Message-ID: <4FF0AEA9.3090900@redhat.com>
Date:	Sun, 01 Jul 2012 22:10:17 +0200
From:	Zdenek Kabelac <zkabelac@...hat.com>
To:	Hugh Dickins <hughd@...gle.com>
CC:	Mike Snitzer <snitzer@...hat.com>,
	Mikulas Patocka <mpatocka@...hat.com>,
	Joe Thornber <ejt@...hat.com>,
	LVM general discussion and development 
	<linux-lvm@...hat.com>, amwang@...hat.com,
	Alasdair G Kergon <agk@...hat.com>,
	linux-kernel@...r.kernel.org, Lukas Czerner <lczerner@...hat.com>
Subject: Re: Regression with FALLOC_FL_PUNCH_HOLE in 3.5-rc kernel

Dne 1.7.2012 20:45, Hugh Dickins napsal(a):
> On Sun, 1 Jul 2012, Zdenek Kabelac wrote:
>> Dne 1.7.2012 01:10, Hugh Dickins napsal(a):
>>> On Sat, 30 Jun 2012, Zdenek Kabelac wrote:
>>>> Dne 30.6.2012 21:55, Hugh Dickins napsal(a):
>>>>> On Sat, 30 Jun 2012, Zdenek Kabelac wrote:
>>>>>>
>>>>>> When I've used 3.5-rc kernels - I've noticed kernel deadlocks.
>>>>>> Ooops log included. After some experimenting - reliable way to hit
>>>>>> this
>>>>>> oops
>>>>>> is to run lvm test suite for 10 minutes. Since 3.5 merge window does
>>>>>> not
>>>>>> included anything related to this oops I've went for bisect.
>>>>>
>>>>> Thanks a lot for reporting, and going to such effort to find
>>>>> a reproducible testcase that you could bisect on.
>>>>>
>>>>>>
>>>>>> Game result is commit: 3f31d07571eeea18a7d34db9af21d2285b807a17
>>>>>>
>>>>>> mm/fs: route MADV_REMOVE to FALLOC_FL_PUNCH_HOLE
>>>>>
>>>>> But this leaves me very puzzled.
>>>>>
>>>>> Is the "lvm test suite" what I find at
>>>>> git.fedorahosted.org/git/lvm2.git
>>>>> under tests/ ?
>>>>
>>>> Yes - that's it -
>>>>
>>>>    make
>>>> as root:
>>>>    cd test
>>>>    make check_local
>>>>
>>>> (inside test subdirectory should be enough, if not - just report any
>>>> problem)
>>>>
>>>>> If you have something else running at the same time, which happens to
>>>>> use
>>>>> madvise(,,MADV_REMOVE) on a filesystem which the commit above now
>>>>> enables
>>>>> it on (I guess ext4 from the =y in your config), then I suppose we
>>>>> should
>>>>> start searching for improper memory freeing or scribbling in its
>>>>> holepunch
>>>>> support: something that might be corrupting the dm_region in your oops.
>>>>
>>>> What the test is doing - it creates file in  LVM_TEST_DIR (default is
>>>> /tmp)
>
> I ran "LVM_TEST_DIR=/tmp make check_local":
> without that it appeared to be using a subdirectory made under test/.
>
> And being a year or two out of date in my userspace, and unfamiliar with
> the syntax and whereabouts of lvm.conf, it was easiest for me to hack
> lib/config/defaults.h to #define DEFAULT_ISSUE_DISCARDS 1
> after I spotted a warning message about issue_discards.
>
>>>> and using loop device to simulate device (small size - it should fit
>>>> bellow
>>>> 200MB)
>>>>
>>>> Within this file second layer through virtual DM devices is created and
>>>> simulates various numbers of PV devices to play with.
>>>
>>> This sounds much easier to set up than I was expecting:
>>> thanks for the info, I'll try it later on today.
>
> Sorry, I never reached it yesterday, but arrived there this morning.
>
>>>
>>>>
>>>> So since everything now support TRIM - such operations should be passed
>>>> down to the backend file - which probably triggers the path.
>>>
>>> What filesystem do you have for /tmp?
>
>  From your later remarks, I inferred tmpfs.
>
>>>
>>> If tmpfs, then it will make much more sense if we assume your bisection
