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Date:   Tue, 18 Jul 2023 13:25:16 +0200
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Subject: Re: [PATCH v5 02/11] block: Block Device Filtering Mechanism
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To:     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>
References: <20230612135228.10702-1-sergei.shtepa@veeam.com>
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 <f935840e-12a7-c37b-183c-27e2d83990ea@huaweicloud.com>
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From:   Sergei Shtepa <sergei.shtepa@veeam.com>
In-Reply-To: <d929eaa7-61d6-c4c4-aabc-0124c3693e10@huaweicloud.com>
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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. 

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
>