_dirty(ptent))
+			pte = pte_mkdirty(pte);
+
+		if (pte_young(ptent))
+			pte = pte_mkyoung(pte);
+	}
+
+	__flush_tlb_range(&vma, start_addr, addr, PAGE_SIZE, true, 3);
+
+	__set_ptes(mm, start_addr, start_ptep, pte, CONT_PTES);
+}
+
+void __contpte_try_unfold(struct mm_struct *mm, unsigned long addr,
+			pte_t *ptep, pte_t pte)
+{
+	/*
+	 * We have already checked that the ptes are contiguous in
+	 * contpte_try_unfold(), so just check that the mm is user space.
+	 */
+	if (!mm_is_user(mm))
+		return;
+
+	pte = pte_mknoncont(pte);
+	contpte_convert(mm, addr, ptep, pte);
+}
+EXPORT_SYMBOL(__contpte_try_unfold);
+
+pte_t contpte_ptep_get(pte_t *ptep, pte_t orig_pte)
+{
+	/*
+	 * Gather access/dirty bits, which may be populated in any of the ptes
+	 * of the contig range. We are guaranteed to be holding the PTL, so any
+	 * contiguous range cannot be unfolded or otherwise modified under our
+	 * feet.
+	 */
+
+	pte_t pte;
+	int i;
+
+	ptep = contpte_align_down(ptep);
+
+	for (i = 0; i < CONT_PTES; i++, ptep++) {
+		pte = __ptep_get(ptep);
+
+		if (pte_dirty(pte))
+			orig_pte = pte_mkdirty(orig_pte);
+
+		if (pte_young(pte))
+			orig_pte = pte_mkyoung(orig_pte);
+	}
+
+	return orig_pte;
+}
+EXPORT_SYMBOL(contpte_ptep_get);
+
+pte_t contpte_ptep_get_lockless(pte_t *orig_ptep)
+{
+	/*
+	 * Gather access/dirty bits, which may be populated in any of the ptes
+	 * of the contig range. We may not be holding the PTL, so any contiguous
+	 * range may be unfolded/modified/refolded under our feet. Therefore we
+	 * ensure we read a _consistent_ contpte range by checking that all ptes
+	 * in the range are valid and have CONT_PTE set, that all pfns are
+	 * contiguous and that all pgprots are the same (ignoring access/dirty).
+	 * If we find a pte that is not consistent, then we must be racing with
+	 * an update so start again. If the target pte does not have CONT_PTE
+	 * set then that is considered consistent on its own because it is not
+	 * part of a contpte range.
+	 */
+
+	pgprot_t orig_prot;
+	unsigned long pfn;
+	pte_t orig_pte;
+	pgprot_t prot;
+	pte_t *ptep;
+	pte_t pte;
+	int i;
+
+retry:
+	orig_pte = __ptep_get(orig_ptep);
+
+	if (!pte_valid_cont(orig_pte))
+		return orig_pte;
+
+	orig_prot = pte_pgprot(pte_mkold(pte_mkclean(orig_pte)));
+	ptep = contpte_align_down(orig_ptep);
+	pfn = pte_pfn(orig_pte) - (orig_ptep - ptep);
+
+	for (i = 0; i < CONT_PTES; i++, ptep++, pfn++) {
+		pte = __ptep_get(ptep);
+		prot = pte_pgprot(pte_mkold(pte_mkclean(pte)));
+
+		if (!pte_valid_cont(pte) ||
+		   pte_pfn(pte) != pfn ||
+		   pgprot_val(prot) != pgprot_val(orig_prot))
+			goto retry;
+
+		if (pte_dirty(pte))
+			orig_pte = pte_mkdirty(orig_pte);
+
+		if (pte_young(pte))
+			orig_pte = pte_mkyoung(orig_pte);
+	}
+
+	return orig_pte;
+}
+EXPORT_SYMBOL(contpte_ptep_get_lockless);
+
+void contpte_set_ptes(struct mm_struct *mm, unsigned long addr,
+					pte_t *ptep, pte_t pte, unsigned int nr)
+{
+	unsigned long next;
+	unsigned long end;
+	unsigned long pfn;
+	pgprot_t prot;
+
+	/*
+	 * The set_ptes() spec guarantees that when nr > 1, the initial state of
+	 * all ptes is not-present. Therefore we never need to unfold or
+	 * otherwise invalidate a range before we set the new ptes.
+	 * contpte_set_ptes() should never be called for nr < 2.
