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Date:	Thu, 06 Nov 2008 15:28:24 -0800
From:	Mingming Cao <cmm@...ibm.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	jack@...e.cz, tytso@....edu, linux-ext4@...r.kernel.org,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH V2 3/3] ext4: quota handling for delayed allocation


On Tue, 2008-11-04 at 17:32 -0800, Andrew Morton wrote:
> On Fri, 31 Oct 2008 14:27:31 -0700
> Mingming Cao <cmm@...ibm.com> wrote:
> 
> > ext4: quota reservation for delayed allocation
> > 
> > Uses quota reservation/claim/release to handle quota properly for delayed
> > allocation in the three steps: 1) quotas are reserved when data being copied
> > to cache when block allocation is defered 2) when new blocks are allocated.
> > reserved quotas are converted to the real allocated quota, 2) over-booked
> > quotas for metadata blocks are released back.
> > 
> > 
> > Signed-off-by: Mingming Cao <cmm@...ibm.com>
> > ---
> >  fs/ext4/inode.c   |   25 ++++++++++++++++++++++++-
> >  fs/ext4/mballoc.c |   18 +++++++++---------
> >  fs/ext4/super.c   |    2 ++
> >  3 files changed, 35 insertions(+), 10 deletions(-)
> > 
> > Index: linux-2.6.28-rc2/fs/ext4/inode.c
> > ===================================================================
> > --- linux-2.6.28-rc2.orig/fs/ext4/inode.c	2008-10-29 13:26:55.000000000 -0700
> > +++ linux-2.6.28-rc2/fs/ext4/inode.c	2008-10-30 14:25:47.000000000 -0700
> > @@ -994,7 +994,9 @@ static void ext4_da_update_reserve_space
> >  {
> >  	struct ext4_sb_info *sbi = EXT4_SB(inode->i_sb);
> >  	int total, mdb, mdb_free;
> > +	int claim_quota, free_quota = 0;
> >  
> > +	claim_quota = used;
> >  	spin_lock(&EXT4_I(inode)->i_block_reservation_lock);
> >  	/* recalculate the number of metablocks still need to be reserved */
> >  	total = EXT4_I(inode)->i_reserved_data_blocks - used;
> > @@ -1007,6 +1009,8 @@ static void ext4_da_update_reserve_space
> >  	if (mdb_free) {
> >  		/* Account for allocated meta_blocks */
> >  		mdb_free -= EXT4_I(inode)->i_allocated_meta_blocks;
> > +		free_quota = mdb_free;
> > +		claim_quota += EXT4_I(inode)->i_allocated_meta_blocks;
> >  
> >  		/* update fs dirty blocks counter */
> >  		percpu_counter_sub(&sbi->s_dirtyblocks_counter, mdb_free);
> > @@ -1017,8 +1021,14 @@ static void ext4_da_update_reserve_space
> >  	/* update per-inode reservations */
> >  	BUG_ON(used  > EXT4_I(inode)->i_reserved_data_blocks);
> >  	EXT4_I(inode)->i_reserved_data_blocks -= used;
> > -
> >  	spin_unlock(&EXT4_I(inode)->i_block_reservation_lock);
> > +
> > +	/*
> > +	 * free those over-booking quota for metadata blocks
> > +	 */
> > +
> > +	if (free_quota)
> > +		DQUOT_RELEASE_RSV_BLOCK(inode, free_quota);
> >  }
> >  
> >  /*
> > @@ -1514,8 +1524,8 @@ static int ext4_journalled_write_end(str
> >  static int ext4_da_reserve_space(struct inode *inode, int nrblocks)
> >  {
> >  	int retries = 0;
> > -       struct ext4_sb_info *sbi = EXT4_SB(inode->i_sb);
> > -       unsigned long md_needed, mdblocks, total = 0;
> > +	struct ext4_sb_info *sbi = EXT4_SB(inode->i_sb);
> > +	unsigned long md_needed, mdblocks, total = 0;
> >  
> >  	/*
> >  	 * recalculate the amount of metadata blocks to reserve
> > @@ -1531,12 +1541,23 @@ repeat:
> >  	md_needed = mdblocks - EXT4_I(inode)->i_reserved_meta_blocks;
> >  	total = md_needed + nrblocks;
> >  
> > +	/*
> > +	 * Make quota reservation here, to prevent quota overflow
> > +	 * later.Real quota accounting is done at pages writeout
> > +	 * time
> 
> "Make the quota reservation here to prevent quota overflow later.  Real
> quota accounting is performed and page writeout time."
> 
> 
Okay.

