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

[Jan Kara <jack@suse.cz>]

On Tue 11-12-07 15:43:10, Andrew Morton wrote:
> 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?
  OK. Thanks for the improvement.

> --- 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) */
> _
> 
									Honza
-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR
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