Re: [PATCH] arm64: mm: fix linear mapping mem access performace degradation

[Mike Rapoport <rppt@kernel.org>]

On Mon, Jun 27, 2022 at 06:46:50PM +0800, guanghui.fgh wrote:
> 在 2022/6/27 17:49, Mike Rapoport 写道:
> > On Mon, Jun 27, 2022 at 05:24:10PM +0800, guanghui.fgh wrote:
> > > 在 2022/6/27 14:34, Mike Rapoport 写道:
> > > 
> > >      On Sun, Jun 26, 2022 at 07:10:15PM +0800, Guanghui Feng wrote:
> > > 
> > >          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
> > >          commit 1a8e1cef7603
> > >          commit 8424ecdde7df
> > >          commit 0a30c53573b0
> > >          commit 2687275a5843
> > > 
> > >      Please include oneline summary of the commit. (See section "Describe your
> > >      changes" in Documentation/process/submitting-patches.rst)
> > > 
> > > OK, I will add oneline summary in the git commit messages.
> > > 
> > >          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.
> > > 
> > >      This already happens when ZONE_DMA/ZONE_DMA32 are disabled. Please explain
> > >      why is it Ok to change the way the memory is mapped with
> > >      ZONE_DMA/ZONE_DMA32 enabled.
> > > 
> > > In short:
> > > 
> > > 1.building all avail mem with block/section mapping（normally 1G/2M） without
> > > inspecting crashkernel
> > > 2. Reserve crashkernel mem as same as previous doing
> > > 3. only change the crashkernle mem mapping to normal mapping(normally 4k).
> > > With this method, there are block/section mapping as more as possible.
> > 
> > This does not answer the question why changing the way the memory is mapped
> > when there is ZONE_DMA/DMA32 and crashkernel won't cause a regression.
> > 
> 1.Quoted messages from arch/arm64/mm/init.c
> 
> "Memory reservation for crash kernel either done early or deferred
> depending on DMA memory zones configs (ZONE_DMA) --
> 
> In absence of ZONE_DMA configs arm64_dma_phys_limit initialized
> here instead of max_zone_phys().  This lets early reservation of
> crash kernel memory which has a dependency on arm64_dma_phys_limit.
> Reserving memory early for crash kernel allows linear creation of block
> mappings (greater than page-granularity) for all the memory bank rangs.
> In this scheme a comparatively quicker boot is observed.
> 
> If ZONE_DMA configs are defined, crash kernel memory reservation
> is delayed until DMA zone memory range size initialization performed in
> zone_sizes_init().  The defer is necessary to steer clear of DMA zone
> memory range to avoid overlap allocation.  So crash kernel memory boundaries
> are not known when mapping all bank memory ranges, which otherwise means not
> possible to exclude crash kernel range from creating block mappings so
> page-granularity mappings are created for the entire memory range."
> 
> Namely, the init order: memblock init--->linear mem mapping(4k mapping for
> crashkernel, requirinig page-granularity changing))--->zone dma
> limit--->reserve crashkernel.
> So when enable ZONE DMA and using crashkernel, the mem mapping using 4k
> mapping.
> 
> 2.As mentioned above, when linear mem use 4k mapping simply, there is high
> dtlb miss(degrade performance).
> This patch use block/section mapping as far as possible with performance
> improvement.
> 
> 3.This patch reserve crashkernel as same as the history(ZONE DMA &
> crashkernel reserving order), and only change the linear mem mapping to
> block/section mapping.

Thank you for the explanation. 

So if I understand correctly, you suggest to reserve the crash kernel
memory late regardless of ZONE_DMA/32 configuration and then map that memory 
using page level mappings after the memory is reserved. 

This makes sense, but I think the patch has a few shortcomings than need to
be addressed.

> Signed-off-by: Guanghui Feng <guanghuifeng@linux.alibaba.com>
> ---
>  arch/arm64/include/asm/mmu.h |   1 +
>  arch/arm64/mm/init.c         |   8 +-
>  arch/arm64/mm/mmu.c          | 274 +++++++++++++++++++++++++++++++++++++++----
>  3 files changed, 253 insertions(+), 30 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/mmu.h b/arch/arm64/include/asm/mmu.h
> index 48f8466..df113cc 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 mapping_crashkernel(void);

I think map_crashkernel() would be a better name.

