Re: [PATCH 3/5] md: add fallback to correct bitmap_ops on version mismatch

[Su Yue <l@damenly.org>]

On Mon 23 Feb 2026 at 10:22, "Yu Kuai" <yukuai@fnnas.com> wrote:

> Hi,
>
> 在 2026/2/17 16:54, Su Yue 写道:
>> On Sat 14 Feb 2026 at 14:10, Yu Kuai <yukuai@fnnas.com> wrote:
>>
>>> If default bitmap version and on-disk version doesn't match, 
>>> and mdadm
>>> is not the latest version to set bitmap_type, set bitmap_ops 
>>> based on
>>> the disk version.
>>>
>> Why not just let old version mdadm fails  since llbitmap is a 
>> new
>> feature.
>
> The original use case is that we found llbitmap array fails to 
> assemble in
> some corner cases, and with the respect I'm not quite familiar 
> with mdadm
> code, so I think this patch is the best solution for now.
>
Would you please elaborate which corner cases that llbitmap array 
fails to assemble
in? Do they happen in mdadm <= 4.5?

> On the other hand, this should also be helpful if we decide to 
> make llbitmap
> the default option in the future.
>
But it's so far, right? llbitmap support is still on the way(mdadm 
4.6 is not released).

I am not opposed to the patch. It just looks strange to me that 
changing kernel code to
let old userspace work with *new* feature.
Maybe the mdadm maintainers have words in another angles?

--
Su
>
>>
>>> Signed-off-by: Yu Kuai <yukuai@fnnas.com>
>>> ---
>>>  drivers/md/md.c | 103 
>>>  +++++++++++++++++++++++++++++++++++++++++++++++-
>>>  1 file changed, 102 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/md/md.c b/drivers/md/md.c
>>> index 59cd303548de..d2607ed5c2e9 100644
>>> --- a/drivers/md/md.c
>>> +++ b/drivers/md/md.c
>>> @@ -6447,15 +6447,116 @@ static void 
>>> md_safemode_timeout(struct
>>> timer_list *t)
>>>
>>>  static int start_dirty_degraded;
>>>
>>> +/*
>>> + * Read bitmap superblock and return the bitmap_id based on 
>>> disk
>>> version.
>>> + * This is used as fallback when default bitmap version and 
>>> on-disk
>>> version
>>> + * doesn't match, and mdadm is not the latest version to set
>>> bitmap_type.
>>> + */
>>> +static enum md_submodule_id md_bitmap_get_id_from_sb(struct 
>>> mddev
>>> *mddev)
>>> +{
>>> +    struct md_rdev *rdev;
>>> +    struct page *sb_page;
>>> +    bitmap_super_t *sb;
>>> +    enum md_submodule_id id = ID_BITMAP_NONE;
>>> +    sector_t sector;
>>> +    u32 version;
>>> +
>>> +    if (!mddev->bitmap_info.offset)
>>> +        return ID_BITMAP_NONE;
>>> +
>>> +    sb_page = alloc_page(GFP_KERNEL);
>>> +    if (!sb_page)
>>> +        return ID_BITMAP_NONE;
>>> +
>>>
>> Personally I don't like the way treating error as 
>> ID_BITMAP_NONE.
>> When wrong things happen everything looks fine, no error code, 
>> no
>> error message.
>
> Ok, sounds reasonable.
>
>>
>>> +    sector = mddev->bitmap_info.offset;
>>> +
>>> +    rdev_for_each(rdev, mddev) {
>>> +        u32 iosize;
>>> +
>>> +        if (!test_bit(In_sync, &rdev->flags) ||
>>> +            test_bit(Faulty, &rdev->flags) ||
>>> +            test_bit(Bitmap_sync, &rdev->flags))
>>> +            continue;
>>> +
>>> +        iosize = roundup(sizeof(bitmap_super_t),
>>> +                 bdev_logical_block_size(rdev->bdev));
>>> +        if (sync_page_io(rdev, sector, iosize, sb_page, 
>>> REQ_OP_READ,
>>> +                 true))
>>> +            goto read_ok;
>>> +    }
>>>
>> And here.
>>
>>> +    goto out;
>>> +
>>> +read_ok:
>>> +    sb = kmap_local_page(sb_page);
>>> +    if (sb->magic != cpu_to_le32(BITMAP_MAGIC))
>>> +        goto out_unmap;
>>> +
>>> +    version = le32_to_cpu(sb->version);
>>> +    switch (version) {
>>> +    case BITMAP_MAJOR_LO:
>>> +    case BITMAP_MAJOR_HI:
>>> +    case BITMAP_MAJOR_CLUSTERED:
>>>
>> For BITMAP_MAJOR_CLUSTERED, why not ID_CLUSTER ?
>
> Because there is no optional bitmap_ops for md-cluster, it's 
> still
> the old bitmap, and llbitmap does not support md-cluster for 
> now.
>
>>
>> --
>> Su
>>> +        id = ID_BITMAP;
>>> +        break;
>>> +    case BITMAP_MAJOR_LOCKLESS:
>>> +        id = ID_LLBITMAP;
>>> +        break;
>>> +    default:
>>> +        pr_warn("md: %s: unknown bitmap version %u\n",
>>> +            mdname(mddev), version);
>>> +        break;
>>> +    }
>>> +
>>> +out_unmap:
>>> +    kunmap_local(sb);
>>> +out:
>>> +    __free_page(sb_page);
>>> +    return id;
>>> +}
>>> +
>>>  static int md_bitmap_create(struct mddev *mddev)
>>>  {
>>> +    enum md_submodule_id orig_id = mddev->bitmap_id;
>>> +    enum md_submodule_id sb_id;
>>> +    int err;
>>> +
>>>      if (mddev->bitmap_id == ID_BITMAP_NONE)
>>>          return -EINVAL;
>>>
>>>      if (!mddev_set_bitmap_ops(mddev))
>>>          return -ENOENT;
>>>
>>> -    return mddev->bitmap_ops->create(mddev);
>>> +    err = mddev->bitmap_ops->create(mddev);
>>> +    if (!err)
>>> +        return 0;
>>>
>>> +
>>> +    /*
>>> +     * Create failed, if default bitmap version and on-disk 
>>> version
>>> +     * doesn't match, and mdadm is not the latest version to 
>>> set
>>> +     * bitmap_type, set bitmap_ops based on the disk version.
>>> +     */
>>> +    mddev_clear_bitmap_ops(mddev);
>>> +
>>> +    sb_id = md_bitmap_get_id_from_sb(mddev);
>>> +    if (sb_id == ID_BITMAP_NONE || sb_id == orig_id)
>>> +        return err;
>>> +
>>> +    pr_info("md: %s: bitmap version mismatch, switching from 
>>> %d to
>>> %d\n",
>>> +        mdname(mddev), orig_id, sb_id);
>>> +
>>> +    mddev->bitmap_id = sb_id;
>>> +    if (!mddev_set_bitmap_ops(mddev)) {
>>> +        mddev->bitmap_id = orig_id;
>>> +        return -ENOENT;
>>> +    }
>>> +
>>> +    err = mddev->bitmap_ops->create(mddev);
>>> +    if (err) {
>>> +        mddev_clear_bitmap_ops(mddev);
>>> +        mddev->bitmap_id = orig_id;
>>> +    }
>>> +
>>> +    return err;
>>>  }
>>>
>>>  static void md_bitmap_destroy(struct mddev *mddev)