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Subject: Re: How best to get the size of a blockdev from a file?
To:     Kanchan Joshi <joshi.k@samsung.com>,
        David Howells <dhowells@redhat.com>
CC:     <hch@lst.de>, <linux-block@vger.kernel.org>
References: <CGME20230418100108epcas5p2d0f2a7a274e78731373986b3d4fced9b@epcas5p2.samsung.com>
 <1609851.1681812012@warthog.procyon.org.uk> <20230418122833.GA14457@green245>
From:   Jason Yan <yanaijie@huawei.com>
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Date:   Tue, 18 Apr 2023 22:05:12 +0800
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On 2023/4/18 20:28, Kanchan Joshi wrote:
> On Tue, Apr 18, 2023 at 11:00:12AM +0100, David Howells wrote:
>> Hi Christoph,
>>
>> It seems that my use of i_size_read(file_inode(in)) in 
>> filemap_splice_read()
>> to get the size of the file to be spliced from doesn't work in the 
>> case of
>> blockdevs and it always returns 0.
>>
>> What would be the best way to get the blockdev size?  Look at
>> file->f_mapping->i_size maybe?
> 
> bdev_nr_bytes(I_BDEV(file->f_mapping->host))
> should work I suppose.
> 

This needs the caller to check if the file is a blockdev. Can we fill 
the upper inode size which belongs to devtmpfs so that the generic code 
do not need to aware the low level inode type?