[PATCH v3] arm64: mm: fix linear mapping mem access performance degradation

From: Guanghui Feng
Date: Thu Jun 30 2022 - 06:50:57 EST

The arm64 can build 2M/1G block/sectiion mapping. When using DMA/DMA32 zone
(enable crashkernel, disable rodata full, disable kfence), the mem_map will
use non block/section mapping(for crashkernel requires to shrink the region
in page granularity). But it will degrade performance when doing larging
continuous mem access in kernel(memcpy/memmove, etc).

There are many changes and discussions:
commit 031495635b46 ("arm64: Do not defer reserve_crashkernel() for
platforms with no DMA memory zones")
commit 0a30c53573b0 ("arm64: mm: Move reserve_crashkernel() into
mem_init()")
commit 2687275a5843 ("arm64: Force NO_BLOCK_MAPPINGS if crashkernel
reservation is required")

This patch changes mem_map to use block/section mapping with crashkernel.
Firstly, do block/section mapping(normally 2M or 1G) for all avail mem at
mem_map, reserve crashkernel memory. And then walking pagetable to split
block/section mapping to non block/section mapping(normally 4K) [[[only]]]
for crashkernel mem. So the linear mem mapping use block/section mapping
as more as possible. We will reduce the cpu dTLB miss conspicuously, and
accelerate mem access about 10-20% performance improvement.

I have tested it with pft(Page Fault Test) and fio, obtained great
performace improvement.

For fio test:
1.prepare ramdisk
modprobe -r brd
modprobe brd rd_nr=1 rd_size=67108864
dmsetup remove_all
wipefs -a --force /dev/ram0
mkfs -t ext4 -E lazy_itable_init=0,lazy_journal_init=0 -q -F /dev/ram0
mkdir -p /fs/ram0
mount -t ext4 /dev/ram0 /fs/ram0

2.prepare fio paremeter in x.fio file:
[global]
bs=4k
ioengine=psync
iodepth=128
size=32G
direct=1
invalidate=1
group_reporting
thread=1
rw=read
directory=/fs/ram0
numjobs=1

[task_0]
cpus_allowed=16
stonewall=1

3.run testcase:
perf stat -e dTLB-load-misses fio x.fio

4.contrast
------------------------
without patch with patch
fio READ aggrb=1493.2MB/s aggrb=1775.3MB/s
dTLB-load-misses 1,818,320,693 438,729,774
time elapsed(s) 70.500326434 62.877316408
user(s) 15.926332000 15.684721000
sys(s) 54.211939000 47.046165000

5.conclusion
Using this patch will reduce dTLB misses and improve performace greatly.

Signed-off-by: Guanghui Feng <guanghuifeng@xxxxxxxxxxxxxxxxx>
---
arch/arm64/include/asm/mmu.h | 1 +
arch/arm64/mm/init.c | 8 +-
arch/arm64/mm/mmu.c | 231 ++++++++++++++++++++++++++++++-------------
3 files changed, 168 insertions(+), 72 deletions(-)

diff --git a/arch/arm64/include/asm/mmu.h b/arch/arm64/include/asm/mmu.h
index 48f8466..1a46b81 100644
--- a/arch/arm64/include/asm/mmu.h
+++ b/arch/arm64/include/asm/mmu.h
@@ -63,6 +63,7 @@ static inline bool arm64_kernel_unmapped_at_el0(void)
extern void arm64_memblock_init(void);
extern void paging_init(void);
extern void bootmem_init(void);
+extern void map_crashkernel(void);
extern void __iomem *early_io_map(phys_addr_t phys, unsigned long virt);
extern void init_mem_pgprot(void);
extern void create_pgd_mapping(struct mm_struct *mm, phys_addr_t phys,
diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
index 339ee84..241d27e 100644
--- a/arch/arm64/mm/init.c
+++ b/arch/arm64/mm/init.c
@@ -190,6 +190,7 @@ static void __init reserve_crashkernel(void)
crashk_res.start = crash_base;
crashk_res.end = crash_base + crash_size - 1;
insert_resource(&iomem_resource, &crashk_res);
+ map_crashkernel();
}

/*
@@ -388,10 +389,6 @@ void __init arm64_memblock_init(void)
}

early_init_fdt_scan_reserved_mem();
-
- if (!IS_ENABLED(CONFIG_ZONE_DMA) && !IS_ENABLED(CONFIG_ZONE_DMA32))
- reserve_crashkernel();
-
high_memory = __va(memblock_end_of_DRAM() - 1) + 1;
}

