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

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Date:	Thu, 7 Jul 2011 09:47:29 +0800
From:	Yongqiang Yang <xiaoqiangnk@...il.com>
To:	Amir Goldstein <amir73il@...il.com>
Cc:	Kyungmin Park <kmpark@...radead.org>,
	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 3:21 PM, Amir Goldstein <amir73il@...il.com> wrote:
> 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?
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.
>
>>
>> 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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