linux-kernel - Re: [PATCH 6/6] zsmalloc: make zsmalloc portable

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Date:	Wed, 25 Apr 2012 10:32:30 -0400
From:	Nitin Gupta <ngupta@...are.org>
To:	Minchan Kim <minchan@...nel.org>
CC:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Seth Jennings <sjenning@...ux.vnet.ibm.com>,
	Dan Magenheimer <dan.magenheimer@...cle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 6/6] zsmalloc: make zsmalloc portable

On 04/25/2012 02:23 AM, Minchan Kim wrote:

> The zsmalloc uses __flush_tlb_one and set_pte.
> It's very lower functions so that it makes arhcitecture dependency
> so currently zsmalloc is used by only x86.
> This patch changes them with map_vm_area and unmap_kernel_range so
> it should work all architecture.
> 
> Signed-off-by: Minchan Kim <minchan@...nel.org>
> ---
>  drivers/staging/zsmalloc/Kconfig         |    4 ----
>  drivers/staging/zsmalloc/zsmalloc-main.c |   27 +++++++++++++++++----------
>  drivers/staging/zsmalloc/zsmalloc_int.h  |    1 -
>  3 files changed, 17 insertions(+), 15 deletions(-)
> 
> diff --git a/drivers/staging/zsmalloc/Kconfig b/drivers/staging/zsmalloc/Kconfig
> index a5ab720..9084565 100644
> --- a/drivers/staging/zsmalloc/Kconfig
> +++ b/drivers/staging/zsmalloc/Kconfig
> @@ -1,9 +1,5 @@
>  config ZSMALLOC
>  	tristate "Memory allocator for compressed pages"
> -	# X86 dependency is because of the use of __flush_tlb_one and set_pte
> -	# in zsmalloc-main.c.
> -	# TODO: convert these to portable functions
> -	depends on X86
>  	default n
>  	help
>  	  zsmalloc is a slab-based memory allocator designed to store
> diff --git a/drivers/staging/zsmalloc/zsmalloc-main.c b/drivers/staging/zsmalloc/zsmalloc-main.c
> index ff089f8..cc017b1 100644
> --- a/drivers/staging/zsmalloc/zsmalloc-main.c
> +++ b/drivers/staging/zsmalloc/zsmalloc-main.c
> @@ -442,7 +442,7 @@ static int zs_cpu_notifier(struct notifier_block *nb, unsigned long action,
>  		area = &per_cpu(zs_map_area, cpu);
>  		if (area->vm)
>  			break;
> -		area->vm = alloc_vm_area(2 * PAGE_SIZE, area->vm_ptes);
> +		area->vm = alloc_vm_area(2 * PAGE_SIZE, NULL);
>  		if (!area->vm)
>  			return notifier_from_errno(-ENOMEM);
>  		break;
> @@ -696,13 +696,22 @@ void *zs_map_object(struct zs_pool *pool, void *handle)
>  	} else {
>  		/* this object spans two pages */
>  		struct page *nextp;
> +		struct page *pages[2];
> +		struct page **page_array = &pages[0];
> +		int err;
>  
>  		nextp = get_next_page(page);
>  		BUG_ON(!nextp);
>  
> +		page_array[0] = page;
> +		page_array[1] = nextp;
>  
> -		set_pte(area->vm_ptes[0], mk_pte(page, PAGE_KERNEL));
> -		set_pte(area->vm_ptes[1], mk_pte(nextp, PAGE_KERNEL));
> +		/*
> +		 * map_vm_area never fail because we already allocated
> +		 * pages for page table in alloc_vm_area.
> +		 */
> +		err = map_vm_area(area->vm, PAGE_KERNEL, &page_array);
> +		BUG_ON(err);
>  
>  		/* We pre-allocated VM area so mapping can never fail */
>  		area->vm_addr = area->vm->addr;
> @@ -730,14 +739,12 @@ void zs_unmap_object(struct zs_pool *pool, void *handle)
>  	off = obj_idx_to_offset(page, obj_idx, class->size);
>  
>  	area = &__get_cpu_var(zs_map_area);
> -	if (off + class->size <= PAGE_SIZE) {
> +	if (off + class->size <= PAGE_SIZE)
>  		kunmap_atomic(area->vm_addr);
> -	} else {
> -		set_pte(area->vm_ptes[0], __pte(0));
> -		set_pte(area->vm_ptes[1], __pte(0));
> -		__flush_tlb_one((unsigned long)area->vm_addr);
> -		__flush_tlb_one((unsigned long)area->vm_addr + PAGE_SIZE);
> -	}
> +	else
> +		unmap_kernel_range((unsigned long)area->vm->addr,
> +					PAGE_SIZE * 2);
> +



This would certainly work but would incur unncessary cost. All we need
to do is to flush the local TLB entry correpsonding to these two pages.
However, unmap_kernel_range --> flush_tlb_kernel_range woule cause TLB
flush on all CPUs. Additionally, implementation of this function
(flush_tlb_kernel_range) on architecutures like x86 seems naive since it
flushes the entire TLB on all the CPUs.

Even with all this penalty, I'm inclined on keeping this change to
remove x86 only dependency, keeping improvements as future work.

I think Seth was working on this improvement but not sure about the
current status. Seth?


>  	put_cpu_var(zs_map_area);
>  }
>  EXPORT_SYMBOL_GPL(zs_unmap_object);
> diff --git a/drivers/staging/zsmalloc/zsmalloc_int.h b/drivers/staging/zsmalloc/zsmalloc_int.h
> index 8f9ce0c..4c11c89 100644
> --- a/drivers/staging/zsmalloc/zsmalloc_int.h
> +++ b/drivers/staging/zsmalloc/zsmalloc_int.h
> @@ -111,7 +111,6 @@ static const int fullness_threshold_frac = 4;
>  
>  struct mapping_area {
>  	struct vm_struct *vm;
> -	pte_t *vm_ptes[2];
>  	char *vm_addr;
>  };
>  


Thanks,
Nitin
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