Re: [PATCH 4/5] xfs: use generic percpu counters for free block counter

[Christoph Hellwig <hch@infradead.org>]

>  	case XFS_SBS_FDBLOCKS:
>  
>  		if (delta > 0) {		/* Putting blocks back */
> +			if (mp->m_resblks == mp->m_resblks_avail) {
> +				percpu_counter_add(&mp->m_sb.sb_fdblocks, delta);
> +				return 0;
> +			}
> +
> +			/* put blocks back into reserve pool first */
> +			spin_lock(&mp->m_sb_lock);
> +			res_used = (long long)
> +					(mp->m_resblks - mp->m_resblks_avail);
> +
>  			if (res_used > delta) {
>  				mp->m_resblks_avail += delta;
>  			} else {
> +				delta -= res_used;
>  				mp->m_resblks_avail = mp->m_resblks;
> +				percpu_counter_add(&mp->m_sb.sb_fdblocks, delta);
>  			}
> +			spin_unlock(&mp->m_sb_lock);
> +			return 0;
>  
> +		}

>  
> +		/*
> +		 * Taking blocks away, need to be more accurate the closer we
> +		 * are to zero.
> +		 *
> +		 * batch size is set to a maximum of 1024 blocks - if we are
> +		 * allocating of freeing extents larger than this then we aren't
> +		 * going to be hammering the counter lock so a lock per update
> +		 * is not a problem.
> +		 *
> +		 * If the counter has a value of less than 2 * max batch size,
> +		 * then make everything serialise as we are real close to
> +		 * ENOSPC.
> +		 */
> +#define __BATCH	1024
> +		if (percpu_counter_compare(&mp->m_sb.sb_fdblocks,
> +					   2 * __BATCH) < 0)
> +			batch = 1;
> +		else
> +			batch = __BATCH;
> +
> +		__percpu_counter_add(&mp->m_sb.sb_fdblocks, delta, batch);
> +		if (percpu_counter_compare(&mp->m_sb.sb_fdblocks,
> +					   XFS_ALLOC_SET_ASIDE(mp)) >= 0) {
> +			/* we had space! */
> +			return 0;
>  		}
>  
> +		/*
> +		 * lock up the sb for dipping into reserves before releasing
> +		 * the space that took us to ENOSPC.
> +		 */
> +		spin_lock(&mp->m_sb_lock);
> +		percpu_counter_add(&mp->m_sb.sb_fdblocks, -delta);
> +		if (!rsvd)
> +			goto fdblocks_enospc;
> +
> +		lcounter = (long long)mp->m_resblks_avail + delta;
> +		if (lcounter >= 0) {
> +			mp->m_resblks_avail = lcounter;
> +			spin_unlock(&mp->m_sb_lock);
> +			return 0;
> +		}
> +		printk_once(KERN_WARNING
> +			"Filesystem \"%s\": reserve blocks depleted! "
> +			"Consider increasing reserve pool size.",
> +			mp->m_fsname);
> +fdblocks_enospc:
> +		spin_unlock(&mp->m_sb_lock);
> +		return -ENOSPC;
> +

This screams for having two different helpers for removing vs adding back
reserved blocks.

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