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SubjectRe: [PATCH v3 1/3] x86/mm: Pre-allocate p4d/pud pages for vmalloc area
On Tue, Jul 21, 2020 at 11:59:51AM +0200, Joerg Roedel wrote:
> From: Joerg Roedel <jroedel@suse.de>
>
> Pre-allocate the page-table pages for the vmalloc area at the level
> which needs synchronization on x86-64, which is P4D for 5-level and
> PUD for 4-level paging.
>
> Doing this at boot makes sure all no synchronization of that area is

Nit: ^all can be dropped

> necessary at runtime. The synchronization takes the pgd_lock and
> iterates over all page-tables in the system, so it can take quite long
> and is better avoided.
>
> Signed-off-by: Joerg Roedel <jroedel@suse.de>
> ---
> arch/x86/mm/init_64.c | 52 +++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 52 insertions(+)
>
> diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
> index dbae185511cd..e76bdb001460 100644
> --- a/arch/x86/mm/init_64.c
> +++ b/arch/x86/mm/init_64.c
> @@ -1238,6 +1238,56 @@ static void __init register_page_bootmem_info(void)
> #endif
> }
>
> +/*
> + * Pre-allocates page-table pages for the vmalloc 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_vmalloc_pages(void)
> +{
> + unsigned long addr;
> + const char *lvl;
> +
> + for (addr = VMALLOC_START; addr <= VMALLOC_END; addr = ALIGN(addr + 1, PGDIR_SIZE)) {
> + pgd_t *pgd = pgd_offset_k(addr);
> + p4d_t *p4d;
> + pud_t *pud;
> +
> + p4d = p4d_offset(pgd, addr);
> + if (p4d_none(*p4d)) {
> + /* Can only happen with 5-level paging */
> + p4d = p4d_alloc(&init_mm, pgd, addr);
> + if (!p4d) {
> + lvl = "p4d";
> + goto failed;
> + }
> + }
> +
> + if (pgtable_l5_enabled())
> + continue;
> +
> + pud = pud_offset(p4d, addr);
> + if (pud_none(*pud)) {
> + /* Ends up here only with 4-level paging */
> + pud = pud_alloc(&init_mm, p4d, addr);
> + if (!pud) {
> + lvl = "pud";
> + 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 vmalloc area\n", lvl);
> +}
> +
> void __init mem_init(void)
> {
> pci_iommu_alloc();
> @@ -1261,6 +1311,8 @@ void __init mem_init(void)
> if (get_gate_vma(&init_mm))
> kclist_add(&kcore_vsyscall, (void *)VSYSCALL_ADDR, PAGE_SIZE, KCORE_USER);
>
> + preallocate_vmalloc_pages();
> +
> mem_init_print_info(NULL);
> }
>
> --
> 2.27.0
>

--
Sincerely yours,
Mike.

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