Re: [PATCH -V3 09/11] ext4: Fix ext4 nomballoc allocator for ENOSPC

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

在 2008-08-27三的 20:58 +0530，Aneesh Kumar K.V写道：
> Make sure we  set windowsize to zero if the free
> blocks left is less that window size. Otherwise
> we skip some group with low freeblock count during
> block allocation
> 

Current code has a way to try to prevent early ENOSPC with old ext3
block reservation. After searching for all block groups and can't do
block reservation and allocation, it will fall back to no block
reservation and scan the block groups from the beginning again. 

But this doesn't work in the case the reservation was turned off in the
first goal block group allocation due to 0 free blocks, and the rest
block groups are skipped due to the check of "free_blocks < windowsz/2",
I think this causes the ENOSPC error you saw.

There are two issues. I am attaching the  fix for two issues here. 

Thanks,

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

ext4: Fix ext4 nomballoc allocator for ENOSPC
    
We run into ENOSPC error on nonmballoc ext4, even when there is free blocks
on the filesystem.

The problem is triggered in the case the goal block group has 0 free blocks
, and the rest block groups are skipped due to the check of "free_blocks 
< windowsz/2". Current code could fall back to non reservation allocation
to prevent early ENOSPC after examing all the block groups with reservation on
, but this code was  bypassed if the reservation window is turned off already,
which is true in this case.

This patch fixed two issues:
1) We don't need to turn off block reservation if the goal block group has 
0 free blocks left and continue search for the rest of block groups.

Current code the intention is to turn off the block reservation if the
 goal allocation group has a few (some) free blocks left (not enough 
for make the desired reservation window),to try to allocation in the
 goal block group, to get better locality. But if the goal blocks have 
0 free blocks,  it should leave the block reservation on, and continues
search for the next block groups,rather than turn off block reservation
completely.

2) we don't need to check the window size if the block reservation is off. 

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

Index: linux-2.6.27-rc3/fs/ext4/balloc.c
===================================================================
--- linux-2.6.27-rc3.orig/fs/ext4/balloc.c	2008-08-28 12:41:55.000000000 -0700
+++ linux-2.6.27-rc3/fs/ext4/balloc.c	2008-08-28 14:40:43.000000000 -0700
@@ -1807,6 +1807,7 @@
 	 * turn off reservation for this allocation
 	 */
 	if (my_rsv && (free_blocks < windowsz)
+		&& (free_blocks > 0)
 		&& (rsv_is_empty(&my_rsv->rsv_window)))
 		my_rsv = NULL;
 
@@ -1843,7 +1844,7 @@
 		 * free blocks is less than half of the reservation
 		 * window size.
 		 */
-		if (free_blocks <= (windowsz/2))
+		if (my_rsv && (free_blocks <= (windowsz/2)))
 			continue;
 
 		brelse(bitmap_bh);


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