> > +	 */
> > +	 if (DQUOT_RESERVE_BLOCK(inode, total)) {
> > +		spin_unlock(&EXT4_I(inode)->i_block_reservation_lock);
> > +		return -EDQUOT;
> > +	}
> 
> Mangled whitespace.  checkpatch seems to miss this (but you didn't run
> checkpatch anyway.  Please do so?)
> 

Attached fixes should fix it. 

> >  	if (ext4_claim_free_blocks(sbi, total)) {
> >  		spin_unlock(&EXT4_I(inode)->i_block_reservation_lock);
> >  		if (ext4_should_retry_alloc(inode->i_sb, &retries)) {
> >  			yield();
> >  			goto repeat;
> >  		}
> > +		DQUOT_RELEASE_RSV_BLOCK(inode,total);
> >  		return -ENOSPC;
> >  	}
> >  	EXT4_I(inode)->i_reserved_data_blocks += nrblocks;
> > @@ -1590,6 +1611,8 @@ static void ext4_da_release_space(struct
> >  	BUG_ON(mdb > EXT4_I(inode)->i_reserved_meta_blocks);
> >  	EXT4_I(inode)->i_reserved_meta_blocks = mdb;
> >  	spin_unlock(&EXT4_I(inode)->i_block_reservation_lock);
> > +
> > +	DQUOT_RELEASE_RSV_BLOCK(inode, release);
> >  }
> 
> DQUOT_RESERVE_BLOCK needs i_block_reservation_lock?  And
> DQUOT_RELEASE_RSV_BLOCK does not?  Seems peculiar.
> 

We need the i_block_reservation_lock to get and calculate the amount of
data/metadata blocks to reserve (freeblocks and quota blocks). 
The quota blocks check/reservation is performed in the middle of
checking-and-update per-inode reserved block counters for delayed
allocation(those data are protected by i_block_reservation_lock), so
that if the request is beyond the quota limits, it will returns without
perform the free blocks reservation. We can't drop the
i_block_reservation_lock before doing quota check/reservation, otherwise
when we get back from the quota check, the free blocks reserved could be
different.

Release the reserved quota blocks  could be performed after the free
blocks reservation is finished, so it doesn't require the lock to be
hold.

> Please see if we can avoid the bad multiple-return-statements?
> 
> This function evaluates EXT4_I(inode) many times.  I'd suggest that you
> look at caching the result of that expression in a local variable, see
> if that results in improved code generation on common architectures.
> 

