Re: [PATCH 00/23] md/llbitmap: md/md-llbitmap: introduce a new lockless bitmap

[Yu Kuai <yukuai1@huaweicloud.com>]

Hi,

在 2025/06/30 11:25, Xiao Ni 写道:
> On Mon, Jun 30, 2025 at 10:34 AM Yu Kuai <yukuai1@huaweicloud.com> wrote:
>>
>> Hi,
>>
>> 在 2025/06/30 9:59, Xiao Ni 写道:
>>>
>>> After reading other patches, I want to check if I understand right.
>>>
>>> The first write sets the bitmap bit. The second write which hits the
>>> same block (one sector, 512 bits) will call llbitmap_infect_dirty_bits
>>> to set all other bits. Then the third write doesn't need to set bitmap
>>> bits. If I'm right, the comments above should say only the first two
>>> writes have additional overhead?
>>
>> Yes, for the same bit, it's twice; For different bit in the same block,
>> it's third, by infect all bits in the block in the second.
> 
> For different bits in the same block, test_and_set_bit(bit,
> pctl->dirty) should be true too, right? So it infects other bits when
> second write hits the same block too.

The dirty will be cleared after bitmap_unplug.
> 
> [946761.035079] llbitmap_set_page_dirty:390 page[0] offset 2024, block 3
> [946761.035430] llbitmap_state_machine:646 delay raid456 initial recovery
> [946761.035802] llbitmap_state_machine:652 bit 1001 state from 0 to 3
> [946761.036498] llbitmap_set_page_dirty:390 page[0] offset 2025, block 3
> [946761.036856] llbitmap_set_page_dirty:403 call llbitmap_infect_dirty_bits
> 
> As the debug logs show, different bits in the same block, the second
> write (offset 2025) infects other bits.
> 
>>
>>    For Reload action, if the bitmap bit is
>>> NeedSync, the changed status will be x. It can't trigger resync/recovery.
>>
>> This is not expected, see llbitmap_state_machine(), if old or new state
>> is need_sync, it will trigger a resync.
>>
>> c = llbitmap_read(llbitmap, start);
>> if (c == BitNeedSync)
>>    need_resync = true;
>> -> for RELOAD case, need_resync is still set.
>>
>> state = state_machine[c][action];
>> if (state == BitNone)
>>    continue
> 
> If bitmap bit is BitNeedSync,
> state_machine[BitNeedSync][BitmapActionReload] returns BitNone, so if
> (state == BitNone) is true, it can't set MD_RECOVERY_NEEDED and it
> can't start sync after assembling the array.

You missed what I said above that llbitmap_read() will trigger resync as
well.
> 
>> if (state == BitNeedSync)
>>    need_resync = true;
>>
>>>
>>> For example:
>>>
>>> cat /sys/block/md127/md/llbitmap/bits
>>> unwritten 3480
>>> clean 2
>>> dirty 0
>>> need sync 510
>>>
>>> It doesn't do resync after aseembling the array. Does it need to modify
>>> the changed status from x to NeedSync?
>>
>> Can you explain in detail how to reporduce this? Aseembling in my VM is
>> fine.
> 
> I added many debug logs, so the sync request runs slowly. The test I do:
> mdadm -CR /dev/md0 -l5 -n3 /dev/loop[0-2] --bitmap=lockless -x 1 /dev/loop3
> dd if=/dev/zero of=/dev/md0 bs=1M count=1 seek=500 oflag=direct
> mdadm --stop /dev/md0 (the sync thread finishes the region that two
> bitmap bits represent, so you can see llbitmap/bits has 510 bits (need
> sync))
> mdadm -As

I don't quite understand, in my case, mdadm -As works fine.
> 
> Regards
> Xiao
>>
>> Thanks,
>> Kuai
>>
>>
> 
> 
> .
>