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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>
References: <20230612135228.10702-1-sergei.shtepa@veeam.com>
 <20230612135228.10702-3-sergei.shtepa@veeam.com>
 <f935840e-12a7-c37b-183c-27e2d83990ea@huaweicloud.com>
 <90f79cf3-86a2-02c0-1887-d3490f9848bb@veeam.com>
From:   Yu Kuai <yukuai1@huaweicloud.com>
Message-ID: <d929eaa7-61d6-c4c4-aabc-0124c3693e10@huaweicloud.com>
Date:   Tue, 18 Jul 2023 09:37:33 +0800
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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.

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