Re: [PATCH v4 14/16] arm64/mm: Wire up PTE_CONT for user mappings

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On 15/01/2024 17:27, Ryan Roberts wrote:
On 15/01/2024 15:14, Alexandre Ghiti wrote:
Hi Ryan,

On 18/12/2023 11:50, Ryan Roberts wrote:
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. Whenever it detects a set of PTEs that meet the
requirements for a contiguous range, the PTEs are re-painted with the
PTE_CONT bit. Use of contpte mappings is intended to be transparent to
the core-mm, which continues to interact with individual ptes.

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.

This initial change provides a baseline that can be optimized in future
commits. That said, fold/unfold operations (which imply tlb
invalidation) are avoided where possible with a few tricks for
access/dirty bit management. Write-protect modifications for contpte
mappings are currently non-optimal, and incure a regression in fork()
performance. This will be addressed in follow-up changes.

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.

Tested-by: John Hubbard <jhubbard@xxxxxxxxxx>
Signed-off-by: Ryan Roberts <ryan.roberts@xxxxxxx>
---
   arch/arm64/Kconfig               |  10 +-
   arch/arm64/include/asm/pgtable.h | 184 +++++++++++++++
   arch/arm64/mm/Makefile           |   1 +
   arch/arm64/mm/contpte.c          | 388 +++++++++++++++++++++++++++++++
   4 files changed, 582 insertions(+), 1 deletion(-)
   create mode 100644 arch/arm64/mm/contpte.c

