linux-kernel - Re: [PATCH 0/3] Ceph fscache: Fix kernel panic due to a race

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Date:	Fri, 27 Dec 2013 09:41:39 +0800
From:	Yunchuan Wen <yunchuanwen@...ntukylin.com>
To:	Milosz Tanski <milosz@...in.com>
Cc:	Li Wang <liwang@...ntukylin.com>,
	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
Subject: Re: [PATCH 0/3] Ceph fscache: Fix kernel panic due to a race

Hi, Milosz
I am not very sure about how to enable caching in write path.

I just compile the latest kernel with "Enable Ceph client caching
support", start cachefilesd, and mount cephfs with -o fsc.
Then, the kernel will panic easily when the script runs to "dd
if=cephfs/foo of=/dev/null bs=8 count=1024"


2013/12/27 Milosz Tanski <milosz@...in.com>:
> 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
> 10 East 53rd Street, 37th floor
> New York, NY 10022
>
> p: 646-253-9055
> e: milosz@...in.com
> --
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