Re: [PATCH 07/23] md/md-bitmap: delay registration of bitmap_ops until creating bitmap

[Xiao Ni <xni@redhat.com>]

在 2025/5/26 下午3:57, Yu Kuai 写道:
> Hi,
>
> 在 2025/05/26 14:52, Xiao Ni 写道:
>> On Sat, May 24, 2025 at 2:18 PM Yu Kuai <yukuai1@huaweicloud.com> wrote:
>>>
>>> From: Yu Kuai <yukuai3@huawei.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.


Thanks for the explanation. Is it ok to add it in the commit log message?

>>
>>>
>>> Signed-off-by: Yu Kuai <yukuai3@huawei.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.


Can we add the check of old when we really want it?

Regards

Xiao

>
> Thanks,
> Kuai
>
>>
>> Regards
>> Xiao
>