On Mon, 29 May 2023 11:00:23 PDT (-0700), bjorn@xxxxxxxxxx wrote:
From: Björn Töpel <bjorn@xxxxxxxxxxxx>
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@xxxxxxxxxx/ # [1]
Signed-off-by: Björn Töpel <bjorn@xxxxxxxxxxxx>
---
arch/riscv/mm/fault.c | 20 +++------------
arch/riscv/mm/init.c | 58 +++++++++++++++++++++++++++++++++++++++++++
2 files changed, 62 insertions(+), 16 deletions(-)
diff --git a/arch/riscv/mm/fault.c b/arch/riscv/mm/fault.c
index 8685f85a7474..6b0b5e517e12 100644
--- a/arch/riscv/mm/fault.c
+++ b/arch/riscv/mm/fault.c
@@ -230,32 +230,20 @@ void handle_page_fault(struct pt_regs *regs)
return;
/*
- * Fault-in kernel-space virtual memory on-demand.
- * The 'reference' page table is init_mm.pgd.
+ * Fault-in kernel-space virtual memory on-demand, for 32-bit
+ * architectures. The 'reference' page table is init_mm.pgd.
That wording seems a little odd to me: I think English allows for these
"add something after the comma to change the meaning of a sentence"
things, but they're kind of complicated. Maybe it's easier to just flip
the order?
That said, it's very early so maybe it's fine...
*
* NOTE! We MUST NOT take any locks for this case. We may
* be in an interrupt or a critical region, and should
* only copy the information from the master page table,
* nothing more.
*/
- if (unlikely((addr >= VMALLOC_START) && (addr < VMALLOC_END))) {
+ if (!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 747e5b1ef02d..38bd4dd95276 100644
--- 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
+
+#ifdef 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
base-commit: ac9a78681b921877518763ba0e89202254349d1b
Reviewed-by: Palmer Dabbelt <palmer@xxxxxxxxxxxx>
aside from the build issue, which seems pretty straight-forward. I'm
going to drop this from patchwork.
