Re: [PATCH v2 4/5] mm, highmem: makes flush_all_zero_pkmaps() return index of first flushed entry

[Minchan Kim <minchan@kernel.org>]

On Thu, Nov 01, 2012 at 01:56:36AM +0900, Joonsoo Kim wrote:
> In current code, after flush_all_zero_pkmaps() is invoked,
> then re-iterate all pkmaps. It can be optimized if flush_all_zero_pkmaps()
> return index of first flushed entry. With this index,
> we can immediately map highmem page to virtual address represented by index.
> So change return type of flush_all_zero_pkmaps()
> and return index of first flushed entry.
> 
> Additionally, update last_pkmap_nr to this index.
> It is certain that entry which is below this index is occupied by other mapping,
> therefore updating last_pkmap_nr to this index is reasonable optimization.
> 
> Cc: Mel Gorman <mel@csn.ul.ie>
> Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
> Cc: Minchan Kim <minchan@kernel.org>
> Signed-off-by: Joonsoo Kim <js1304@gmail.com>
> 
> diff --git a/include/linux/highmem.h b/include/linux/highmem.h
> index ef788b5..97ad208 100644
> --- a/include/linux/highmem.h
> +++ b/include/linux/highmem.h
> @@ -32,6 +32,7 @@ static inline void invalidate_kernel_vmap_range(void *vaddr, int size)
>  
>  #ifdef CONFIG_HIGHMEM
>  #include <asm/highmem.h>
> +#define PKMAP_INVALID_INDEX (LAST_PKMAP)
>  
>  /* declarations for linux/mm/highmem.c */
>  unsigned int nr_free_highpages(void);
> diff --git a/mm/highmem.c b/mm/highmem.c
> index d98b0a9..b365f7b 100644
> --- a/mm/highmem.c
> +++ b/mm/highmem.c
> @@ -106,10 +106,10 @@ struct page *kmap_to_page(void *vaddr)
>  	return virt_to_page(addr);
>  }
>  
> -static void flush_all_zero_pkmaps(void)
> +static unsigned int flush_all_zero_pkmaps(void)
>  {
>  	int i;
> -	int need_flush = 0;
> +	unsigned int index = PKMAP_INVALID_INDEX;
>  
>  	flush_cache_kmaps();
>  
> @@ -141,10 +141,13 @@ static void flush_all_zero_pkmaps(void)
>  			  &pkmap_page_table[i]);
>  
>  		set_page_address(page, NULL);
> -		need_flush = 1;
> +		if (index == PKMAP_INVALID_INDEX)
> +			index = i;
>  	}
> -	if (need_flush)
> +	if (index != PKMAP_INVALID_INDEX)
>  		flush_tlb_kernel_range(PKMAP_ADDR(0), PKMAP_ADDR(LAST_PKMAP));
> +
> +	return index;
>  }
>  
>  /**
> @@ -152,14 +155,19 @@ static void flush_all_zero_pkmaps(void)
>   */
>  void kmap_flush_unused(void)
>  {
> +	unsigned int index;
> +
>  	lock_kmap();
> -	flush_all_zero_pkmaps();
> +	index = flush_all_zero_pkmaps();
> +	if (index != PKMAP_INVALID_INDEX && (index < last_pkmap_nr))
> +		last_pkmap_nr = index;

I don't know how kmap_flush_unused is really fast path so how my nitpick
is effective. Anyway,
What problem happens if we do following as?

lock()
index = flush_all_zero_pkmaps();
if (index != PKMAP_INVALID_INDEX)
        last_pkmap_nr = index;
unlock();

Normally, last_pkmap_nr is increased with searching empty slot in
map_new_virtual. So I expect return value of flush_all_zero_pkmaps
in kmap_flush_unused normally become either less than last_pkmap_nr
or last_pkmap_nr + 1.

 
>  	unlock_kmap();
>  }
>  
>  static inline unsigned long map_new_virtual(struct page *page)
>  {
>  	unsigned long vaddr;
> +	unsigned int index = PKMAP_INVALID_INDEX;
>  	int count;
>  
>  start:
> @@ -168,40 +176,45 @@ start:
>  	for (;;) {
>  		last_pkmap_nr = (last_pkmap_nr + 1) & LAST_PKMAP_MASK;
>  		if (!last_pkmap_nr) {
> -			flush_all_zero_pkmaps();
> -			count = LAST_PKMAP;
> +			index = flush_all_zero_pkmaps();
> +			break;
>  		}
> -		if (!pkmap_count[last_pkmap_nr])
> +		if (!pkmap_count[last_pkmap_nr]) {
> +			index = last_pkmap_nr;
>  			break;	/* Found a usable entry */
> -		if (--count)
> -			continue;
> -
> -		/*
> -		 * Sleep for somebody else to unmap their entries
> -		 */
> -		{
> -			DECLARE_WAITQUEUE(wait, current);
> -
> -			__set_current_state(TASK_UNINTERRUPTIBLE);
> -			add_wait_queue(&pkmap_map_wait, &wait);
> -			unlock_kmap();
> -			schedule();
> -			remove_wait_queue(&pkmap_map_wait, &wait);
> -			lock_kmap();
> -
> -			/* Somebody else might have mapped it while we slept */
> -			if (page_address(page))
> -				return (unsigned long)page_address(page);
> -
> -			/* Re-start */
> -			goto start;
>  		}
> +		if (--count == 0)
> +			break;
>  	}
> -	vaddr = PKMAP_ADDR(last_pkmap_nr);
> +
> +	/*
> +	 * Sleep for somebody else to unmap their entries
> +	 */
> +	if (index == PKMAP_INVALID_INDEX) {
> +		DECLARE_WAITQUEUE(wait, current);
> +
> +		__set_current_state(TASK_UNINTERRUPTIBLE);
> +		add_wait_queue(&pkmap_map_wait, &wait);
> +		unlock_kmap();
> +		schedule();
> +		remove_wait_queue(&pkmap_map_wait, &wait);
> +		lock_kmap();
> +
> +		/* Somebody else might have mapped it while we slept */
> +		vaddr = (unsigned long)page_address(page);
> +		if (vaddr)
> +			return vaddr;
> +
> +		/* Re-start */
> +		goto start;
> +	}
> +
> +	vaddr = PKMAP_ADDR(index);
>  	set_pte_at(&init_mm, vaddr,
> -		   &(pkmap_page_table[last_pkmap_nr]), mk_pte(page, kmap_prot));
> +		   &(pkmap_page_table[index]), mk_pte(page, kmap_prot));
>  
> -	pkmap_count[last_pkmap_nr] = 1;
> +	pkmap_count[index] = 1;
> +	last_pkmap_nr = index;
>  	set_page_address(page, (void *)vaddr);
>  
>  	return vaddr;
> -- 
> 1.7.9.5
> 
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-- 
Kind regards,
Minchan Kim