linux-kernel - Re: [PATCH] fs, jbd: use a more generic hash function

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Message-ID: <20141022080218.GA26827@quack.suse.cz>
Date:	Wed, 22 Oct 2014 10:02:18 +0200
From:	Jan Kara <jack@...e.cz>
To:	Sasha Levin <sasha.levin@...cle.com>
Cc:	akpm@...ux-foundation.org, jack@...e.cz,
	linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org,
	a.ryabinin@...sung.com
Subject: Re: [PATCH] fs, jbd: use a more generic hash function

On Tue 21-10-14 16:43:55, Sasha Levin wrote:
> While the hash function used by the revoke hashtable is good somewhere else,
> it's not really good here.
> 
> The default hash shift (8) means that one third of the hashing function
> gets lost (and is undefined anyways (8 - 12 = negative shift)):
> 
> 	"(block << (hash_shift - 12))) & (table->hash_size - 1)"
> 
> Instead, just use the kernel's generic hash function that gets used everywhere
> else.
  Thanks. Merged to my tree.

								Honza
> 
> Signed-off-by: Sasha Levin <sasha.levin@...cle.com>
> ---
>  fs/jbd/revoke.c |    7 ++-----
>  1 file changed, 2 insertions(+), 5 deletions(-)
> 
> diff --git a/fs/jbd/revoke.c b/fs/jbd/revoke.c
> index 8898bbd..dcead63 100644
> --- a/fs/jbd/revoke.c
> +++ b/fs/jbd/revoke.c
> @@ -93,6 +93,7 @@
>  #include <linux/bio.h>
>  #endif
>  #include <linux/log2.h>
> +#include <linux/hash.h>
>  
>  static struct kmem_cache *revoke_record_cache;
>  static struct kmem_cache *revoke_table_cache;
> @@ -129,15 +130,11 @@ static void flush_descriptor(journal_t *, struct journal_head *, int, int);
>  
>  /* Utility functions to maintain the revoke table */
>  
> -/* Borrowed from buffer.c: this is a tried and tested block hash function */
>  static inline int hash(journal_t *journal, unsigned int block)
>  {
>  	struct jbd_revoke_table_s *table = journal->j_revoke;
> -	int hash_shift = table->hash_shift;
>  
> -	return ((block << (hash_shift - 6)) ^
> -		(block >> 13) ^
> -		(block << (hash_shift - 12))) & (table->hash_size - 1);
> +	return hash_32(block, table->hash_shift);
>  }
>  
>  static int insert_revoke_hash(journal_t *journal, unsigned int blocknr,
> -- 
> 1.7.10.4
> 
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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