>  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..0e7540b 100644
> --- a/arch/arm64/mm/init.c
> +++ b/arch/arm64/mm/init.c
> @@ -388,10 +388,6 @@ 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 +434,8 @@ 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();
> +	mapping_crashkernel();

This can be called from reserve_crashkernel() directly.
 
>  	memblock_dump_all();
>  }
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index 626ec32..0672afd 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -498,6 +498,256 @@ static int __init enable_crash_mem_map(char *arg)
>  }
>  early_param("crashkernel", enable_crash_mem_map);
>  
> +#ifdef CONFIG_KEXEC_CORE
> +static phys_addr_t __init early_crashkernel_pgtable_alloc(int shift)
> +{
> +	phys_addr_t phys;
> +	void *ptr;
> +
> +	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;
> +}
> +
> +static void init_crashkernel_pte(pmd_t *pmdp, unsigned long addr,
> +				 unsigned long end,
> +				 phys_addr_t phys, pgprot_t prot)
> +{
> +	pte_t *ptep;
> +	ptep = pte_offset_kernel(pmdp, addr);
> +	do {
> +		set_pte(ptep, pfn_pte(__phys_to_pfn(phys), prot));
> +		phys += PAGE_SIZE;
> +	} while (ptep++, addr += PAGE_SIZE, addr != end);
> +}
> +
> +static void alloc_crashkernel_cont_pte(pmd_t *pmdp, 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 pmd = READ_ONCE(*pmdp);
> +
> +	BUG_ON(pmd_sect(pmd));
> +	if (pmd_none(pmd)) {
> +		pmdval_t pmdval = PMD_TYPE_TABLE | PMD_TABLE_UXN;
> +		phys_addr_t pte_phys;
> +
> +		if (flags & NO_EXEC_MAPPINGS)
> +			pmdval |= PMD_TABLE_PXN;
> +		BUG_ON(!pgtable_alloc);
> +		pte_phys = pgtable_alloc(PAGE_SHIFT);
> +		__pmd_populate(pmdp, pte_phys, pmdval);
> +		pmd = READ_ONCE(*pmdp);
> +	}
> +	BUG_ON(pmd_bad(pmd));
> +
> +	do {
> +		pgprot_t __prot = prot;
> +		next = pte_cont_addr_end(addr, end);
> +		init_crashkernel_pte(pmdp, addr, next, phys, __prot);
> +		phys += next - addr;
> +	} while (addr = next, addr != end);
> +}
> +
> +static void init_crashkernel_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)
> +{
> +	phys_addr_t map_offset;
> +	unsigned long next;
> +	pmd_t *pmdp;
> +	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);
> +
> +			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);
> +		}
> +		alloc_crashkernel_cont_pte(pmdp, addr, next, phys, prot,
> +					   pgtable_alloc, flags);
> +		phys += next - addr;
> +	} while (pmdp++, addr = next, addr != end);
> +}
> +
> +static void alloc_crashkernel_cont_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)
> +{
> +	unsigned long next;
> +	pud_t pud = READ_ONCE(*pudp);
> +
> +	/*
> +	 * Check for initial section mappings in the pgd/pud.
> +	 */
> +	BUG_ON(pud_sect(pud));
> +	if (pud_none(pud)) {
> +		pudval_t pudval = PUD_TYPE_TABLE | PUD_TABLE_UXN;
> +		phys_addr_t pmd_phys;
> +
> +		if (flags & NO_EXEC_MAPPINGS)
> +			pudval |= PUD_TABLE_PXN;
> +		BUG_ON(!pgtable_alloc);
> +		pmd_phys = pgtable_alloc(PMD_SHIFT);
> +		__pud_populate(pudp, pmd_phys, pudval);
> +		pud = READ_ONCE(*pudp);
> +	}
> +	BUG_ON(pud_bad(pud));
> +
> +	do {
> +		pgprot_t __prot = prot;
> +		next = pmd_cont_addr_end(addr, end);
> +		init_crashkernel_pmd(pudp, addr, next, phys, __prot,
> +				     pgtable_alloc, flags);
> +		phys += next - addr;
> +	} while (addr = next, addr != end);
> +}
> +
> +static void alloc_crashkernel_pud(pgd_t *pgdp, unsigned long addr,
> +				  unsigned long end, phys_addr_t phys,
> +				  pgprot_t prot,
> +				  phys_addr_t (*pgtable_alloc)(int),
> +				  int flags)
> +{
> +	phys_addr_t map_offset;
> +	unsigned long next;
> +	pud_t *pudp;
> +	p4d_t *p4dp = p4d_offset(pgdp, addr);
> +	p4d_t p4d = READ_ONCE(*p4dp);
> +	pudval_t pudval;
> +
> +	if (p4d_none(p4d)) {
> +		p4dval_t p4dval = P4D_TYPE_TABLE | P4D_TABLE_UXN;
> +		phys_addr_t pud_phys;
> +
> +		if (flags & NO_EXEC_MAPPINGS)
> +			p4dval |= P4D_TABLE_PXN;
> +		BUG_ON(!pgtable_alloc);
> +		pud_phys = pgtable_alloc(PUD_SHIFT);
> +		__p4d_populate(p4dp, pud_phys, p4dval);
> +		p4d = READ_ONCE(*p4dp);
> +	}
> +	BUG_ON(p4d_bad(p4d));
> +
> +	/*
> +	 * No need for locking during early boot. And it doesn't work as
> +	 * expected with KASLR enabled.
> +	 */
> +	if (system_state != SYSTEM_BOOTING)
> +		mutex_lock(&fixmap_lock);
> +	pudp = pud_offset(p4dp, addr);
> +	do {
> +		next = pud_addr_end(addr, end);
> +		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);
> +
> +			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);
> +		}
> +		alloc_crashkernel_cont_pmd(pudp, addr, next, phys, prot,
> +					   pgtable_alloc, flags);
> +		phys += next - addr;
> +	} while (pudp++, addr = next, addr != end);
> +
> +	if (system_state != SYSTEM_BOOTING)
> +		mutex_unlock(&fixmap_lock);
> +}
> +
> +static void __create_crashkernel_mapping(pgd_t *pgdir, phys_addr_t phys,
> +					 unsigned long virt, phys_addr_t size,
> +					 pgprot_t prot,
> +					 phys_addr_t (*pgtable_alloc)(int),
> +					 int flags)
> +{
> +	unsigned long addr, end, next;
> +	pgd_t *pgdp = pgd_offset_pgd(pgdir, virt);
> +
> +	/*
> +	 * If the virtual and physical address don't have the same offset
> +	 * within a page, we cannot map the region as the caller expects.
> +	 */
> +	if (WARN_ON((phys ^ virt) & ~PAGE_MASK))
> +		return;
> +
> +	phys &= PAGE_MASK;
> +	addr = virt & PAGE_MASK;
> +	end = PAGE_ALIGN(virt + size);
> +
> +	do {
> +		next = pgd_addr_end(addr, end);
> +		alloc_crashkernel_pud(pgdp, addr, next, phys, prot,
> +				      pgtable_alloc, flags);
> +		phys += next - addr;
> +	} while (pgdp++, addr = next, addr != end);
> +}

