On Thu, Aug 28, 2008 at 02:57:49PM -0700, Mingming Cao wrote:
>
> From: Mingming Cao <cmm@xxxxxxxxxx>
>
> 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".
The goal block group had free blocks < windowsz .
>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.
I don't see how this change is going to make a difference. The goal group
had free blocks < windowsz and that made my_rsv = NULL. I guess we
should not make my_rsv in the first loop. Or in otherwords we can remove
/*
* if there is not enough free blocks to make a new
* resevation
* 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;
And since we have the below check in the for loop
if (my_rsv && (free_blocks <= (windowsz/2)))
continue;
We would skip all the groups that have low free block count.
Now if we are not able to allocate any blocks (ENOSPC)
we loop back because of
if (my_rsv) {
my_rsv = NULL;
windowsz = 0;
group_no = goal_group;
goto retry_alloc;
}
and that would allocate blocks from the first group available.
This also give a chance to scan all the groups to make sure
if we have any of them left with enough free blocks to
add to the reservation.
>
> 2) we don't need to check the window size if the block reservation is off.
This change i have already tested.
>
> Signed-off-by: Mingming Cao <cmm@xxxxxxxxxx>
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@xxxxxxxxxxxxxxxxxx>
>
> 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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