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Date:	Wed, 3 Dec 2008 18:33:30 +0100
From:	Jan Kara <jack@...e.cz>
To:	Mingming Cao <cmm@...ibm.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, tytso <tytso@....edu>,
	linux-ext4 <linux-ext4@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH V3 1/3] quota: Add reservation support for delayed
	block allocation

  Hi Mingming,

On Thu 06-11-08 15:34:35, Mingming Cao wrote:
> Quota: Add quota reservation support
> 
> Delayed allocation defers the block allocation at the dirty pages
> flush-out time, doing quota charge/check at that time is too late.
> But we can't charge the quota blocks until blocks are really allocated,
> otherwise users could get overcharged after reboot from system crash.
> 
> This patch adds quota reservation for delayed allocation. Quota blocks
> are reserved in memory, inode and quota won't gets dirtied until later
> block allocation time.
> 
> Signed-off-by: Mingming Cao <cmm@...ibm.com>
  I've finally got to reading your patches (I hope this is the last version
- at least it's the latest version I have ;). Sorry it took so long.
Here are some comments:

> Index: linux-2.6.28-rc2/fs/dquot.c
> ===================================================================
> --- linux-2.6.28-rc2.orig/fs/dquot.c	2008-11-06 13:36:21.000000000 -0800
> +++ linux-2.6.28-rc2/fs/dquot.c	2008-11-06 14:04:10.000000000 -0800
> @@ -841,6 +841,11 @@ static inline void dquot_incr_space(stru
>  	dquot->dq_dqb.dqb_curspace += number;
>  }
>  
> +static inline void dquot_resv_space(struct dquot *dquot, qsize_t number)
> +{
> +	dquot->dq_dqb.dqb_rsvspace += number;
> +}
> +
>  static inline void dquot_decr_inodes(struct dquot *dquot, qsize_t number)
>  {
>  	if (dquot->dq_dqb.dqb_curinodes > number)
> @@ -1069,13 +1074,18 @@ static int check_idq(struct dquot *dquot
>  /* needs dq_data_lock */
>  static int check_bdq(struct dquot *dquot, qsize_t space, int prealloc, char *warntype)
>  {
> +	qsize_t tspace;
> +
>  	*warntype = QUOTA_NL_NOWARN;
>  	if (!sb_has_quota_limits_enabled(dquot->dq_sb, dquot->dq_type) ||
>  	    test_bit(DQ_FAKE_B, &dquot->dq_flags))
>  		return QUOTA_OK;
>  
> +	tspace = dquot->dq_dqb.dqb_curspace + dquot->dq_dqb.dqb_rsvspace
> +		+ space;
> +
>  	if (dquot->dq_dqb.dqb_bhardlimit &&
> -	    dquot->dq_dqb.dqb_curspace + space > dquot->dq_dqb.dqb_bhardlimit &&
> +	    tspace > dquot->dq_dqb.dqb_bhardlimit &&
>              !ignore_hardlimit(dquot)) {
>  		if (!prealloc)
>  			*warntype = QUOTA_NL_BHARDWARN;
> @@ -1083,7 +1093,7 @@ static int check_bdq(struct dquot *dquot
>  	}
>  
>  	if (dquot->dq_dqb.dqb_bsoftlimit &&
> -	    dquot->dq_dqb.dqb_curspace + space > dquot->dq_dqb.dqb_bsoftlimit &&
> +	    tspace > dquot->dq_dqb.dqb_bsoftlimit &&
>  	    dquot->dq_dqb.dqb_btime && get_seconds() >= dquot->dq_dqb.dqb_btime &&
>              !ignore_hardlimit(dquot)) {
>  		if (!prealloc)
> @@ -1092,7 +1102,7 @@ static int check_bdq(struct dquot *dquot
>  	}
>  
>  	if (dquot->dq_dqb.dqb_bsoftlimit &&
> -	    dquot->dq_dqb.dqb_curspace + space > dquot->dq_dqb.dqb_bsoftlimit &&
> +	    tspace > dquot->dq_dqb.dqb_bsoftlimit &&
>  	    dquot->dq_dqb.dqb_btime == 0) {
>  		if (!prealloc) {
>  			*warntype = QUOTA_NL_BSOFTWARN;
> @@ -1227,49 +1237,85 @@ void vfs_dq_drop(struct inode *inode)
>  /*
>   * This operation can block, but only after everything is updated
