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Re: [linux-lvm] 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And being a year or two out of date in my userspace, and unfamiliar with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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