Re: questions on handling the size of the last block group

On Aug 16, 2006  16:03 -0500, Eric Sandeen wrote:
>  From my understanding, it's possible that the last block group can have 
> fewer blocks than blocks_per_group, if
> 
> total blocks < groups * blocks_per_group
> 
> I found one or two places in e2fsprogs where this is explicitly handled, 
> and in the patch series I just sent I put it in a few more places, but 
> there are still some spots I wonder about, such as:
> 
> static int alloc_new_reservation(struct ext3_reserve_window_node *my_rsv,
>                  ext3_grpblk_t grp_goal, struct super_block *sb,
>                  unsigned int group, struct buffer_head *bitmap_bh)
> {
>          group_first_block = ext3_group_first_block_no(sb, group);
>          group_end_block = group_first_block + 
> (EXT3_BLOCKS_PER_GROUP(sb) - 1);
> 
> 	<...>
> 
>          ret = find_next_reservable_window(search_head, my_rsv, sb,
>                                                  start_block, 
> group_end_block)
> 
> Here group_end_block may be off the end of the filesystem for the last 
> group.
> 
> Is this safe for allocations in the last block group?  Could the 
> allocator be looking for blocks past the end of the filesystem...? 
> Surely not, I expect that more than one such filesystem has been filled 
> to the last block, but I'm not sure where this gets caught... perhaps 
> when searching the bitmaps?

These are indeed searching the bitmaps, and the bitmaps are always a full
block regardless of how many blocks are in the last group.  The only
difference is that the bits past the end of the filesystem are always marked
"used".

Cheers, Andreas
--
Andreas Dilger
Principal Software Engineer
Cluster File Systems, Inc.


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