Re: [RFC PATCH v4 29/34] ext4: fall back to buffer_head path for defrag

[Zhang Yi]

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@xxxxxxxxxx>
>>
>> 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.