linux-kernel - Re: PROBLEM: udf mount takes forever to fail + proposed solution

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Message-ID: <CAG3eYYRi=c7-vfkW9zayu4LYv_ty3kXJ8SB4T6pgAyDUXzmcYQ@mail.gmail.com>
Date:	Fri, 11 Oct 2013 23:46:37 +0200
From:	Péter András Felvégi <petschy@...il.com>
To:	Jan Kara <jack@...e.cz>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: PROBLEM: udf mount takes forever to fail + proposed solution

OK, I'll prepare a new patch with explicit ID checks (BEA01, BOOT2,
CDROM, CD001, CDW02, NSR02, NSR03, TEA01, that's all I think) and the
needed changelog in a few days. Do you think that a hard upper limit
on the sector offset is desirable? The ISO9660 driver looks for the
first 100 sectors, but I didn't find anything in the specs suggesting
a max length for the udf volume recognition area.

Regards, Peter


On 11 October 2013 17:18, Jan Kara <jack@...e.cz> wrote:
>   Hello,
>
> On Thu 10-10-13 23:23:11, Péter András Felvégi wrote:
>> recently I made the mistake trying to mount an unformatted ssd
>> partition. The mount command 'hang', was unable to kill it. Top showed
>> the process is in the uninterruptible D state. However, iotop showed
>> slight activity, about 4M/s read from the disk that noone else used.
>> This was 100% reproducible. sync froze, too, if was given out after
>> the mount cmd. When trying to shut down the machine, it didn't stop,
>> just waited for something to happen.
>>
>> I narrowed down the problem to the UDF filesystem driver. In
>> fs/udf/super.c, udf_check_vsd() reads the sectors in a for loop, with
>> the following exit conditions:
>> - NSR02 or NSR03 descriptor is found
>> - the read fails
>> - vsd->stdIdent[0] == 0
>>
>> Browsed through the UDF 2.6 spec, ECMA 167 and 119. As I understand,
>> the descriptors should start at offset 32768, forming a contiguous
>> sequence. In ECMA 167 it is stated that the sequence is terminated by
>> an invalid descriptor: unrecorded, or blank (all zeros). However, this
>> presupposes that the filesystem is UDF.
>>
>> Since the ssd partition was not formatted, it contained only 0xff
>> bytes, thus none of the exit conditions were met, and the function
>> read through the whole, in two passes. The runtime was pathetic, it
>> took the mount 350 minutes to fail. I have no clue why this was so
>> slow, reading through the partition with dd gives 482 secs for the
>> 220G, ~450M/s. Setting the blocksize to 512 or 2048 didn't make much
>> of a difference.
>>
>> I peppered the code with some messages to see what happens:
>> # time mount -t udf /dev/sdb3 /media/floppy
>> UDF-fs: check_vsd: sectorsize=2048
>> UDF-fs: check_vsd: sector offs=32768, s_blocksize=512, s_blocksize_bits=9
>> UDF-fs: read 107989660 sectors of total size 55290705920 bytes
>> UDF-fs: warning (device sdb3): udf_load_vrs: No VRS found
>> UDF-fs: Rescanning with blocksize 2048
>> UDF-fs: check_vsd: sectorsize=2048
>> UDF-fs: check_vsd: sector offs=32768, s_blocksize=2048, s_blocksize_bits=11
>> UDF-fs: read 107989660 sectors of total size 221162823680 bytes
>> UDF-fs: warning (device sdb3): udf_load_vrs: No VRS found
>> UDF-fs: warning (device sdb3): udf_fill_super: No partition found (1)
>> mount: wrong fs type, bad option, bad superblock on /dev/sdb3,
>>        missing codepage or helper program, or other error
>>        In some cases useful info is found in syslog - try
>>        dmesg | tail  or so
>> real    352m4.740s
>> user    0m0.000s
>> sys     27m23.560s
>>
>> Tried to mount other partitions, too, formatted to ext3, ext4, btrfs
>> and ntfs. The mount failed with those sooner, accidentally just
>> because there were some blocks near to the beginning with a zero byte
>> just at the right place.
>>
>> Then I prepared an 'all 0xff' 4G image, and burnt it to a DVD. The
>> mount failed, but took only 25 minutes. 'Only', compared to the case
>> with the ssd. This truely doesn't reflect the throughput of the
>> devices, hopefully someone with more experience will have a clue.
>   Thanks for the report and detailed analysis. Frankly, instead of your
> function checking the identifier, I'd rather follow the standard in detail
> and add handling (meaning ignore) of the remaining specified descriptors
> (CDW02, BOOT2) and bail out if anything else is found. If someone complains
> because some broken medium stops mounting, we can try something more
> elaborate but for now I'd go with the simple solution.
>
> Also please read Documentation/SubmittingPatches - your patch was missing a
> changelog entry (you can basically take your somewhat shortened email for
> that) and a Signed-off-by line. Thanks!
>
>                                                                 Honza
> --
> Jan Kara <jack@...e.cz>
> SUSE Labs, CR
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