linux-kernel - Re: udf: allow implicit blocksize specification during mount

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Date:   Thu, 15 Jun 2017 15:49:39 +0200
From:   Pali Rohár <pali.rohar@...il.com>
To:     Jan Kara <jack@...e.cz>
Cc:     Fabian Frederick <fabf@...net.be>, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org
Subject: Re: udf: allow implicit blocksize specification during mount

On Thursday 15 June 2017 10:34:27 Jan Kara wrote:
> On Wed 14-06-17 21:36:45, Pali Rohár wrote:
> > On Tuesday 13 June 2017 14:59:55 Jan Kara wrote:
> > > Hi,
> > > 
> > > On Mon 12-06-17 22:40:14, Pali Rohár wrote:
> > > > Hi! I found that following UDF patch was included into linus tree:
> > > > https://patchwork.kernel.org/patch/9524557/
> > > > 
> > > > It is really a good improvement to recognize UDF file system which
> > > > have block size different from disk sector size and also different
> > > > from 2048.
> > > > 
> > > > But should not detection on 4K native disks (4096/4096) try to also
> > > > use block size of 512 bytes? Because current loop is from logical
> > > > sector size to 4096.
> > > 
> > > By definition, bdev_logical_block_size() is the smallest block size a
> > > device can support. So if it is larger than 512, the device driver
> > > had explicitely declared that it cannot handle smaller blocks...
> > 
> > Ok, but it is a really problem when trying to read data from filesystem 
> > which has smaller blocks as the smallest block size of a device?
> > 
> > In the worst case filesystem driver needs to read 512 bytes, but device 
> > can send only block of 4096 bytes (as it does not support smaller 
> > block). Driver receives 4096 bytes, then it process just first 512 bytes 
> > and do not care about remaining data...
> 
> Well, as much as I agree this is possible in principle, the block layer,
> block device page cache etc. don't handle this so it would be a non-trivial
> effort to support this.

Ok, so it is not a problem in UDF driver nor hardware, just it is
limitation of kernel block layer which do not support it.

-- 
Pali Rohár
pali.rohar@...il.com