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

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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/ # [1]
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>
This commit is contained in:
Björn Töpel 2023-05-31 11:38:17 +02:00 committed by Palmer Dabbelt
parent 16252e018a
commit 7d3332be01
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GPG Key ID: 2E1319F35FBB1889
2 changed files with 60 additions and 14 deletions
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16
arch/riscv/mm/fault.c
View File

@ -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();

58
arch/riscv/mm/init.c
View File

@ -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