linux-kernel - Re: [PATCH 07/23] md/md-bitmap: delay registration of bitmap

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Message-ID: <12a61dcf-ad39-48e8-132f-c49979b9012b@huaweicloud.com>
Date: Mon, 26 May 2025 15:57:37 +0800
From: Yu Kuai <yukuai1@...weicloud.com>
To: Xiao Ni <xni@...hat.com>, Yu Kuai <yukuai1@...weicloud.com>
Cc: hch@....de, colyli@...nel.org, song@...nel.org,
 linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-raid@...r.kernel.org, yi.zhang@...wei.com, yangerkun@...wei.com,
 johnny.chenyi@...wei.com, "yukuai (C)" <yukuai3@...wei.com>
Subject: Re: [PATCH 07/23] md/md-bitmap: delay registration of bitmap_ops
 until creating bitmap

Hi,

在 2025/05/26 14:52, Xiao Ni 写道:
> On Sat, May 24, 2025 at 2:18 PM Yu Kuai <yukuai1@...weicloud.com> wrote:
>>
>> From: Yu Kuai <yukuai3@...wei.com>
>>
>> Currently bitmap_ops is registered while allocating mddev, this is fine
>> when there is only one bitmap_ops, however, after introduing a new
>> bitmap_ops, user space need a time window to choose which bitmap_ops to
>> use while creating new array.
> 
> Could you give more explanation about what the time window is? Is it
> between setting llbitmap by bitmap_type and md_bitmap_create?

The window after this patch is that user can write the new sysfs after
allocating mddev, and before running the array.
> 
>>
>> Signed-off-by: Yu Kuai <yukuai3@...wei.com>
>> ---
>>   drivers/md/md.c | 86 +++++++++++++++++++++++++++++++------------------
>>   1 file changed, 55 insertions(+), 31 deletions(-)
>>
>> diff --git a/drivers/md/md.c b/drivers/md/md.c
>> index 4eb0c6effd5b..dc4b85f30e13 100644
>> --- a/drivers/md/md.c
>> +++ b/drivers/md/md.c
>> @@ -674,39 +674,50 @@ static void no_op(struct percpu_ref *r) {}
>>
>>   static bool mddev_set_bitmap_ops(struct mddev *mddev)
>>   {
>> +       struct bitmap_operations *old = mddev->bitmap_ops;
>> +       struct md_submodule_head *head;
>> +
>> +       if (mddev->bitmap_id == ID_BITMAP_NONE ||
>> +           (old && old->head.id == mddev->bitmap_id))
>> +               return true;
>> +
>>          xa_lock(&md_submodule);
>> -       mddev->bitmap_ops = xa_load(&md_submodule, mddev->bitmap_id);
>> +       head = xa_load(&md_submodule, mddev->bitmap_id);
>>          xa_unlock(&md_submodule);
>>
>> -       if (!mddev->bitmap_ops) {
>> -               pr_warn_once("md: can't find bitmap id %d\n", mddev->bitmap_id);
>> +       if (WARN_ON_ONCE(!head || head->type != MD_BITMAP)) {
>> +               pr_err("md: can't find bitmap id %d\n", mddev->bitmap_id);
>>                  return false;
>>          }
>>
>> +       if (old && old->group)
>> +               sysfs_remove_group(&mddev->kobj, old->group);
> 
> I think you're handling a competition problem here. But I don't know
> how the old/old->group is already created when creating an array.
> Could you explain this?

It's not possible now, this is because I think we want to be able to
switch existing array with old bitmap to new bitmap.

Thanks,
Kuai

> 
> Regards
> Xiao