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

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Message-Id: <201706142136.45895@pali>
Date:   Wed, 14 Jun 2017 21:36:45 +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 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...

> Look at the comment before "blk_queue_logical_block_size":
> 
> /**
>  * blk_queue_logical_block_size - set logical block size for the
> queue * @q:  the request queue for the device
>  * @size:  the logical block size, in bytes
>  *
>  * Description:
>  *   This should be set to the lowest possible block size that the
>  *   storage device can address.  The default of 512 covers most
>  *   hardware.
>  **/
> 
> > Anyway, macro UDF_DEFAULT_BLOCKSIZE is not used in udf source code
> > anymore, so it can be deleted. I think its name was incorrect as
> > default block size for UDF should be logical block size of disk,
> > not hardcoded value 2048 which is logical block size for optical
> > media.
> 
> Thanks. Removed.
> 
> 								Honza


-- 
Pali Rohár
pali.rohar@...il.com

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