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Subject: Re: [RFC PATCH v4 29/34] ext4: fall back to buffer_head path for
 defrag
To: Dave Chinner <david@fromorbit.com>
Cc: linux-ext4@vger.kernel.org, linux-fsdevel@vger.kernel.org,
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 hch@infradead.org, djwong@kernel.org, willy@infradead.org,
 zokeefe@google.com, yi.zhang@huawei.com, chengzhihao1@huawei.com,
 yukuai3@huawei.com, wangkefeng.wang@huawei.com
References: <20240410142948.2817554-1-yi.zhang@huaweicloud.com>
 <20240410150313.2820364-1-yi.zhang@huaweicloud.com>
 <ZjIMQTAtxZ0NhCD2@dread.disaster.area>
From: Zhang Yi <yi.zhang@huaweicloud.com>
Message-ID: <5dbb3021-b92e-2e53-7eee-5a6595a5ad03@huaweicloud.com>
Date: Mon, 6 May 2024 21:05:22 +0800
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On 2024/5/1 17:32, Dave Chinner wrote:
> On Wed, Apr 10, 2024 at 11:03:08PM +0800, Zhang Yi wrote:
>> From: Zhang Yi <yi.zhang@huawei.com>
>>
>> Online defrag doesn't support iomap path yet, we have to fall back to
>> buffer_head path for the inode which has been using iomap. Changing
>> active inode is dangerous, before we start, we must hold the inode lock
>> and the mapping->invalidate_lock, and writeback all dirty folios and
>> drop the inode's pagecache.
> 
> Even then, I don't think this is obviously safe. We went through
> this with DAX and we couldn't make it work safely.
> 
> Just return EOPNOTSUPP to the online defrag ioctl if iomap is in use
> - that avoids all the excitement involved in doing dangerous things
> like swapping aops structures on actively referenced inodes...
> 

Okay, this is just a temporary solution to support defrag. I've been
looking at how to support defrag for iomap recently, I hope it could
be supported in the near future, so let's drop this dangerous
operation.

Thanks,
Yi.