@@ -438,8 +435,7 @@ void __init bootmem_init(void)
* request_standard_resources() depends on crashkernel's memory being
* reserved, so do it here.
*/
- if (IS_ENABLED(CONFIG_ZONE_DMA) || IS_ENABLED(CONFIG_ZONE_DMA32))
- reserve_crashkernel();
+ reserve_crashkernel();

memblock_dump_all();
}
diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
index 626ec32..4b779cf 100644
--- a/arch/arm64/mm/mmu.c
+++ b/arch/arm64/mm/mmu.c
@@ -42,6 +42,7 @@
#define NO_BLOCK_MAPPINGS BIT(0)
#define NO_CONT_MAPPINGS BIT(1)
#define NO_EXEC_MAPPINGS BIT(2) /* assumes FEAT_HPDS is not used */
+#define NO_SEC_REMAPPINGS BIT(3) /* rebuild with non block/sec mapping*/

u64 idmap_t0sz = TCR_T0SZ(VA_BITS_MIN);
u64 idmap_ptrs_per_pgd = PTRS_PER_PGD;
@@ -156,11 +157,12 @@ static bool pgattr_change_is_safe(u64 old, u64 new)
}

static void init_pte(pmd_t *pmdp, unsigned long addr, unsigned long end,
- phys_addr_t phys, pgprot_t prot)
+ phys_addr_t phys, pgprot_t prot, int flags)
{
pte_t *ptep;

- ptep = pte_set_fixmap_offset(pmdp, addr);
+ ptep = (flags & NO_SEC_REMAPPINGS) ? pte_offset_kernel(pmdp, addr) :
+ pte_set_fixmap_offset(pmdp, addr);
do {
pte_t old_pte = READ_ONCE(*ptep);

@@ -176,7 +178,8 @@ static void init_pte(pmd_t *pmdp, unsigned long addr, unsigned long end,
phys += PAGE_SIZE;
} while (ptep++, addr += PAGE_SIZE, addr != end);

- pte_clear_fixmap();
+ if (!(flags & NO_SEC_REMAPPINGS))
+ pte_clear_fixmap();
}

static void alloc_init_cont_pte(pmd_t *pmdp, unsigned long addr,
@@ -208,16 +211,59 @@ static void alloc_init_cont_pte(pmd_t *pmdp, unsigned long addr,
next = pte_cont_addr_end(addr, end);

/* use a contiguous mapping if the range is suitably aligned */
- if ((((addr | next | phys) & ~CONT_PTE_MASK) == 0) &&
+ if (!(flags & NO_SEC_REMAPPINGS) &&
+ (((addr | next | phys) & ~CONT_PTE_MASK) == 0) &&
(flags & NO_CONT_MAPPINGS) == 0)
__prot = __pgprot(pgprot_val(prot) | PTE_CONT);

- init_pte(pmdp, addr, next, phys, __prot);
+ init_pte(pmdp, addr, next, phys, __prot, flags);

phys += next - addr;
} while (addr = next, addr != end);
}

+static void init_pmd_remap(pud_t *pudp, unsigned long addr, unsigned long end,
+ phys_addr_t phys, pgprot_t prot,
+ phys_addr_t (*pgtable_alloc)(int), int flags)
+{
+ unsigned long next;
+ pmd_t *pmdp;
+ phys_addr_t map_offset;
+ pmdval_t pmdval;
+
+ pmdp = pmd_offset(pudp, addr);
+ do {
+ next = pmd_addr_end(addr, end);
+
+ if (!pmd_none(*pmdp) && pmd_sect(*pmdp)) {
+ phys_addr_t pte_phys = pgtable_alloc(PAGE_SHIFT);
+ pmd_clear(pmdp);
+ pmdval = PMD_TYPE_TABLE | PMD_TABLE_UXN;
+ if (flags & NO_EXEC_MAPPINGS)
+ pmdval |= PMD_TABLE_PXN;
+ __pmd_populate(pmdp, pte_phys, pmdval);
+ flush_tlb_kernel_range(addr, addr + PAGE_SIZE);
+
+ map_offset = addr - (addr & PMD_MASK);
+ if (map_offset)
+ alloc_init_cont_pte(pmdp, addr & PMD_MASK, addr,
+ phys - map_offset, prot,
+ pgtable_alloc,
+ flags & (~NO_SEC_REMAPPINGS));
+
+ if (next < (addr & PMD_MASK) + PMD_SIZE)
+ alloc_init_cont_pte(pmdp, next,
+ (addr & PUD_MASK) + PUD_SIZE,
+ next - addr + phys,
+ prot, pgtable_alloc,
+ flags & (~NO_SEC_REMAPPINGS));
+ }
+ alloc_init_cont_pte(pmdp, addr, next, phys, prot,
+ pgtable_alloc, flags);
+ phys += next - addr;
+ } while (pmdp++, addr = next, addr != end);
+}
+
static void init_pmd(pud_t *pudp, unsigned long addr, unsigned long end,
phys_addr_t phys, pgprot_t prot,
phys_addr_t (*pgtable_alloc)(int), int flags)
@@ -286,16 +332,87 @@ static void alloc_init_cont_pmd(pud_t *pudp, unsigned long addr,
next = pmd_cont_addr_end(addr, end);