>>> endpoint was off by one.  Your bisection log was not quite complete;
>>> and even if it did appear to converge on the commit you cite, you might
>>> have got (un)lucky when testing the commit before it, and concluded
>>> "good" when more attempts would have said "bad".
>>>
>>> The commit before, 83e4fa9c16e4af7122e31be3eca5d57881d236fe
>>> "tmpfs: support fallocate FALLOC_FL_PUNCH_HOLE", would be a
>>> much more likely first bad commit if your /tmp is on tmpfs:
>>> that does indeed wire up loop to pass TRIM down to tmpfs by
>>> fallocate - that indeed played a part in my own testing.
>>>
>>> Whereas if your /tmp is on ext4, loop has been passing TRIM down
>>> with fallocate since v3.0.  And whichever, madvise(,,MADV_REMOVE)
>>> should be completely irrelevant.
>>
>> While I've been aware of the fact that tmpfs was enhanced with trim support -
>> I've not tried to run on real ext4 filesystem since for my tests I'm using
>> tmpfs for quite some time to safe rewrites of SSD :)
>>
>> So now I've checked with real ext4 - and the bug is there as well
>> so I've went back - it crashes on 3.4, 3.3 and 3.2 as well.
>>
>> 3.1 is the first kernel which does survive (checked 5 repeated runs)
>
> Very useful research, thank you.
>
>>
>> And you are correct, the first commit which causes crash really is
>> 83e4fa9c16e4af  when I use  tmpfs as backend storage - the problem why I've
>> missed to properly identify this commit in my bisect is that crash usually
>> happens on the second pass of the lvm test suite 'make check_local' execution
>> - and I've been running test just once. To be sure I've run 5 run passes on
>> 3.4.0-08568-gec9516f - which is OK, but 3.4.0-08569-g83e4fa9 is crashing
>> usually on second run, with commit 3f31d07571e  the crash always happens in
>> the first pass.
>>
>> I've also checked some rawhide kernel vmlinuz-3.5.0-0.rc2.git0.1.fc18.x86_64
>> and it's crashing as well - so it's probably not uniqueness of my config.
>>
>> So is there any primary suspect in 3.2 which is worth to check - or I need
>> another day to play another bisect game ?
>
> No need for a further bisect: 3.2 is when the disard/trim/fallocate
> support went into drivers/block/loop.c, so these tests would be unable
> to show if DM was right or wrong before then.

Well I've played meanwhile the game with minimized kernel config
and the outcome is:

last working kernel is: 3.1.0-rc1-00008-g548ef6c
first broken: 3.1.0-rc1-00009-gdfaa2ef

dfaa2ef68e80c378e610e3c8c536f1c239e8d3ef
loop: add discard support for loop devices

> I don't have Fedora Rawhide on, but after hacking ISSUE_DISCARDS
> I did quickly crash around where you expected; though in my case
> it was in dm_rh_dec() called from mirror_end_io().

Change of Issue Discards option in lvm.conf is not needed.
I'm able to get these oopses with this setting turned off.

> But I've not taken it any further than that.  You've shown that it's
> as much a problem with ext4 as with tmpfs, and has been a problem
> ever since these tests' use of discard reached DM.
>
> I think it's fair to assume that there's something wrong with DM's
> handling of REQ_DISCARD.  (Perhaps it was all correct when written,
> but I think there has been a series of modifications to REQ_DISCARD
> handling in the block layer, it's been rather troublesome.)
>

So does anyone has some idea what should be checked next ?

Zdenek
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