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 7b071a00425d..de76e484ff3a 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -2209,6 +2209,15 @@ config UNWIND_PATCH_PAC_INTO_SCS
       select UNWIND_TABLES
       select DYNAMIC_SCS
   +config ARM64_CONTPTE
+    bool "Contiguous PTE mappings for user memory" if EXPERT
+    depends on TRANSPARENT_HUGEPAGE
+    default y
+    help
+      When enabled, user mappings are configured using the PTE contiguous
+      bit, for any mappings that meet the size and alignment requirements.
+      This reduces TLB pressure and improves performance.
+
   endmenu # "Kernel Features"
     menu "Boot options"
@@ -2318,4 +2327,3 @@ endmenu # "CPU Power Management"
   source "drivers/acpi/Kconfig"
     source "arch/arm64/kvm/Kconfig"
-
diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
index 6930c14f062f..e64120452301 100644
--- a/arch/arm64/include/asm/pgtable.h
+++ b/arch/arm64/include/asm/pgtable.h
@@ -133,6 +133,10 @@ static inline pteval_t __phys_to_pte_val(phys_addr_t phys)
    */
   #define pte_valid_not_user(pte) \
       ((pte_val(pte) & (PTE_VALID | PTE_USER | PTE_UXN)) == (PTE_VALID |
PTE_UXN))
+/*
+ * Returns true if the pte is valid and has the contiguous bit set.
+ */
+#define pte_valid_cont(pte)    (pte_valid(pte) && pte_cont(pte))
   /*
    * Could the pte be present in the TLB? We must check mm_tlb_flush_pending
    * so that we don't erroneously return false for pages that have been
@@ -1116,6 +1120,184 @@ extern void ptep_modify_prot_commit(struct
vm_area_struct *vma,
                       unsigned long addr, pte_t *ptep,
                       pte_t old_pte, pte_t new_pte);
   +#ifdef CONFIG_ARM64_CONTPTE
+
+/*
+ * The contpte APIs are used to transparently manage the contiguous bit in ptes
+ * where it is possible and makes sense to do so. The PTE_CONT bit is considered
+ * a private implementation detail of the public ptep API (see below).
+ */
+extern void __contpte_try_fold(struct mm_struct *mm, unsigned long addr,
+                pte_t *ptep, pte_t pte);
+extern void __contpte_try_unfold(struct mm_struct *mm, unsigned long addr,
+                pte_t *ptep, pte_t pte);
+extern pte_t contpte_ptep_get(pte_t *ptep, pte_t orig_pte);
+extern pte_t contpte_ptep_get_lockless(pte_t *orig_ptep);
+extern void contpte_set_ptes(struct mm_struct *mm, unsigned long addr,
+                pte_t *ptep, pte_t pte, unsigned int nr);
+extern int contpte_ptep_test_and_clear_young(struct vm_area_struct *vma,
+                unsigned long addr, pte_t *ptep);
+extern int contpte_ptep_clear_flush_young(struct vm_area_struct *vma,
+                unsigned long addr, pte_t *ptep);
+extern int contpte_ptep_set_access_flags(struct vm_area_struct *vma,
+                unsigned long addr, pte_t *ptep,
+                pte_t entry, int dirty);
+
+static inline void contpte_try_fold(struct mm_struct *mm, unsigned long addr,
+                    pte_t *ptep, pte_t pte)
+{
+    /*
+     * Only bother trying if both the virtual and physical addresses are
+     * aligned and correspond to the last entry in a contig range. The core
+     * code mostly modifies ranges from low to high, so this is the likely
+     * the last modification in the contig range, so a good time to fold.
+     * We can't fold special mappings, because there is no associated folio.
+     */
+
+    const unsigned long contmask = CONT_PTES - 1;
+    bool valign = (((unsigned long)ptep >> 3) & contmask) == contmask;
+    bool palign = (pte_pfn(pte) & contmask) == contmask;
+
+    if (valign && palign &&
+        pte_valid(pte) && !pte_cont(pte) && !pte_special(pte))
+        __contpte_try_fold(mm, addr, ptep, pte);
+}
+
+static inline void contpte_try_unfold(struct mm_struct *mm, unsigned long addr,
+                    pte_t *ptep, pte_t pte)
+{
+    if (pte_valid_cont(pte))
+        __contpte_try_unfold(mm, addr, ptep, pte);
+}
+
+/*
+ * The below functions constitute the public API that arm64 presents to the
+ * core-mm to manipulate PTE entries within their page tables (or at least this
+ * is the subset of the API that arm64 needs to implement). These public
+ * versions will automatically and transparently apply the contiguous bit where
+ * it makes sense to do so. Therefore any users that are contig-aware (e.g.
+ * hugetlb, kernel mapper) should NOT use these APIs, but instead use the
+ * private versions, which are prefixed with double underscore. All of these
+ * APIs except for ptep_get_lockless() are expected to be called with the PTL
+ * held.
+ */
+
+#define ptep_get ptep_get
+static inline pte_t ptep_get(pte_t *ptep)
+{
+    pte_t pte = __ptep_get(ptep);
+
+    if (!pte_valid_cont(pte))
+        return pte;
+
+    return contpte_ptep_get(ptep, pte);
+}
+
+#define ptep_get_lockless ptep_get_lockless
+static inline pte_t ptep_get_lockless(pte_t *ptep)
+{
+    pte_t pte = __ptep_get(ptep);
+
+    if (!pte_valid_cont(pte))
+        return pte;
+
+    return contpte_ptep_get_lockless(ptep);
+}
+
+static inline void set_pte(pte_t *ptep, pte_t pte)
+{
+    /*
+     * We don't have the mm or vaddr so cannot unfold or fold contig entries
+     * (since it requires tlb maintenance). set_pte() is not used in core
+     * code, so this should never even be called. Regardless do our best to
+     * service any call and emit a warning if there is any attempt to set a
+     * pte on top of an existing contig range.
+     */
+    pte_t orig_pte = __ptep_get(ptep);
+
+    WARN_ON_ONCE(pte_valid_cont(orig_pte));
+    __set_pte(ptep, pte_mknoncont(pte));
+}
+
+#define set_ptes set_ptes
+static inline void set_ptes(struct mm_struct *mm, unsigned long addr,
+                pte_t *ptep, pte_t pte, unsigned int nr)
+{
+    pte = pte_mknoncont(pte);
+
+    if (nr == 1) {
+        contpte_try_unfold(mm, addr, ptep, __ptep_get(ptep));
+        __set_ptes(mm, addr, ptep, pte, 1);
+        contpte_try_fold(mm, addr, ptep, pte);
+    } else
+        contpte_set_ptes(mm, addr, ptep, pte, nr);
+}
+
+static inline void pte_clear(struct mm_struct *mm,
+                unsigned long addr, pte_t *ptep)
+{
+    contpte_try_unfold(mm, addr, ptep, __ptep_get(ptep));
+    __pte_clear(mm, addr, ptep);
+}
+
+#define __HAVE_ARCH_PTEP_GET_AND_CLEAR
+static inline pte_t ptep_get_and_clear(struct mm_struct *mm,
+                unsigned long addr, pte_t *ptep)
+{
+    contpte_try_unfold(mm, addr, ptep, __ptep_get(ptep));
+    return __ptep_get_and_clear(mm, addr, ptep);
+}
+
+#define __HAVE_ARCH_PTEP_TEST_AND_CLEAR_YOUNG
+static inline int ptep_test_and_clear_young(struct vm_area_struct *vma,
+                unsigned long addr, pte_t *ptep)
+{
+    pte_t orig_pte = __ptep_get(ptep);
