Re: [PATCH] Ext4: Fix delalloc enospace handling counter update race

[Mingming Cao <cmm@us.ibm.com>]

在 2008-07-14一的 22:23 +0530，Aneesh Kumar K.V写道：
> On Mon, Jul 14, 2008 at 09:28:17AM -0700, Mingming Cao wrote:
> > 
> > Ext4: Fix delalloc enospace handling counter update race
> > 
> > From: Mingming Cao <cmm@us.ibm.com>

> > Also in the case of truncate, we should not clear the per-allocation allocated
> > metablocks counter as that may be in-use by parallel allocation. The patch
> > only clear the per-allocation allocated metablocks when allocation is successfully returned.
> > 
> > 
> > Signed-off-by: Mingming Cao <cmm@us.ibm.com>
> 
> Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>


> > @@ -1519,7 +1529,8 @@ void ext4_da_release_space(struct inode 
> > 
> >  	BUG_ON(mdb > EXT4_I(inode)->i_reserved_meta_blocks);
> >  	EXT4_I(inode)->i_reserved_meta_blocks = mdb;
> > -	EXT4_I(inode)->i_allocated_meta_blocks = 0;
> > +	if (used)
> > +		EXT4_I(inode)->i_allocated_meta_blocks = 0;
> >  	spin_unlock(&EXT4_I(inode)->i_block_reservation_lock);
> >  }
> > 

After think again, this part of race fix may not needed, when the first
part of patch is applied. Since truncate is also hold the i_data_sem, so
it could be assured that when truncate is releasing the page's  reserved
blocks, there is no parallel block allocation.

Updated patch with only the first race fix, 

If no objection I will fold this to the parent patch
delalloc-ext4-ENOSPC-handling.patch in the patch queue.

Regards,
Mingming

Ext4: Fix delalloc enospace handling counter update race

From: Mingming Cao <cmm@us.ibm.com>

Ext4 delalloc reserve meta blocks ahead of time so later when real block
allocation fs will not short of free blocks for allocating meta blocks.
It keeps track of the real number of new allocated meta data blocks, so
after block allocation it will update how much meta data blocks still
need to be reserved for that inode.

Both per inode reserved metadata blocks and per-allocation allocated metablocks
are protected by the per inode delalloc reservation lock. The per-allocation
allocated metablocks counter should be protected by the i_data_sem as well,
so that it could avoid race with other block allocation to the same inode
in parallel. The patch moves the code under the i_data_sem protection.

Signed-off-by: Mingming Cao <cmm@us.ibm.com>

---
 fs/ext4/inode.c |   16 +++++++++++-----
 1 file changed, 11 insertions(+), 5 deletions(-)

Index: linux-2.6.26-rc9/fs/ext4/inode.c
===================================================================
--- linux-2.6.26-rc9.orig/fs/ext4/inode.c	2008-07-14 08:54:25.000000000 -0700
+++ linux-2.6.26-rc9/fs/ext4/inode.c	2008-07-14 10:51:00.000000000 -0700
@@ -1060,8 +1060,18 @@ int ext4_get_blocks_wrap(handle_t *handl
 							~EXT4_EXT_MIGRATE;
 		}
 	}
-	if (flag)
+
+	if (flag) {
 		EXT4_I(inode)->i_delalloc_reserved_flag = 0;
+		/*
+		 * Update reserved blocks/metadata blocks
+		 * after successful block allocation
+		 * which were deferred till now
+		 */
+		if ((retval > 0) && buffer_delay(bh))
+			ext4_da_release_space(inode, retval, 0);
+	}
+
 	up_write((&EXT4_I(inode)->i_data_sem));
 	return retval;
 }
@@ -2005,10 +2015,6 @@ static int ext4_da_get_block_write(struc
 	if (ret > 0) {
 		bh_result->b_size = (ret << inode->i_blkbits);
 
-		/* release reserved-but-unused meta blocks */
-		if (buffer_delay(bh_result))
-			ext4_da_release_space(inode, ret, 0);