Re: Re-2: Strange problems with lseek in qemu-img map

Wen Congyang
2015-06-06T03:53:50Z

At 2015/6/5 23:14, Lukáš Czerner Wrote:
> On Fri, 5 Jun 2015, Stefan Hajnoczi wrote:
>
>> Date: Fri, 5 Jun 2015 15:05:15 +0100
>> From: Stefan Hajnoczi <stefanha@gmail.com>
>> To: David Weber <wb@munzinger.de>
>> Cc: qemu-devel@nongnu.org, linux-ext4@vger.kernel.org,
>> Lukas Czerner <lczerner@redhat.com>, wency@cn.fujitsu.com
>> Subject: Re: [Qemu-devel] Re-2: Strange problems with lseek in qemu-img map
>>
>> On Wed, Jun 03, 2015 at 03:09:40PM +0000, David Weber wrote:
>>>>> I then startet a fedora 22 live system and I saw the same problem. It
>>>>> happens
>>>>> on both the ramdisk and a ext4 filesystem.
>>>>
>>>> "it" == qemu-img map hangs or takes a very long time?
>>> I never waited for it to complete but I guess it just takes very long.
>>>
>>>>
>>>> Can you post a shell script that reproduces this with a ramdisk? That
>>>> seems like the easiest way to get people debugging it.
>>>
>>> You can use the following commands.
>>>
>>> mkfs.ext4 /dev/ram0
>>> mkdir -p /mnt/tmp
>>> mount /dev/ram0 /mnt/tmp
>>> cd /mnt/tmp
>>> qemu-img create test 500G
>>> time qemu-img map test
>>>
>>> This takes foreover on all my systems.
>>
>> I experience the same thing on Linux 4.1.0-rc5 with ext4 (4 KB file
>> system block size, 512B device sector size).
>>
>> The lseek() calls take *seconds* to complete on a completely empty
>> (sparse) 500G file.
>>
>> Here is the perf-top(1) output:
>>
>> 34.08% [kernel] [k] _raw_read_lock
>> 21.11% [kernel] [k] ext4_es_lookup_extent
>> 20.67% [kernel] [k] ext4_es_find_delayed_extent_range
>> 8.68% [kernel] [k] ext4_map_blocks
>> 4.99% [kernel] [k] ext4_llseek
>> 1.90% [kernel] [k] rb_next
>> 0.14% [kernel] [k] __fget
>>
>> Yikes!
>>
>> The syscall causing this is just:
>>
>> lseek(7, 0, SEEK_DATA)
>>
>> Lukas: I've CCed you because you have helped with ext4 issues in the
>> past. Not sure if you have time or if this is your area.
>>
>> Stefan
>
> Hi,
>
> this is definitely an issue with extent status tree and I have not seen
> it before. I'll look at it first thing next week, but meanwhile I'll
> add Zheng Liu who is the author of the extent status tree so he
> might be able to help you.

It is an known bug, and Dmitry Monakhov tried to fix it:
http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=14516bb7bb6ffbd49f35389f9ece3b2045ba5815

But this patch introduced another problem, and have been reverted.

Thanks
Wen Congyang

>
> Regards,
> -Lukas
>
>