Palmer Dabbelt <palmer@xxxxxxxxxxx> writes: > 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. Hmm, you applied the V2 a couple of days ago [1], which fixes the build issue. Did you drop the V2 from the queue? [1] https://lore.kernel.org/linux-riscv/168727442024.569.16572247474971535604.git-patchwork-notify@xxxxxxxxxx/ Björn
