Re: ext4: why init the unused block group at batched discard?

On Wed, Jul 6, 2011 at 3:21 PM, Amir Goldstein [off-list ref] wrote:

On Wed, Jul 6, 2011 at 9:09 AM, Kyungmin Park [off-list ref] wrote:

quoted

Hi Lukas,

During code review batched discard support at ext4. I wonder why do
you init the uninitialized block group during batched discard.
As you know uninitialized block group mean that there's no operation
at these blocks.

Hi Kyungmin,

EXT4_MB_GRP_NEED_INIT() is not the same as EXT4_BG_BLOCK_UNINIT.
The former means that no allocation/free was attempted from the block group
since mount time and the latter means that no allocation happened since mkfs.

quoted

So no need to trim it at all.

So I think it is possible that blocks deleted in previous mount time would
need to be trimmed in this mount time.

Am I right, Lukas?

It seems that you both are right.

if EXT4_MB_GRP_NEED_INIT() retruns true, it means buddy allocator has
not been initialized, and ext4_mb_init_group() initializes the buddy
allocator for specified group.

Actually, ext4_mb_init_group() initializes UNINIT block bitmap to
generate buddy bitmaps.

Maybe in trim case, we should consider BLOCK_UNINIT as well.

Yongqiang.

Amir.

quoted

How do you think?

int ext4_trim_fs(struct super_block *sb, struct fstrim_range *range)
{
       ...

       for (group = first_group; group <= last_group; group++) {
               grp = ext4_get_group_info(sb, group);
               /* We only do this if the grp has never been initialized */
               if (unlikely(EXT4_MB_GRP_NEED_INIT(grp))) {
                       ret = ext4_mb_init_group(sb, group);
                       if (ret)
                               break;
               }
       ...
       }
       ...
}

Thank you,
Kyungmin Park
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-- 
Best Wishes
Yongqiang Yang
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