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

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Message-ID: <20170615083427.GE1764@quack2.suse.cz>
Date:   Thu, 15 Jun 2017 10:34:27 +0200
From:   Jan Kara <jack@...e.cz>
To:     Pali Rohár <pali.rohar@...il.com>
Cc:     Jan Kara <jack@...e.cz>, 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 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.

								Honza

-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR