Unverified Commit 7d3332be authored by Björn Töpel's avatar Björn Töpel Committed by Palmer Dabbelt
Browse files

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

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: default avatarBjörn Töpel <bjorn@rivosinc.com>
Reviewed-by: default avatarAlexandre Ghiti <alexghiti@rivosinc.com>
Link: https://lore.kernel.org/r/20230531093817.665799-1-bjorn@kernel.org


Signed-off-by: default avatarPalmer Dabbelt <palmer@rivosinc.com>
parent 16252e01

arch/riscv/mm/fault.c

+2 −14
Original line number Diff line number Diff line
@@ -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();

arch/riscv/mm/init.c

+58 −0
Original line number Diff line number Diff line
@@ -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