+
+    if (!pte_valid_cont(orig_pte))
+        return __ptep_test_and_clear_young(vma, addr, ptep);
+
+    return contpte_ptep_test_and_clear_young(vma, addr, ptep);
+}
+
+#define __HAVE_ARCH_PTEP_CLEAR_YOUNG_FLUSH
+static inline int ptep_clear_flush_young(struct vm_area_struct *vma,
+                unsigned long addr, pte_t *ptep)
+{
+    pte_t orig_pte = __ptep_get(ptep);
+
+    if (!pte_valid_cont(orig_pte))
+        return __ptep_clear_flush_young(vma, addr, ptep);
+
+    return contpte_ptep_clear_flush_young(vma, addr, ptep);
+}
+
+#define __HAVE_ARCH_PTEP_SET_WRPROTECT
+static inline void ptep_set_wrprotect(struct mm_struct *mm,
+                unsigned long addr, pte_t *ptep)
+{
+    contpte_try_unfold(mm, addr, ptep, __ptep_get(ptep));
+    __ptep_set_wrprotect(mm, addr, ptep);
+    contpte_try_fold(mm, addr, ptep, __ptep_get(ptep));
+}
+
+#define __HAVE_ARCH_PTEP_SET_ACCESS_FLAGS
+static inline int ptep_set_access_flags(struct vm_area_struct *vma,
+                unsigned long addr, pte_t *ptep,
+                pte_t entry, int dirty)
+{
+    pte_t orig_pte = __ptep_get(ptep);
+
+    entry = pte_mknoncont(entry);
+
+    if (!pte_valid_cont(orig_pte))
+        return __ptep_set_access_flags(vma, addr, ptep, entry, dirty);
+
+    return contpte_ptep_set_access_flags(vma, addr, ptep, entry, dirty);
+}
+
+#else /* CONFIG_ARM64_CONTPTE */
+
   #define ptep_get                __ptep_get
   #define set_pte                    __set_pte
   #define set_ptes                __set_ptes
@@ -1131,6 +1313,8 @@ extern void ptep_modify_prot_commit(struct
vm_area_struct *vma,
   #define __HAVE_ARCH_PTEP_SET_ACCESS_FLAGS
   #define ptep_set_access_flags            __ptep_set_access_flags
   +#endif /* CONFIG_ARM64_CONTPTE */
+
   #endif /* !__ASSEMBLY__ */
     #endif /* __ASM_PGTABLE_H */
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..69c36749dd98
--- /dev/null
+++ b/arch/arm64/mm/contpte.c
@@ -0,0 +1,388 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Copyright (C) 2023 ARM Ltd.
+ */
+
+#include <linux/mm.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.
+     */
+    return mm != &init_mm;
+}
+
+static inline pte_t *contpte_align_down(pte_t *ptep)
+{
+    return (pte_t *)(ALIGN_DOWN((unsigned long)ptep >> 3, CONT_PTES) << 3);
+}
+
+static void ptep_clear_flush_range(struct mm_struct *mm, unsigned long addr,
+                pte_t *ptep, int nr)
+{
+    struct vm_area_struct vma = TLB_FLUSH_VMA(mm, 0);
+    unsigned long start_addr = addr;
+    int i;
+
+    for (i = 0; i < nr; i++, ptep++, addr += PAGE_SIZE)
+        __pte_clear(mm, addr, ptep);
+
+    __flush_tlb_range(&vma, start_addr, addr, PAGE_SIZE, true, 3);
+}
+
+static bool ptep_any_valid(pte_t *ptep, int nr)
+{
+    int i;
+
+    for (i = 0; i < nr; i++, ptep++) {
+        if (pte_valid(__ptep_get(ptep)))
+            return true;
+    }
+
+    return false;
+}
+
+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_fold(struct mm_struct *mm, unsigned long addr,
+            pte_t *ptep, pte_t pte)
+{
+    /*
+     * We have already checked that the virtual and pysical addresses are
+     * correctly aligned for a contpte mapping in contpte_try_fold() so the
+     * remaining checks are to ensure that the contpte range is fully
+     * covered by a single folio, and ensure that all the ptes are valid
+     * with contiguous PFNs and matching prots. We ignore the state of the
+     * access and dirty bits for the purpose of deciding if its a contiguous
+     * range; the folding process will generate a single contpte entry which
+     * has a single access and dirty bit. Those 2 bits are the logical OR of
+     * their respective bits in the constituent pte entries. In order to
+     * ensure the contpte range is covered by a single folio, we must
+     * recover the folio from the pfn, but special mappings don't have a
+     * folio backing them. Fortunately contpte_try_fold() already checked
+     * that the pte is not special - we never try to fold special mappings.
+     * Note we can't use vm_normal_page() for this since we don't have the
+     * vma.
+     */
+
+    unsigned long folio_saddr;
+    unsigned long folio_eaddr;
+    unsigned long cont_saddr;
+    unsigned long cont_eaddr;
+    struct folio *folio;
+    struct page *page;
+    unsigned long pfn;
+    pte_t *orig_ptep;
+    pgprot_t prot;
+    pte_t subpte;
+    int i;
+
+    if (!mm_is_user(mm))
+        return;
+
+    page = pte_page(pte);
+    folio = page_folio(page);
+    folio_saddr = addr - (page - &folio->page) * PAGE_SIZE;
+    folio_eaddr = folio_saddr + folio_nr_pages(folio) * PAGE_SIZE;
+    cont_saddr = ALIGN_DOWN(addr, CONT_PTE_SIZE);
+    cont_eaddr = cont_saddr + CONT_PTE_SIZE;
+
+    if (folio_saddr > cont_saddr || folio_eaddr < cont_eaddr)
+        return;
+
+    pfn = pte_pfn(pte) - ((addr - cont_saddr) >> PAGE_SHIFT);
+    prot = pte_pgprot(pte_mkold(pte_mkclean(pte)));
+    orig_ptep = ptep;
+    ptep = contpte_align_down(ptep);
+
+    for (i = 0; i < CONT_PTES; i++, ptep++, pfn++) {
+        subpte = __ptep_get(ptep);
+        subpte = pte_mkold(pte_mkclean(subpte));
+
+        if (!pte_valid(subpte) ||
+            pte_pfn(subpte) != pfn ||
+            pgprot_val(pte_pgprot(subpte)) != pgprot_val(prot))
+            return;
+    }
+
+    pte = pte_mkcont(pte);
+    contpte_convert(mm, addr, orig_ptep, pte);
+}
+EXPORT_SYMBOL(__contpte_try_fold);
+
+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 guarranteed 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;
+    pte_t orig_pte;
+
+    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);
+
+        /*
+         * If operating on a partial contiguous range then we must first
+         * unfold the contiguous range if it was previously folded.
+         * Otherwise we could end up with overlapping tlb entries.
+         */
+        if (nr != CONT_PTES)
+            contpte_try_unfold(mm, addr, ptep, __ptep_get(ptep));
+
+        /*
+         * If we are replacing ptes that were contiguous or if the new
+         * ptes are contiguous and any of the ptes being replaced are
+         * valid, we need to clear and flush the range to prevent
+         * overlapping tlb entries.
+         */
+        orig_pte = __ptep_get(ptep);
+        if (pte_valid_cont(orig_pte) ||
+            (pte_cont(pte) && ptep_any_valid(ptep, nr)))
+            ptep_clear_flush_range(mm, addr, ptep, nr);
+
+        __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/fold the
+     * contig range. But if the write bit is changing, we need to go through
+     * the full unfold/fold cycle.
+     */
+    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);