>   */
> -int dquot_alloc_space(struct inode *inode, qsize_t number, int warn)
> +int __dquot_alloc_space(struct inode *inode, qsize_t number,
> +			int warn, int reserve)
>  {
> -	int cnt, ret = NO_QUOTA;
> +	int cnt, ret = QUOTA_OK;
>  	char warntype[MAXQUOTAS];
>  
> -	/* First test before acquiring mutex - solves deadlocks when we
> -         * re-enter the quota code and are already holding the mutex */
> -	if (IS_NOQUOTA(inode)) {
> -out_add:
> -		inode_add_bytes(inode, number);
> -		return QUOTA_OK;
> -	}
>  	for (cnt = 0; cnt < MAXQUOTAS; cnt++)
>  		warntype[cnt] = QUOTA_NL_NOWARN;
>  
> -	down_read(&sb_dqopt(inode->i_sb)->dqptr_sem);
> -	if (IS_NOQUOTA(inode)) {	/* Now we can do reliable test... */
> -		up_read(&sb_dqopt(inode->i_sb)->dqptr_sem);
> -		goto out_add;
> -	}
>  	spin_lock(&dq_data_lock);
>  	for (cnt = 0; cnt < MAXQUOTAS; cnt++) {
>  		if (inode->i_dquot[cnt] == NODQUOT)
>  			continue;
> -		if (check_bdq(inode->i_dquot[cnt], number, warn, warntype+cnt) == NO_QUOTA)
> -			goto warn_put_all;
> +		if (check_bdq(inode->i_dquot[cnt], number, warn, warntype+cnt)
> +		    == NO_QUOTA) {
> +			ret = NO_QUOTA;
> +			goto out_unlock;
> +		}
>  	}
>  	for (cnt = 0; cnt < MAXQUOTAS; cnt++) {
>  		if (inode->i_dquot[cnt] == NODQUOT)
>  			continue;
> -		dquot_incr_space(inode->i_dquot[cnt], number);
> +		if (reserve)
> +			dquot_resv_space(inode->i_dquot[cnt], number);
> +		else
> +			dquot_incr_space(inode->i_dquot[cnt], number);
>  	}
> -	inode_add_bytes(inode, number);
> -	ret = QUOTA_OK;
> -warn_put_all:
> +out_unlock:
> +	flush_warnings(inode->i_dquot, warntype);
>  	spin_unlock(&dq_data_lock);
> +	return ret;
> +}
> +
> +int dquot_alloc_space(struct inode *inode, qsize_t number, int warn)
> +{
> +	int cnt, ret = QUOTA_OK;
> +
> +	/*
> +	 * First test before acquiring mutex - solves deadlocks when we
> +	 * re-enter the quota code and are already holding the mutex
> +	 */
> +	if (IS_NOQUOTA(inode))
> +		goto out;
> +
> +	down_read(&sb_dqopt(inode->i_sb)->dqptr_sem);
> +	if (IS_NOQUOTA(inode))
> +		goto out_unlock;
> +
> +	ret = __dquot_alloc_space(inode, number, warn, 0);
> +	if (ret == NO_QUOTA)
> +		goto out_unlock;
> +
> +	/* Dirtify all the dquots - this can block when journalling */
> +	for (cnt = 0; cnt < MAXQUOTAS; cnt++)
> +		if (inode->i_dquot[cnt])
> +			mark_dquot_dirty(inode->i_dquot[cnt]);
> +out_unlock:
> +	up_read(&sb_dqopt(inode->i_sb)->dqptr_sem);
> +out:
>  	if (ret == QUOTA_OK)
> -		/* Dirtify all the dquots - this can block when journalling */
> -		for (cnt = 0; cnt < MAXQUOTAS; cnt++)
> -			if (inode->i_dquot[cnt])
> -				mark_dquot_dirty(inode->i_dquot[cnt]);
> -	flush_warnings(inode->i_dquot, warntype);
> +		inode_add_bytes(inode, number);
> +	return ret;
> +}
  Doing inode_add_bytes() here can cause some trouble - we have to make
sure that the amount of space quota knows (in dqb_space) about and the amount
of space in i_blocks is always the same. We have to keep this invariant
because of concurrent write() and chown/chgrp() calls. So we use dq_data_lock
and make both the change to quota usage and i_blocks under that spinlock.
And you've moved the change of i_blocks outside of dq_data_lock. In
practice this need not be a problem since other locks (i_mutex) might
provide enough synchronization but I'd rather see inode_add_bytes() inside
the dq_data_lock together with the quota usage change.

