riscv: mm: Pre-allocate PGD entries for vmalloc/modules area

The RISC-V port requires that kernel PGD entries are to be
synchronized between MMs. This is done via the vmalloc_fault()
function, that simply copies the PGD entries from init_mm to the
faulting one.

Historically, faulting in PGD entries have been a source for both bugs
[1], and poor performance.

One way to get rid of vmalloc faults is by pre-allocating the PGD
entries. Pre-allocating the entries potientially wastes 64 * 4K (65 on
SV39). The pre-allocation function is pulled from Jörg Rödel's x86
work, with the addition of 3-level page tables (PMD allocations).

The pmd_alloc() function needs the ptlock cache to be initialized
(when split page locks is enabled), so the pre-allocation is done in a
RISC-V specific pgtable_cache_init() implementation.

Pre-allocate the kernel PGD entries for the vmalloc/modules area, but
only for 64b platforms.

Link: https://lore.kernel.org/lkml/20200508144043.13893-1-joro@8bytes.org/
Signed-off-by: Björn Töpel <bjorn@rivosinc.com>
Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Link: https://lore.kernel.org/r/20230531093817.665799-1-bjorn@kernel.org
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

diff --git a/arch/riscv/mm/fault.c b/arch/riscv/mm/fault.c
index 8685f85..b023fb3 100644 (file)
--- a/arch/riscv/mm/fault.c
+++ b/arch/riscv/mm/fault.c
@@ -238,24 +238,12 @@ void handle_page_fault(struct pt_regs *regs)
         * only copy the information from the master page table,
         * nothing more.
         */
-       if (unlikely((addr >= VMALLOC_START) && (addr < VMALLOC_END))) {
+       if ((!IS_ENABLED(CONFIG_MMU) || !IS_ENABLED(CONFIG_64BIT)) &&
+           unlikely(addr >= VMALLOC_START && addr < VMALLOC_END)) {
                vmalloc_fault(regs, code, addr);
                return;
        }
 
-#ifdef CONFIG_64BIT
-       /*
-        * Modules in 64bit kernels lie in their own virtual region which is not
-        * in the vmalloc region, but dealing with page faults in this region
-        * or the vmalloc region amounts to doing the same thing: checking that
-        * the mapping exists in init_mm.pgd and updating user page table, so
-        * just use vmalloc_fault.
-        */
-       if (unlikely(addr >= MODULES_VADDR && addr < MODULES_END)) {
-               vmalloc_fault(regs, code, addr);
-               return;
-       }
-#endif
        /* Enable interrupts if they were enabled in the parent context. */
        if (!regs_irqs_disabled(regs))
                local_irq_enable();

diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
index 747e5b1..45ceaff 100644 (file)
--- a/arch/riscv/mm/init.c
+++ b/arch/riscv/mm/init.c
@@ -1363,3 +1363,61 @@ int __meminit vmemmap_populate(unsigned long start, unsigned long end, int node,
        return vmemmap_populate_basepages(start, end, node, NULL);
 }
 #endif
+
+#if defined(CONFIG_MMU) && defined(CONFIG_64BIT)
+/*
+ * Pre-allocates page-table pages for a specific area in the kernel
+ * page-table. Only the level which needs to be synchronized between
+ * all page-tables is allocated because the synchronization can be
+ * expensive.
+ */
+static void __init preallocate_pgd_pages_range(unsigned long start, unsigned long end,
+                                              const char *area)
+{
+       unsigned long addr;
+       const char *lvl;
+
+       for (addr = start; addr < end && addr >= start; addr = ALIGN(addr + 1, PGDIR_SIZE)) {
+               pgd_t *pgd = pgd_offset_k(addr);
+               p4d_t *p4d;
+               pud_t *pud;
+               pmd_t *pmd;
+
+               lvl = "p4d";
+               p4d = p4d_alloc(&init_mm, pgd, addr);
+               if (!p4d)
+                       goto failed;
+
+               if (pgtable_l5_enabled)
+                       continue;
+
+               lvl = "pud";
+               pud = pud_alloc(&init_mm, p4d, addr);
+               if (!pud)
+                       goto failed;
+
+               if (pgtable_l4_enabled)
+                       continue;
+
+               lvl = "pmd";
+               pmd = pmd_alloc(&init_mm, pud, addr);
+               if (!pmd)
+                       goto failed;
+       }
+       return;
+
+failed:
+       /*
+        * The pages have to be there now or they will be missing in
+        * process page-tables later.
+        */
+       panic("Failed to pre-allocate %s pages for %s area\n", lvl, area);
+}
+
+void __init pgtable_cache_init(void)
+{
+       preallocate_pgd_pages_range(VMALLOC_START, VMALLOC_END, "vmalloc");
+       if (IS_ENABLED(CONFIG_MODULES))
+               preallocate_pgd_pages_range(MODULES_VADDR, MODULES_END, "bpf/modules");
+}
+#endif