Re: [PATCH v4 14/16] arm64/mm: Wire up PTE_CONT for user mappings
From: Alexandre Ghiti
Date: Tue Jan 16 2024 - 17:15:55 EST
On 16/01/2024 15:44, Ryan Roberts wrote:
On 15/01/2024 21:23, Alexandre Ghiti wrote:
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,
I'm not familiar with riscv HW specs or the Linux implementation, so its hard
for me to say for sure. If your __ptep_set_access_flags() implementation is
writing all of entry to the PTE, then it looks like its probably wrong to me -
you will be writing the same PFN to every PTE in the contpte block. Certainly on
Arm that would be wrong.
On riscv, ptes in the same contiguous range are expected to have the
same pfn because the lsbs of the pfn are used to advertise the number of
ptes in the mapping. The HW will use the virtual bits to address the
correct page (see page 89 of the riscv privileged specification
https://drive.google.com/file/d/1EMip5dZlnypTk7pt4WWUKmtjUKTOkBqh/view).
On Arm, __ptep_set_access_flags() is just folding the access flags in entry into
the PTE without changing anything else. That's needed in order to deal with
racing HW updates of the access/dirty bits.
Ok I have to think more about that, we don't do that (neither does x86),
so maybe I'm missing something.
Thanks,
Alex
Thanks,
Ryan
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);