Re: [PATCH] Improve inode list scanning in add_dquot_ref()

[Andrew Morton <akpm@linux-foundation.org>]

On Tue, 11 Dec 2007 14:43:19 +0100
Jan Kara <jack@suse.cz> wrote:

>   Hi,
> 
>   attached patch makes add_dquot_ref() quite faster if used on a life
> filesystem. It has survived some testing on my machine but there could be
> some subtle races I've missed so please leave it in -mm for a while (not
> that I'd think that people use quotas with -mm kernels but at least it
> should go into vanilla in -rc1...). Thanks.
> 
> 								Honza
> 
> ---
> We restarted scan of sb->s_inodes list whenever we had to drop inode_lock in
> add_dquot_ref(). This leads to overall quadratic running time and thus
> add_dquot_ref() can take several minutes when called on a life filesystem.  We
> fix the problem by using the fact that inode cannot be removed from s_inodes
> list while we hold a reference to it and thus we can safely restart the scan if
> we don't drop the reference. Here we use the fact that inodes freshly added to
> s_inodes list are already guaranteed to have quotas properly initialized and
> the ordering of inodes on s_inodes list does not change so we cannot skip any
> inode.  Thanks goes to Nick <gentuu@gmail.com> for analyzing the problem and
> testing the fix.
> 
> Signed-off-by: Jan Kara <jack@suse.cz>
> CC: Nick <gentuu@gmail.com>
> 
> diff --git a/fs/dquot.c b/fs/dquot.c
> index 2809768..ecea4be 100644
> --- a/fs/dquot.c
> +++ b/fs/dquot.c
> @@ -696,9 +696,8 @@ static int dqinit_needed(struct inode *inode, int type)
>  /* This routine is guarded by dqonoff_mutex mutex */
>  static void add_dquot_ref(struct super_block *sb, int type)
>  {
> -	struct inode *inode;
> +	struct inode *inode, *old_inode = NULL;
>  
> -restart:
>  	spin_lock(&inode_lock);
>  	list_for_each_entry(inode, &sb->s_inodes, i_sb_list) {
>  		if (!atomic_read(&inode->i_writecount))
> @@ -711,12 +710,20 @@ restart:
>  		__iget(inode);
>  		spin_unlock(&inode_lock);
>  
> +		if (old_inode)
> +			iput(old_inode);
>  		sb->dq_op->initialize(inode, type);
> -		iput(inode);
> -		/* As we may have blocked we had better restart... */
> -		goto restart;
> +		/* We hold a reference to 'inode' so it couldn't have been
> +		 * removed from s_inodes list while we dropped the inode_lock.
> +		 * We cannot iput the inode now as we can be holding the last
> +		 * reference and we cannot iput it under inode_lock. So we
> +		 * keep the reference and iput it later. */
> +		old_inode = inode;
> +		spin_lock(&inode_lock);
>  	}
>  	spin_unlock(&inode_lock);
> +	if (old_inode)
> +		iput(old_inode);
>  }
>  

iput(NULL) is legal.  In the great majority of cases old_inode!=NULL so I
think we should use that feature here?


--- a/fs/dquot.c~quota-improve-inode-list-scanning-in-add_dquot_ref-fix
+++ a/fs/dquot.c
@@ -710,8 +710,7 @@ static void add_dquot_ref(struct super_b
 		__iget(inode);
 		spin_unlock(&inode_lock);
 
-		if (old_inode)
-			iput(old_inode);
+		iput(old_inode);
 		sb->dq_op->initialize(inode, type);
 		/* We hold a reference to 'inode' so it couldn't have been
 		 * removed from s_inodes list while we dropped the inode_lock.
@@ -722,8 +721,7 @@ static void add_dquot_ref(struct super_b
 		spin_lock(&inode_lock);
 	}
 	spin_unlock(&inode_lock);
-	if (old_inode)
-		iput(old_inode);
+	iput(old_inode);
 }
 
 /* Return 0 if dqput() won't block (note that 1 doesn't necessarily mean blocking) */
_