__create_crashkernel_mapping() and the helpers it uses resemble very much
__create_pgd_mapping() and it's helpers. I think a cleaner way would be to
clear section mappings and than call __create_pgd_mapping() for crash
kernel memory with block/section mappings disabled.

> +static void __init map_crashkernel(pgd_t *pgdp, phys_addr_t start,
> +				   phys_addr_t end, pgprot_t prot, int flags)
> +{
> +	__create_crashkernel_mapping(pgdp, start, __phys_to_virt(start),
> +				     end - start, prot,
> +				     early_crashkernel_pgtable_alloc, flags);
> +}

No need for this, you can call __create_crashkernel_mapping() directly from
mapping_crashkernel().

> +
> +void __init mapping_crashkernel(void)
> +{
> +	if (can_set_direct_map() || IS_ENABLED(CONFIG_KFENCE))
> +	    return;
> +
> +	if (!crash_mem_map || !crashk_res.end)
> +	    return;
> +
> +	map_crashkernel(swapper_pg_dir, crashk_res.start,
> +			crashk_res.end + 1, PAGE_KERNEL,
> +			NO_BLOCK_MAPPINGS | NO_CONT_MAPPINGS);
> +}
> +#else
> +void __init mapping_crashkernel(void) {}
> +#endif
> +
>  static void __init map_mem(pgd_t *pgdp)
>  {
>  	static const u64 direct_map_end = _PAGE_END(VA_BITS_MIN);
> @@ -527,17 +777,6 @@ 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 +809,6 @@ 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


-- 
Sincerely yours,
Mike.

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