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Date:	Fri, 3 Jan 2014 09:43:21 -0500
From:	Milosz Tanski <milosz@...in.com>
To:	Li Wang <liwang@...ntukylin.com>
Cc:	ceph-devel <ceph-devel@...r.kernel.org>,
	Sage Weil <sage@...tank.com>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	linux-kernel@...r.kernel.org,
	Yunchuan Wen <yunchuanwen@...ntukylin.com>
Subject: Re: [PATCH 0/3] Ceph fscache: Fix kernel panic due to a race

I'm going to look the patches and the issue in full detail. In the
meantime do you guys have the oops back trace. I have some other
fscache patches that haven't made it upstream yet that might have been
masking this issue for me.

On Fri, Dec 27, 2013 at 10:51 PM, Li Wang <liwang@...ntukylin.com> wrote:
> Hi Milosz,
>   As far as I know, logically, currently fscache does not play
> as write cache for Ceph, except that there is a
> call to ceph_readpage_to_fscache() in ceph_writepage(), but that
> is nothing related to our test case. According to our observation,
> our test case never goes through ceph_writepage(), instead, it goes
> through ceph_writepages(). So in other words, I donot think this
> is related to caching in write path.
>   May I try to explain the panic in more detail,
>
> (1) dd if=/dev/zero of=cephfs/foo bs=8 count=512
> (2) echo 3 > /proc/sys/vm/drop_caches
> (3) dd if=cephfs/foo of=/dev/null bs=8 count=1024
>
> For statement (1), it is frequently appending a file, so
> ceph_aio_write() frequently updates the inode->i_size,
> however, these updates did not immediately reflected to
> object->store_limit_l. For statement (3), when we
> start reading the second page at [4096, 8192), ceph find that the page
> does not be cached in fscache, then it decides to write this page into
> fscache, during this process in cachefiles_write_page(), it found that
> object->store_limit_l < 4096 (page->index << 12), it causes panic. Does
> it make sense?
>
> Cheers,
> Li Wang
>
>
> On 2013/12/27 6:51, Milosz Tanski wrote:
>>
>> Li,
>>
>> I looked at the patchset am I correct that this only happens when we
>> enable caching in the write path?
>>
>> - Milosz
>>
>> On Thu, Dec 26, 2013 at 9:29 AM, Li Wang <liwang@...ntukylin.com> wrote:
>>>
>>> From: Yunchuan Wen <yunchuanwen@...ntukylin.com>
>>>
>>> The following scripts could easily panic the kernel,
>>>
>>> #!/bin/bash
>>> mount -t ceph -o fsc MONADDR:/ cephfs
>>> rm -rf cephfs/foo
>>> dd if=/dev/zero of=cephfs/foo bs=8 count=512
>>> echo 3 > /proc/sys/vm/drop_caches
>>> dd if=cephfs/foo of=/dev/null bs=8 count=1024
>>>
>>> This is due to when writing a page into fscache, the code will
>>> assert that the write position does not exceed the
>>> object->store_limit_l, which is supposed to be equal to inode->i_size.
>>> However, for current implementation, after file writing, the
>>> object->store_limit_l is not synchronized with new
>>> inode->i_size immediately, which introduces a race that if writing
>>> a new page into fscache, will reach the ASSERT that write position
>>> has exceeded the object->store_limit_l, and cause kernel panic.
>>> This patch fixes it.
>>>
>>> Yunchuan Wen (3):
>>>    Ceph fscache: Add an interface to synchronize object store limit
>>>    Ceph fscache: Update object store limit after writing
>>>    Ceph fscache: Wait for completion of object initialization
>>>
>>>   fs/ceph/cache.c |    1 +
>>>   fs/ceph/cache.h |   10 ++++++++++
>>>   fs/ceph/file.c  |    3 +++
>>>   3 files changed, 14 insertions(+)
>>>
>>> --
>>> 1.7.9.5
>>>
>>
>>
>>
>



-- 
Milosz Tanski
CTO
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p: 646-253-9055
e: milosz@...in.com
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