Re: [PATCH] fs/buffer: simplify the code flow of LRU management algorithm

[Minfei Huang]

Ping, Could you someone help to review this patch?

Thanks
Minfei

On 09/10/15 at 04:09pm, Minfei Huang wrote:
> From: Minfei Huang <mnfhuang@xxxxxxxxx>
> 
> There is a buffer_head lru list cache in local cpu to accelerate the
> speed. The LRU management algorithm is simple enough in
> bh_lru_install().
> 
> There are three situtaions we should deal with.
> 1) All/part of the lru cache is NULL.
> 2) The new buffer_head hitts the lru cache.
> 3) The new buffer_head does hit the lru cache.
> 
> We put the new buffer_head at the head of lru cache, then copy the
> buffer_head from the original lru cache, and drop the spare.
> 
> Signed-off-by: Minfei Huang <mnfhuang@xxxxxxxxx>
> ---
>  fs/buffer.c | 32 ++++++++++++++++++++------------
>  1 file changed, 20 insertions(+), 12 deletions(-)
> 
> diff --git a/fs/buffer.c b/fs/buffer.c
> index 1cf7a53..2139574 100644
> --- a/fs/buffer.c
> +++ b/fs/buffer.c
> @@ -1287,8 +1287,6 @@ static inline void check_irqs_on(void)
>   */
>  static void bh_lru_install(struct buffer_head *bh)
>  {
> -	struct buffer_head *evictee = NULL;
> -
>  	check_irqs_on();
>  	bh_lru_lock();
>  	if (__this_cpu_read(bh_lrus.bhs[0]) != bh) {
> @@ -1302,25 +1300,35 @@ static void bh_lru_install(struct buffer_head *bh)
>  			struct buffer_head *bh2 =
>  				__this_cpu_read(bh_lrus.bhs[in]);
>  
> -			if (bh2 == bh) {
> +			if (bh2 == NULL) {
> +				/* Rest value in bh_lrus.bhs always is NULL */
> +				break;
> +			} else if (bh2 == bh) {
>  				__brelse(bh2);
>  			} else {
> -				if (out >= BH_LRU_SIZE) {
> -					BUG_ON(evictee != NULL);
> -					evictee = bh2;
> +				if (out == BH_LRU_SIZE) {
> +					/*
> +					 * this condition will be happened,
> +					 * only if none of entry in
> +					 * bh_lrus.bhs hits the new bh,
> +					 * so the last bh should be released.
> +					 */
> +					BUG_ON(in != BH_LRU_SIZE - 1);
> +					__brelse(bh2);
> +					break;
>  				} else {
>  					bhs[out++] = bh2;
>  				}
>  			}
>  		}
> -		while (out < BH_LRU_SIZE)
> -			bhs[out++] = NULL;
> -		memcpy(this_cpu_ptr(&bh_lrus.bhs), bhs, sizeof(bhs));
> +		/*
> +		 * it is fine that the value out may be smaller than
> +		 * BH_LRU_SIZE. The rest of the value in bh_lrus.bhs is NULL.
> +		 */
> +		memcpy(this_cpu_ptr(&bh_lrus.bhs), bhs,
> +				sizeof(struct buffer_head *) * out);
>  	}
>  	bh_lru_unlock();
> -
> -	if (evictee)
> -		__brelse(evictee);
>  }
>  
>  /*
> -- 
> 2.1.0
> 
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