Message-ID: <877hbkpo7v.fsf@devron.myhome.or.jp>
Date:	Mon, 28 Mar 2011 00:39:16 +0900
From:	OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
To:	Kyungmin Park <kmpark@...radead.org>
Cc:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	Lukas Czerner <lczerner@...hat.com>
Subject: Re: [PATCH v5] fat: Batched discard support for fat

Kyungmin Park <kmpark@...radead.org> writes:

> From: Kyungmin Park <kyungmin.park@...sung.com>
>
> FAT supports batched discard as ext4.
>
> Cited from Lukas words.
> "The current solution is not ideal because of its bad performance impact.
> So basic idea to improve things is to avoid discarding every time some
> blocks are freed. and instead batching is together into bigger trims,
> which tends to be more effective."
>
> You can find an information in detail at following URLs.
> http://lwn.net/Articles/397538/
> http://lwn.net/Articles/383933/
>
> Clearify the meaning of "len" (Cited form Lukas mail)
>
> Let the "O" be free (bytes, blocks, whatever), and "=" be used.
> Now, we have a filesystem like this.
>
>   OOOO==O===OO===OOOOO==O===O===OOOOOOO===
>   ^                                      ^
>   0                                      40
>
> This is how it supposed to wotk if you have called FITIRM with parameters:
>
> start = 0
> minlen = 2
> len = 20
>
> So you will go through (blocks, bytes...) 0 -> 20
>
>   OOOO==O===OO===OOOOO==O===O===OOOOOOO===
>   ^                   ^
>   0                   20
>
> So, you will call discard on extents:
>
> 0-3
> You'll skip 6 because is smaller than minlen
> 10-11
> 15-19
>
> instead of 
>
> 0-3
> 10-11
> 15-19
> 30-36
>
> Signed-off-by: Kyungmin Park <kyungmin.park@...sung.com>
> ---
> Changelog v5:
> 	Exit when start cluster is grater than max cluster
>
> Changelog v4:
> 	Simplify the exit condition
>
> Changelog v3:
> 	Adjust the minlen from queue discard_granularity
> 	Use the corrent len usage
> Changelog v2:
> 	Use the given start and len as Lukas comments
> 	Check the queue supports discard feature
> ---
> diff --git a/fs/fat/fat.h b/fs/fat/fat.h
> index f504089..08b53e1 100644
> --- a/fs/fat/fat.h
> +++ b/fs/fat/fat.h
> @@ -299,6 +299,7 @@ extern int fat_alloc_clusters(struct inode *inode, int *cluster,
>  			      int nr_cluster);
>  extern int fat_free_clusters(struct inode *inode, int cluster);
>  extern int fat_count_free_clusters(struct super_block *sb);
> +extern int fat_trim_fs(struct super_block *sb, struct fstrim_range *range);
>  
>  /* fat/file.c */
>  extern long fat_generic_ioctl(struct file *filp, unsigned int cmd,
> diff --git a/fs/fat/fatent.c b/fs/fat/fatent.c
> index b47d2c9..cc89ea6 100644
> --- a/fs/fat/fatent.c
> +++ b/fs/fat/fatent.c
> @@ -1,6 +1,8 @@
>  /*
>   * Copyright (C) 2004, OGAWA Hirofumi
>   * Released under GPL v2.
> + *
> + * Batched discard support by Kyungmin Park <kyungmin.park@...sung.com>
>   */
>  
>  #include <linux/module.h>
> @@ -541,6 +543,16 @@ out:
>  	return err;
>  }
>  
> +static int fat_issue_discard(struct super_block *sb, int cluster, int nr_clus)
> +{
> +	struct msdos_sb_info *sbi = MSDOS_SB(sb);
> +	sector_t block, nr_blocks;
> +
> +	block = fat_clus_to_blknr(sbi, cluster);
> +	nr_blocks = nr_clus * sbi->sec_per_clus;
> +	return sb_issue_discard(sb, block, nr_blocks, GFP_NOFS, 0);
> +}
> +
>  int fat_free_clusters(struct inode *inode, int cluster)
>  {
>  	struct super_block *sb = inode->i_sb;
> @@ -575,11 +587,7 @@ int fat_free_clusters(struct inode *inode, int cluster)
>  			if (cluster != fatent.entry + 1) {
>  				int nr_clus = fatent.entry - first_cl + 1;
>  
> -				sb_issue_discard(sb,
> -					fat_clus_to_blknr(sbi, first_cl),
> -					nr_clus * sbi->sec_per_clus,
> -					GFP_NOFS, 0);
> -
> +				fat_issue_discard(sb, first_cl, nr_clus);
>  				first_cl = cluster;
>  			}
>  		}
> @@ -683,3 +691,86 @@ out:
>  	unlock_fat(sbi);
>  	return err;
>  }
> +
> +int fat_trim_fs(struct super_block *sb, struct fstrim_range *range)
> +{
> +	struct msdos_sb_info *sbi = MSDOS_SB(sb);
> +	struct fatent_operations *ops = sbi->fatent_ops;
> +	struct fat_entry fatent;
> +	unsigned long reada_blocks, reada_mask, cur_block;
> +	int err, free, count, entry;
> +	int start, len, minlen, trimmed;
> +
> +	start = range->start >> sb->s_blocksize_bits;
> +	start = start / sbi->sec_per_clus;

start is round-down, I think it's strange interface. E.g. user specified
the range as "start=10 len=1024". So the range should be 10-1034,
i.e. (assume cluster-size is 512) 512-1024, right?