> +
> +int dquot_reserve_space(struct inode *inode, qsize_t number, int warn)
> +{
> +	int ret = QUOTA_OK;
> +
> +	if (IS_NOQUOTA(inode))
> +		goto out;
> +
> +	down_read(&sb_dqopt(inode->i_sb)->dqptr_sem);
> +	if (IS_NOQUOTA(inode))
> +		goto out_unlock;
> +
> +	ret = __dquot_alloc_space(inode, number, warn, 1);
> +out_unlock:
>  	up_read(&sb_dqopt(inode->i_sb)->dqptr_sem);
> +out:
>  	return ret;
>  }
>  
> @@ -1958,7 +2004,7 @@ static void do_get_dqblk(struct dquot *d
>  	spin_lock(&dq_data_lock);
>  	di->dqb_bhardlimit = stoqb(dm->dqb_bhardlimit);
>  	di->dqb_bsoftlimit = stoqb(dm->dqb_bsoftlimit);
> -	di->dqb_curspace = dm->dqb_curspace;
> +	di->dqb_curspace = dm->dqb_curspace + dm->dqb_rsvspace;
>  	di->dqb_ihardlimit = dm->dqb_ihardlimit;
>  	di->dqb_isoftlimit = dm->dqb_isoftlimit;
>  	di->dqb_curinodes = dm->dqb_curinodes;
  OK, but you should probably subtract dqb_rsvspace from the amount
provided by userspace in do_set_dqblk(). Setting usage is inherently
dangerous thing if fs is used and admin should know what he's doing but
I think setting it so that "total used space" is what admin meant is the
way of the least surprise.

