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Message-ID: <87ilbfjy1k.fsf@all.your.base.are.belong.to.us>
Date: Thu, 22 Jun 2023 17:51:03 +0200
From: Björn Töpel <bjorn@...nel.org>
To: Palmer Dabbelt <palmer@...belt.com>
Cc: Paul Walmsley <paul.walmsley@...ive.com>, aou@...s.berkeley.edu,
linux-riscv@...ts.infradead.org, Bjorn Topel <bjorn@...osinc.com>,
linux-kernel@...r.kernel.org, linux-mm@...ck.org,
linux@...osinc.com, alexghiti@...osinc.com, joro@...tes.org
Subject: Re: [PATCH] riscv: mm: Pre-allocate PGD entries vmalloc/modules area
Palmer Dabbelt <palmer@...belt.com> writes:
> On Mon, 29 May 2023 11:00:23 PDT (-0700), bjorn@...nel.org wrote:
>> From: Björn Töpel <bjorn@...osinc.com>
>>
>> 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@...osinc.com>
>> ---
>> 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@...osinc.com>
>
> 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@kernel.org/
Björn
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