Ok, I will see if I could put together two cleanup patches.
> >  static void ext4_da_page_release_reservation(struct page *page,
> > Index: linux-2.6.28-rc2/fs/ext4/super.c
> > ===================================================================
> > --- linux-2.6.28-rc2.orig/fs/ext4/super.c	2008-10-29 13:26:55.000000000 -0700
> > +++ linux-2.6.28-rc2/fs/ext4/super.c	2008-10-29 14:00:27.000000000 -0700
> > @@ -795,6 +795,9 @@ static struct dquot_operations ext4_quot
> >  	.initialize	= ext4_dquot_initialize,
> >  	.drop		= ext4_dquot_drop,
> >  	.alloc_space	= dquot_alloc_space,
> > +	.reserve_space	= dquot_reserve_space,
> > +	.claim_space	= dquot_claim_space,
> > +	.release_rsv	= dquot_release_reserved_space,
> >  	.alloc_inode	= dquot_alloc_inode,
> >  	.free_space	= dquot_free_space,
> >  	.free_inode	= dquot_free_inode,
> > Index: linux-2.6.28-rc2/fs/ext4/mballoc.c
> > ===================================================================
> > --- linux-2.6.28-rc2.orig/fs/ext4/mballoc.c	2008-10-29 13:26:55.000000000 -0700
> > +++ linux-2.6.28-rc2/fs/ext4/mballoc.c	2008-10-30 14:30:39.000000000 -0700
> > @@ -2887,9 +2887,11 @@ ext4_mb_mark_diskspace_used(struct ext4_
> >  	if (!(ac->ac_flags & EXT4_MB_DELALLOC_RESERVED))
> >  		/* release all the reserved blocks if non delalloc */
> >  		percpu_counter_sub(&sbi->s_dirtyblocks_counter, reserv_blks);
> > -	else
> > +	else {
> >  		percpu_counter_sub(&sbi->s_dirtyblocks_counter,
> >  						ac->ac_b_ex.fe_len);
> > +		DQUOT_CLAIM_BLOCK(ac->ac_inode, ac->ac_b_ex.fe_len);
> > +	}
> >  
> >  	if (sbi->s_log_groups_per_flex) {
> >  		ext4_group_t flex_group = ext4_flex_group(sbi,
> > @@ -4286,15 +4288,24 @@ ext4_fsblk_t ext4_mb_new_blocks(handle_t
> >  	struct ext4_sb_info *sbi;
> >  	struct super_block *sb;
> >  	ext4_fsblk_t block = 0;
> > -	unsigned long inquota;
> > +	unsigned long inquota = 0;
> >  	unsigned long reserv_blks = 0;
> >  
> >  	sb = ar->inode->i_sb;
> >  	sbi = EXT4_SB(sb);
> >  
> > -	if (!EXT4_I(ar->inode)->i_delalloc_reserved_flag) {
> > +	/*
> > +	 * For delayed allocation, we could skip the ENOSPC and
> > +	 * EDQUOT check, as blocks and quotas have been already
> > +	 * reserved when data being copied to cache
> 
> "when data was copied into pagecache"?
> 
Sure.

> > +	 */
> > +	if (EXT4_I(ar->inode)->i_delalloc_reserved_flag)
> > +		ar->flags |= EXT4_MB_DELALLOC_RESERVED;
> > +	else {
> >  		/*
> > -		 * With delalloc we already reserved the blocks
> > +		 * Without delayed allocation we need to verify
> > +		 * there is enough free blocks to do block allocation
> > +		 * and under the quota limits
> 
> I don't understand the "and under the quota limits" bit.  What is this
> referring to?
> 

Sorry, better to be "and verify allocation is under the quota limits"

> The comment needs a bit of attention.

> >  		 */
> >  		while (ar->len && ext4_claim_free_blocks(sbi, ar->len)) {
> >  			/* let others to free the space */
> > @@ -4306,19 +4317,16 @@ ext4_fsblk_t ext4_mb_new_blocks(handle_t
> >  			return 0;
> >  		}
> >  		reserv_blks = ar->len;
> > +		while (ar->len && DQUOT_ALLOC_BLOCK(ar->inode, ar->len)) {
> > +			ar->flags |= EXT4_MB_HINT_NOPREALLOC;
> > +			ar->len--;
> > +		}
> 
> What's this doing?  Trying increasingly small quota allocations until
> one of them succeeds?  This sounds like a good way to burn up a large
> number of cycles.
> 
> Isn't there some way we can be smarter about this?  Surely the quota
> layer knows exactly how many blocks we can reserve.

Yep, you are right, there is a sb operation get_dqblk() fs could use to
get the quota disk usage out. I will optimize it in a separete patch.

> 
> > +		if (ar->len == 0) {
> > +			*errp = -EDQUOT;
> > +			return 0;
> > +		}
> > +		inquota = ar->len;
> >  	}
> > -	while (ar->len && DQUOT_ALLOC_BLOCK(ar->inode, ar->len)) {
> > -		ar->flags |= EXT4_MB_HINT_NOPREALLOC;
> > -		ar->len--;
> > -	}
> 
> Oh, it already sucked :)
> 
> > -	if (ar->len == 0) {
> > -		*errp = -EDQUOT;
> > -		return 0;
> > -	}
> > -	inquota = ar->len;
> > -
> > -	if (EXT4_I(ar->inode)->i_delalloc_reserved_flag)
> > -		ar->flags |= EXT4_MB_DELALLOC_RESERVED;
> >  
> >  	ac = kmem_cache_alloc(ext4_ac_cachep, GFP_NOFS);
> >  	if (!ac) {
> > @@ -4380,7 +4388,7 @@ repeat:
> >  out2:
> >  	kmem_cache_free(ext4_ac_cachep, ac);
> >  out1:
> > -	if (ar->len < inquota)
> > +	if (inquota && ar->len < inquota)
> >  		DQUOT_FREE_BLOCK(ar->inode, inquota - ar->len);
> >  
> >  	return block;
> > 

Thanks for your comments, here is the incremental fix

---
 fs/ext4/inode.c   |   10 +++++-----
 fs/ext4/mballoc.c |    4 ++--
 2 files changed, 7 insertions(+), 7 deletions(-)

Index: linux-2.6.28-rc2/fs/ext4/inode.c
===================================================================
--- linux-2.6.28-rc2.orig/fs/ext4/inode.c	2008-11-06 14:02:57.000000000 -0800
+++ linux-2.6.28-rc2/fs/ext4/inode.c	2008-11-06 14:03:01.000000000 -0800
@@ -1542,11 +1542,11 @@ repeat:
 	total = md_needed + nrblocks;
 
 	/*
-	 * Make quota reservation here, to prevent quota overflow
-	 * later.Real quota accounting is done at pages writeout
-	 * time
+	 * Make quota reservation here to prevent quota overflow
+	 * later. Real quota accounting is done at pages writeout
+	 * time.
 	 */
-	 if (DQUOT_RESERVE_BLOCK(inode, total)) {
+	if (DQUOT_RESERVE_BLOCK(inode, total)) {
 		spin_unlock(&EXT4_I(inode)->i_block_reservation_lock);
 		return -EDQUOT;
 	}
@@ -1557,7 +1557,7 @@ repeat:
 			yield();
 			goto repeat;
 		}
-		DQUOT_RELEASE_RSV_BLOCK(inode,total);
+		DQUOT_RELEASE_RSV_BLOCK(inode, total);
 		return -ENOSPC;
 	}
 	EXT4_I(inode)->i_reserved_data_blocks += nrblocks;
Index: linux-2.6.28-rc2/fs/ext4/mballoc.c
===================================================================
--- linux-2.6.28-rc2.orig/fs/ext4/mballoc.c	2008-11-06 14:02:57.000000000 -0800
+++ linux-2.6.28-rc2/fs/ext4/mballoc.c	2008-11-06 14:03:01.000000000 -0800
@@ -4297,7 +4297,7 @@ ext4_fsblk_t ext4_mb_new_blocks(handle_t
 	/*
 	 * For delayed allocation, we could skip the ENOSPC and
 	 * EDQUOT check, as blocks and quotas have been already
-	 * reserved when data being copied to cache
+	 * reserved when data being copied into pagecache.
 	 */
 	if (EXT4_I(ar->inode)->i_delalloc_reserved_flag)
 		ar->flags |= EXT4_MB_DELALLOC_RESERVED;
@@ -4305,7 +4305,7 @@ ext4_fsblk_t ext4_mb_new_blocks(handle_t
 		/*
 		 * Without delayed allocation we need to verify
 		 * there is enough free blocks to do block allocation
-		 * and under the quota limits
+		 * and verify allocation doesn't exceed the quota limits.
 		 */
 		while (ar->len && ext4_claim_free_blocks(sbi, ar->len)) {
 			/* let others to free the space */


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