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

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Date:	Wed, 6 Jul 2011 10:21:23 +0300
From:	Amir Goldstein <amir73il@...il.com>
To:	Kyungmin Park <kmpark@...radead.org>
Cc:	Lukas Czerner <lczerner@...hat.com>,
	Linux Filesystem Mailing List <linux-fsdevel@...r.kernel.org>,
	Ext4 Developers List <linux-ext4@...r.kernel.org>
Subject: Re: ext4: why init the unused block group at batched discard?

On Wed, Jul 6, 2011 at 9:09 AM, Kyungmin Park <kmpark@...radead.org> wrote:
> 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.

> 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?

Amir.

>
> 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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