> @@ -2297,6 +2343,7 @@ EXPORT_SYMBOL(dquot_alloc_space);
>  EXPORT_SYMBOL(dquot_alloc_inode);
>  EXPORT_SYMBOL(dquot_free_space);
>  EXPORT_SYMBOL(dquot_free_inode);
> +EXPORT_SYMBOL(dquot_reserve_space);
>  EXPORT_SYMBOL(dquot_transfer);
>  EXPORT_SYMBOL(vfs_dq_transfer);
>  EXPORT_SYMBOL(vfs_dq_quota_on_remount);
> Index: linux-2.6.28-rc2/include/linux/quota.h
> ===================================================================
> --- linux-2.6.28-rc2.orig/include/linux/quota.h	2008-11-06 13:36:21.000000000 -0800
> +++ linux-2.6.28-rc2/include/linux/quota.h	2008-11-06 14:04:01.000000000 -0800
> @@ -186,6 +186,7 @@ struct mem_dqblk {
>  	qsize_t dqb_bhardlimit;	/* absolute limit on disk blks alloc */
>  	qsize_t dqb_bsoftlimit;	/* preferred limit on disk blks */
>  	qsize_t dqb_curspace;	/* current used space */
> +	qsize_t dqb_rsvspace;   /* current reserved space for delalloc*/
>  	qsize_t dqb_ihardlimit;	/* absolute limit on allocated inodes */
>  	qsize_t dqb_isoftlimit;	/* preferred inode limit */
>  	qsize_t dqb_curinodes;	/* current # allocated inodes */
> @@ -291,6 +292,7 @@ struct dquot_operations {
>  	int (*release_dquot) (struct dquot *);		/* Quota is going to be deleted from disk */
>  	int (*mark_dirty) (struct dquot *);		/* Dquot is marked dirty */
>  	int (*write_info) (struct super_block *, int);	/* Write of quota "superblock" */
> +	int (*reserve_space) (struct inode *, qsize_t, int); /* reserve quota for delayed block allocation */
>  };
>  
>  /* Operations handling requests from userspace */
> Index: linux-2.6.28-rc2/include/linux/quotaops.h
> ===================================================================
> --- linux-2.6.28-rc2.orig/include/linux/quotaops.h	2008-11-06 13:36:21.000000000 -0800
> +++ linux-2.6.28-rc2/include/linux/quotaops.h	2008-11-06 14:04:10.000000000 -0800
> @@ -176,6 +176,16 @@ static inline int vfs_dq_alloc_space(str
>  	return ret;
>  }
>  
> +static inline int vfs_dq_reserve_space(struct inode *inode, qsize_t nr)
> +{
> +	if (sb_any_quota_active(inode->i_sb)) {
> +		/* Used space is updated in alloc_space() */
> +		if (inode->i_sb->dq_op->reserve_space(inode, nr, 0) == NO_QUOTA)
> +			return 1;
> +	}
> +	return 0;
> +}
> +
>  static inline int vfs_dq_alloc_inode(struct inode *inode)
>  {
>  	if (sb_any_quota_active(inode->i_sb)) {
> @@ -327,6 +337,11 @@ static inline int vfs_dq_alloc_space(str
>  	return 0;
>  }
>  
> +static inline int vfs_dq_reserve_space(struct inode *inode, qsize_t nr)
> +{
> +	return 0;
> +}
> +
>  static inline void vfs_dq_free_space_nodirty(struct inode *inode, qsize_t nr)
>  {
>  	inode_sub_bytes(inode, nr);
> @@ -358,12 +373,19 @@ static inline int vfs_dq_alloc_block_nod
>  			nr << inode->i_sb->s_blocksize_bits);
>  }
>  
> +
>  static inline int vfs_dq_alloc_block(struct inode *inode, qsize_t nr)
>  {
>  	return vfs_dq_alloc_space(inode,
>  			nr << inode->i_sb->s_blocksize_bits);
>  }
>  
> +static inline int vfs_dq_reserve_block(struct inode *inode, qsize_t nr)
> +{
> +	return vfs_dq_reserve_space(inode,
> +			nr << inode->i_blkbits);
> +}
> +
>  static inline void vfs_dq_free_block_nodirty(struct inode *inode, qsize_t nr)
>  {
>  	vfs_dq_free_space_nodirty(inode, nr << inode->i_sb->s_blocksize_bits);
> @@ -392,6 +414,7 @@ static inline void vfs_dq_free_block(str
>  #define DQUOT_ALLOC_BLOCK_NODIRTY(inode, nr) \
>  				vfs_dq_alloc_block_nodirty(inode, nr)
>  #define DQUOT_ALLOC_BLOCK(inode, nr) vfs_dq_alloc_block(inode, nr)
> +#define DQUOT_RESERVE_BLOCK(inode, nr) vfs_dq_reserve_block(inode, nr)
  These defines should go away as soon as I get to grepping all the
filesystems and converting them to new lowercase function names. So there's
no need of adding this one.

>  #define DQUOT_ALLOC_INODE(inode) vfs_dq_alloc_inode(inode)
>  #define DQUOT_FREE_SPACE_NODIRTY(inode, nr) \
>  				vfs_dq_free_space_nodirty(inode, nr)
> 

									Honza
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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