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Subject: Re: [PATCH v5 02/11] block: Block Device Filtering Mechanism
To:     Sergei Shtepa <sergei.shtepa@veeam.com>,
        Yu Kuai <yukuai1@huaweicloud.com>, axboe@kernel.dk,
        hch@infradead.org, corbet@lwn.net, snitzer@kernel.org
Cc:     viro@zeniv.linux.org.uk, brauner@kernel.org, dchinner@redhat.com,
        willy@infradead.org, dlemoal@kernel.org, linux@weissschuh.net,
        jack@suse.cz, ming.lei@redhat.com, linux-block@vger.kernel.org,
        linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-fsdevel@vger.kernel.org,
        Donald Buczek <buczek@molgen.mpg.de>,
        "yukuai (C)" <yukuai3@huawei.com>
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 <686b9999-c903-cff1-48ba-21324031da17@veeam.com>
From:   Yu Kuai <yukuai1@huaweicloud.com>
Message-ID: <fc740cf1-93a7-e438-e784-5209808981dc@huaweicloud.com>
Date:   Tue, 18 Jul 2023 20:32:01 +0800
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Hi,

在 2023/07/18 19:25, Sergei Shtepa 写道:
> Hi.
> 
> On 7/18/23 03:37, Yu Kuai wrote:
>> Subject:
>> Re: [PATCH v5 02/11] block: Block Device Filtering Mechanism
>> From:
>> Yu Kuai <yukuai1@huaweicloud.com>
>> Date:
>> 7/18/23, 03:37
>>
>> To:
>> Sergei Shtepa <sergei.shtepa@veeam.com>, Yu Kuai <yukuai1@huaweicloud.com>, axboe@kernel.dk, hch@infradead.org, corbet@lwn.net, snitzer@kernel.org
>> CC:
>> viro@zeniv.linux.org.uk, brauner@kernel.org, dchinner@redhat.com, willy@infradead.org, dlemoal@kernel.org, linux@weissschuh.net, jack@suse.cz, ming.lei@redhat.com, linux-block@vger.kernel.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, Donald Buczek <buczek@molgen.mpg.de>, "yukuai (C)" <yukuai3@huawei.com>
>>
>>
>> Hi,
>>
>> 在 2023/07/17 22:39, Sergei Shtepa 写道:
>>>
>>>
>>> On 7/11/23 04:02, Yu Kuai wrote:
>>>> bdev_disk_changed() is not handled, where delete_partition() and
>>>> add_partition() will be called, this means blkfilter for partiton will
>>>> be removed after partition rescan. Am I missing something?
>>>
>>> Yes, when the bdev_disk_changed() is called, all disk block devices
>>> are deleted and new ones are re-created. Therefore, the information
>>> about the attached filters will be lost. This is equivalent to
>>> removing the disk and adding it back.
>>>
>>> For the blksnap module, partition rescan will mean the loss of the
>>> change trackers data. If a snapshot was created, then such
>>> a partition rescan will cause the snapshot to be corrupted.
>>>
>>
>> I haven't review blksnap code yet, but this sounds like a problem.
> 
> I can't imagine a case where this could be a problem.
> Partition rescan is possible only if the file system has not been
> mounted on any of the disk partitions. Ioctl BLKRRPART will return
> -EBUSY. Therefore, during normal operation of the system, rescan is
> not performed.
> And if the file systems have not been mounted, it is possible that
> the disk partition structure has changed or the disk in the media
> device has changed. In this case, it is better to detach the
> filter, otherwise it may lead to incorrect operation of the module.
> 
> We can add prechange/postchange callback functions so that the
> filter can track rescan process. But at the moment, this is not
> necessary for the blksnap module.

So you mean that blkfilter is only used for the case that partition
is mounted? (Or you mean that partition is opened)

Then, I think you mean that filter should only be used for the partition
that is opended? Otherwise, filter can be gone at any time since
partition rescan can be gone.

//user
1. attach filter
		// other context rescan partition
2. mount fs
// user will found filter is gone.

Thanks,
Kuai

> 
> Therefore, I will refrain from making changes for now.
> 
>>
>> possible solutions I have in mind:
>>
>> 1. Store blkfilter for each partition from bdev_disk_changed() before
>> delete_partition(), and add blkfilter back after add_partition().
>>
>> 2. Store blkfilter from gendisk as a xarray, and protect it by
>> 'open_mutex' like 'part_tbl', block_device can keep the pointer to
>> reference blkfilter so that performance from fast path is ok, and the
>> lifetime of blkfiter can be managed separately.
>>
>>> There was an idea to do filtering at the disk level,
>>> but I abandoned it.
>>> .
>>>
>> I think it's better to do filtering at the partition level as well.
>>
>> Thanks,
>> Kuai
>>
> .
>