/* use a contiguous mapping if the range is suitably aligned */
- if ((((addr | next | phys) & ~CONT_PMD_MASK) == 0) &&
+ if (!(flags & NO_SEC_REMAPPINGS) &&
+ (((addr | next | phys) & ~CONT_PMD_MASK) == 0) &&
(flags & NO_CONT_MAPPINGS) == 0)
__prot = __pgprot(pgprot_val(prot) | PTE_CONT);

- init_pmd(pudp, addr, next, phys, __prot, pgtable_alloc, flags);
+ if (flags & NO_SEC_REMAPPINGS)
+ init_pmd_remap(pudp, addr, next, phys, __prot,
+ pgtable_alloc, flags);
+ else
+ init_pmd(pudp, addr, next, phys, __prot,
+ pgtable_alloc, flags);

phys += next - addr;
} while (addr = next, addr != end);
}

+static void init_pud_remap(pud_t *pudp, unsigned long addr, unsigned long next,
+ phys_addr_t phys, pgprot_t prot,
+ phys_addr_t (*pgtable_alloc)(int),
+ int flags)
+{
+ pudval_t pudval;
+ phys_addr_t map_offset;
+
+ if (!pud_none(*pudp) && pud_sect(*pudp)) {
+ phys_addr_t pmd_phys = pgtable_alloc(PMD_SHIFT);
+ pud_clear(pudp);
+ pudval = PUD_TYPE_TABLE | PUD_TABLE_UXN;
+ if (flags & NO_EXEC_MAPPINGS)
+ pudval |= PUD_TABLE_PXN;
+
+ __pud_populate(pudp, pmd_phys, pudval);
+ flush_tlb_kernel_range(addr, addr + PAGE_SIZE);
+
+ map_offset = addr - (addr & PUD_MASK);
+ if (map_offset)
+ alloc_init_cont_pmd(pudp, addr & PUD_MASK,
+ addr, phys - map_offset,
+ prot, pgtable_alloc,
+ flags & (~NO_SEC_REMAPPINGS));
+
+ if (next < (addr & PUD_MASK) + PUD_SIZE)
+ alloc_init_cont_pmd(pudp, next,
+ (addr & PUD_MASK) + PUD_SIZE,
+ next - addr + phys,
+ prot, pgtable_alloc,
+ flags & (~NO_SEC_REMAPPINGS));
+ }
+ alloc_init_cont_pmd(pudp, addr, next, phys, prot,
+ pgtable_alloc, flags);
+}
+
+static void init_pud(pud_t *pudp, unsigned long addr, unsigned long next,
+ phys_addr_t phys, pgprot_t prot,
+ phys_addr_t (*pgtable_alloc)(int),
+ int flags)
+{
+ pud_t old_pud = READ_ONCE(*pudp);
+ /*
+ * For 4K granule only, attempt to put down a 1GB block
+ */
+ if (pud_sect_supported() &&
+ ((addr | next | phys) & ~PUD_MASK) == 0 &&
+ (flags & NO_BLOCK_MAPPINGS) == 0) {
+ pud_set_huge(pudp, phys, prot);
+
+ /*
+ * After the PUD entry has been populated once, we
+ * only allow updates to the permission attributes.
+ */
+ BUG_ON(!pgattr_change_is_safe(pud_val(old_pud),
+ READ_ONCE(pud_val(*pudp))));
+ } else {
+ alloc_init_cont_pmd(pudp, addr, next, phys, prot,
+ pgtable_alloc, flags);
+
+ BUG_ON(pud_val(old_pud) != 0 &&
+ pud_val(old_pud) != READ_ONCE(pud_val(*pudp)));
+ }
+}
+
static void alloc_init_pud(pgd_t *pgdp, unsigned long addr, unsigned long end,
phys_addr_t phys, pgprot_t prot,
phys_addr_t (*pgtable_alloc)(int),
@@ -325,37 +442,24 @@ static void alloc_init_pud(pgd_t *pgdp, unsigned long addr, unsigned long end,
*/
if (system_state != SYSTEM_BOOTING)
mutex_lock(&fixmap_lock);
- pudp = pud_set_fixmap_offset(p4dp, addr);
- do {
- pud_t old_pud = READ_ONCE(*pudp);

+ pudp = (flags & NO_SEC_REMAPPINGS) ? pud_offset(p4dp, addr) :
+ pud_set_fixmap_offset(p4dp, addr);
+ do {
next = pud_addr_end(addr, end);