+	 */
+	VM_WARN_ON(nr == 1);
+
+	if (!mm_is_user(mm))
+		return __set_ptes(mm, addr, ptep, pte, nr);
+
+	end = addr + (nr << PAGE_SHIFT);
+	pfn = pte_pfn(pte);
+	prot = pte_pgprot(pte);
+
+	do {
+		next = pte_cont_addr_end(addr, end);
+		nr = (next - addr) >> PAGE_SHIFT;
+		pte = pfn_pte(pfn, prot);
+
+		if (((addr | next | (pfn << PAGE_SHIFT)) & ~CONT_PTE_MASK) == 0)
+			pte = pte_mkcont(pte);
+		else
+			pte = pte_mknoncont(pte);
+
+		__set_ptes(mm, addr, ptep, pte, nr);
+
+		addr = next;
+		ptep += nr;
+		pfn += nr;
+
+	} while (addr != end);
+}
+EXPORT_SYMBOL(contpte_set_ptes);
+
+int contpte_ptep_test_and_clear_young(struct vm_area_struct *vma,
+					unsigned long addr, pte_t *ptep)
+{
+	/*
+	 * ptep_clear_flush_young() technically requires us to clear the access
+	 * flag for a _single_ pte. However, the core-mm code actually tracks
+	 * access/dirty per folio, not per page. And since we only create a
+	 * contig range when the range is covered by a single folio, we can get
+	 * away with clearing young for the whole contig range here, so we avoid
+	 * having to unfold.
+	 */
+
+	int young = 0;
+	int i;
+
+	ptep = contpte_align_down(ptep);
+	addr = ALIGN_DOWN(addr, CONT_PTE_SIZE);
+
+	for (i = 0; i < CONT_PTES; i++, ptep++, addr += PAGE_SIZE)
+		young |= __ptep_test_and_clear_young(vma, addr, ptep);
+
+	return young;
+}
+EXPORT_SYMBOL(contpte_ptep_test_and_clear_young);
+
+int contpte_ptep_clear_flush_young(struct vm_area_struct *vma,
+					unsigned long addr, pte_t *ptep)
+{
+	int young;
+
+	young = contpte_ptep_test_and_clear_young(vma, addr, ptep);
+
+	if (young) {
+		/*
+		 * See comment in __ptep_clear_flush_young(); same rationale for
+		 * eliding the trailing DSB applies here.
+		 */
+		addr = ALIGN_DOWN(addr, CONT_PTE_SIZE);
+		__flush_tlb_range_nosync(vma, addr, addr + CONT_PTE_SIZE,
+					 PAGE_SIZE, true, 3);
+	}
+
+	return young;
+}
+EXPORT_SYMBOL(contpte_ptep_clear_flush_young);
+
+int contpte_ptep_set_access_flags(struct vm_area_struct *vma,
+					unsigned long addr, pte_t *ptep,
+					pte_t entry, int dirty)
+{
+	unsigned long start_addr;
+	pte_t orig_pte;
+	int i;
+
+	/*
+	 * Gather the access/dirty bits for the contiguous range. If nothing has
+	 * changed, its a noop.
+	 */
+	orig_pte = pte_mknoncont(ptep_get(ptep));
+	if (pte_val(orig_pte) == pte_val(entry))
+		return 0;
+
+	/*
+	 * We can fix up access/dirty bits without having to unfold the contig
+	 * range. But if the write bit is changing, we must unfold.
+	 */
+	if (pte_write(orig_pte) == pte_write(entry)) {
+		/*
+		 * For HW access management, we technically only need to update
+		 * the flag on a single pte in the range. But for SW access
+		 * management, we need to update all the ptes to prevent extra
+		 * faults. Avoid per-page tlb flush in __ptep_set_access_flags()
+		 * and instead flush the whole range at the end.
+		 */
+		ptep = contpte_align_down(ptep);
+		start_addr = addr = ALIGN_DOWN(addr, CONT_PTE_SIZE);
+
+		for (i = 0; i < CONT_PTES; i++, ptep++, addr += PAGE_SIZE)
+			__ptep_set_access_flags(vma, addr, ptep, entry, 0);
+
+		if (dirty)
+			__flush_tlb_range(vma, start_addr, addr,
+							PAGE_SIZE, true, 3);
+	} else {
+		__contpte_try_unfold(vma->vm_mm, addr, ptep, orig_pte);
+		__ptep_set_access_flags(vma, addr, ptep, entry, dirty);
+	}
+
+	return 1;
+}
+EXPORT_SYMBOL(contpte_ptep_set_access_flags);