entry was pte_mknoncont() in ptep_set_access_flags() so here you lose the
contpte range, is that intentional? Or am I mistaken?
entry doesn't have PTE_CONT bit set, that's correct. I intentionally strip that
bit at the interface boundary, because it is the implementation's job to decide
whether its a contpte block, not the caller's. But there are situations where
the caller can end up with a pte that has PTE_CONT set (by having done a
previous ptep_get() for example) and then it forwards the pte to a setter. So
stripping it is required; It would probably be cleaner to strip it before
returning it from ptep_get(), but that would be problematic for pte_leaf_size()
which is called from perf_get_pgtable_size().

In this particular case, __ptep_set_access_flags() only modifies the PTE's
access flags, so CONT_PTE will remain as it is in the page table. The fact that
entry has it cleared is not a problem.


I see, I had not checked the arm64 implementation of ptep_set_access_flags(). For context, I'm merging the arm64 contpte support with the riscv napot support, the implementation being quite similar (although riscv is a bit different as it uses bits from the pfn to advertise the number of contiguous ptes).

Anyway,  our implementation of ptep_set_access_flags() actually sets the ptep with entry, so we would actually lose the cont bit. I would simply do the following (I will in my patchset, no need for you to worry about this):

__ptep_set_access_flags(vma, addr, ptep, pte_mkcont(entry), 0);

Let me know if you think this is not right,

Thanks,

Alex



Thanks,
Ryan



+
+        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);
+        contpte_try_fold(vma->vm_mm, addr, ptep, entry);
+    }
+
+    return 1;
+}
+EXPORT_SYMBOL(contpte_ptep_set_access_flags);




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