> +	len = range->len >> sb->s_blocksize_bits;
> +	len = len / sbi->sec_per_clus;

And the end cluster should be round_down(start+len), right?
i.e. start=10 len=1014, the end should be 1024 not 512.

> +	minlen = range->minlen >> sb->s_blocksize_bits;
> +	minlen = minlen / sbi->sec_per_clus;
> +	trimmed = 0;
> +	count = 0;
> +	err = -EINVAL;
> +
> +	lock_fat(sbi);
> +	if (sbi->free_clusters != -1 && sbi->free_clus_valid)
> +		goto out;

free clusters count validation doesn't matter here. If you want to check
free cluster count, you should check free_clusters==0 or not (after
validation).

> +	if (start >= sbi->max_cluster)
> +		goto out;

This check can be done outside lock. And don't we need to check other
parameters from userland?

> +	if (start < FAT_START_ENT)
> +		start = FAT_START_ENT;

Valid data cluster is 2 - max_cluster, but it should be mapped to 0 -
(max_cluster - FAT_START_ENT). Otherwise this interface's abstraction is
useless, right?

> +	fatent_set_entry(&fatent, start);
> +
> +	while (count < sbi->max_cluster) {
> +		if (fatent.entry >= sbi->max_cluster)
> +			fatent.entry = FAT_START_ENT;

Why do we cyclic this?

> +		/* readahead of fat blocks */
> +		if ((cur_block & reada_mask) == 0) {
> +			unsigned long rest = sbi->fat_length - cur_block;
> +			fat_ent_reada(sb, &fatent, min(reada_blocks, rest));
> +		}
> +		cur_block++;
> +
> +		err = fat_ent_read_block(sb, &fatent);
> +		if (err)
> +			goto out;
> +
> +		do {
> +			if (ops->ent_get(&fatent) == FAT_ENT_FREE) {
> +				free++;
> +				if (!entry)
> +					entry = fatent.entry;
> +			} else if (entry) {
> +				if (free >= minlen) {
> +					fat_issue_discard(sb, entry, free);
> +					trimmed += free;
> +				}
> +				free = 0;
> +				entry = 0;
> +			}
> +			count++;
> +			if (count >= len)
> +				goto done;
> +		} while (fat_ent_next(sbi, &fatent));
> +	}
> +done:
> +	if (free >= minlen) {
> +		fat_issue_discard(sb, entry, free);
> +		trimmed += free;
> +	}
> +	range->len = (trimmed * sbi->sec_per_clus) << sb->s_blocksize_bits;

this doesn't need cast? 

	range->len = (u64)(trimmed * sbi->sec_per_clus) << sb->s_blocksize_bits;

> +	fatent_brelse(&fatent);
> +out:
> +	unlock_fat(sbi);
> +	return err;
> +}
> diff --git a/fs/fat/file.c b/fs/fat/file.c
> index 7257752..9910aba 100644
> --- a/fs/fat/file.c
> +++ b/fs/fat/file.c
> @@ -125,6 +125,36 @@ long fat_generic_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
>  		return fat_ioctl_get_attributes(inode, user_attr);
>  	case FAT_IOCTL_SET_ATTRIBUTES:
>  		return fat_ioctl_set_attributes(filp, user_attr);
> +	case FITRIM:
> +	{
> +		struct super_block *sb = inode->i_sb;
> +		struct request_queue *q = bdev_get_queue(sb->s_bdev);
> +		struct fstrim_range range;
> +		int ret = 0;
> +
> +		if (!capable(CAP_SYS_ADMIN))
> +			return -EPERM;
> +
> +		if (!blk_queue_discard(q))
> +			return -EOPNOTSUPP;
> +
> +		if (copy_from_user(&range, (struct fstrim_range *)arg,
> +					sizeof(range)))
> +			return -EFAULT;

Please use __user annotation.

> +		range.minlen = max((unsigned int)range.minlen,
> +					q->limits.discard_granularity);

Please use max_t() instead.

> +		ret = fat_trim_fs(sb, &range);
> +		if (ret < 0)
> +			return ret;
> +
> +		if (copy_to_user((struct fstrim_range *)arg, &range,
> +					sizeof(range)))
> +			return -EFAULT;
> +
> +		return 0;
> +	}
> +
>  	default:
>  		return -ENOTTY;	/* Inappropriate ioctl for device */
>  	}

This doesn't need compat_ioctl?

Thanks.
-- 
OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
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