- /*
- * For 4K granule only, attempt to put down a 1GB block
- */
- if (pud_sect_supported() &&
- ((addr | next | phys) & ~PUD_MASK) == 0 &&
- (flags & NO_BLOCK_MAPPINGS) == 0) {
- pud_set_huge(pudp, phys, prot);
-
- /*
- * After the PUD entry has been populated once, we
- * only allow updates to the permission attributes.
- */
- BUG_ON(!pgattr_change_is_safe(pud_val(old_pud),
- READ_ONCE(pud_val(*pudp))));
- } else {
- alloc_init_cont_pmd(pudp, addr, next, phys, prot,
- pgtable_alloc, flags);
-
- BUG_ON(pud_val(old_pud) != 0 &&
- pud_val(old_pud) != READ_ONCE(pud_val(*pudp)));
- }
+ if (flags & NO_SEC_REMAPPINGS)
+ init_pud_remap(pudp, addr, next, phys, prot,
+ pgtable_alloc, flags);
+ else
+ init_pud(pudp, addr, next, phys, prot, pgtable_alloc,
+ flags);
phys += next - addr;
} while (pudp++, addr = next, addr != end);

- pud_clear_fixmap();
+ if (!(flags & NO_SEC_REMAPPINGS))
+ pud_clear_fixmap();
+
if (system_state != SYSTEM_BOOTING)
mutex_unlock(&fixmap_lock);
}
@@ -483,20 +587,39 @@ void __init mark_linear_text_alias_ro(void)
PAGE_KERNEL_RO);
}

-static bool crash_mem_map __initdata;
+#ifdef CONFIG_KEXEC_CORE
+static phys_addr_t __init early_crashkernel_pgtable_alloc(int shift)
+{
+ phys_addr_t phys;
+ void *ptr;

-static int __init enable_crash_mem_map(char *arg)
+ phys = memblock_phys_alloc_range(PAGE_SIZE, PAGE_SIZE, 0,
+ MEMBLOCK_ALLOC_NOLEAKTRACE);
+ if (!phys)
+ panic("Failed to allocate page table page\n");
+
+ ptr = (void *)__phys_to_virt(phys);
+ memset(ptr, 0, PAGE_SIZE);
+ return phys;
+}
+
+void __init map_crashkernel(void)
{
- /*
- * Proper parameter parsing is done by reserve_crashkernel(). We only
- * need to know if the linear map has to avoid block mappings so that
- * the crashkernel reservations can be unmapped later.
- */
- crash_mem_map = true;
+ if (can_set_direct_map() || IS_ENABLED(CONFIG_KFENCE))
+ return;

- return 0;
+ if (!crashk_res.end)
+ return;
+
+ __create_pgd_mapping(swapper_pg_dir, crashk_res.start,
+ __phys_to_virt(crashk_res.start),
+ crashk_res.end + 1 - crashk_res.start, PAGE_KERNEL,
+ early_crashkernel_pgtable_alloc,
+ NO_EXEC_MAPPINGS | NO_SEC_REMAPPINGS);
}
-early_param("crashkernel", enable_crash_mem_map);
+#else
+void __init map_crashkernel(void) {}
+#endif

static void __init map_mem(pgd_t *pgdp)
{
@@ -527,17 +650,6 @@ static void __init map_mem(pgd_t *pgdp)
*/
memblock_mark_nomap(kernel_start, kernel_end - kernel_start);

-#ifdef CONFIG_KEXEC_CORE
- if (crash_mem_map) {
- if (IS_ENABLED(CONFIG_ZONE_DMA) ||
- IS_ENABLED(CONFIG_ZONE_DMA32))
- flags |= NO_BLOCK_MAPPINGS | NO_CONT_MAPPINGS;
- else if (crashk_res.end)
- memblock_mark_nomap(crashk_res.start,
- resource_size(&crashk_res));
- }
-#endif
-
/* map all the memory banks */
for_each_mem_range(i, &start, &end) {
if (start >= end)
@@ -570,19 +682,6 @@ static void __init map_mem(pgd_t *pgdp)
* in page granularity and put back unused memory to buddy system
* through /sys/kernel/kexec_crash_size interface.
*/
-#ifdef CONFIG_KEXEC_CORE
- if (crash_mem_map &&
- !IS_ENABLED(CONFIG_ZONE_DMA) && !IS_ENABLED(CONFIG_ZONE_DMA32)) {
- if (crashk_res.end) {
- __map_memblock(pgdp, crashk_res.start,
- crashk_res.end + 1,
- PAGE_KERNEL,
- NO_BLOCK_MAPPINGS | NO_CONT_MAPPINGS);
- memblock_clear_nomap(crashk_res.start,
- resource_size(&crashk_res));
- }
- }
-#endif
}

void mark_rodata_ro(void)
--
1.8.3.1