linux-ext4 - Re: [PATCH 02/12] container quota: lock/unlock mount namespace when performing quotactl.

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Date:	Thu, 31 May 2012 20:40:21 +0800
From:	Jeff Liu <jeff.liu@...cle.com>
To:	Glauber Costa <glommer@...allels.com>
CC:	containers@...ts.linux-foundation.org, cgroups@...r.kernel.org,
	jack@...e.cz, daniel.lezcano@...e.fr, tytso@....edu, bpm@....com,
	chris.mason@...cle.com, hch@...radead.org,
	christopher.jones@...cle.com, david@...morbit.com,
	tinguely@....com, tm@....ma, linux-ext4@...r.kernel.org,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 02/12] container quota: lock/unlock mount namespace when
 performing quotactl.

On 05/31/2012 05:04 PM, Glauber Costa wrote:

> On 05/30/2012 06:58 PM, jeff.liu@...cle.com wrote:
>> Introduce two functions to lock/unlock a particular mount namespace
>> when container quota operations is performed.
>>
>> Signed-off-by: Jie Liu<jeff.liu@...cle.com>
>> ---
>>   fs/namespace.c                |   17 +++++++++++++++++
>>   include/linux/mnt_namespace.h |   13 +++++++++++++
>>   2 files changed, 30 insertions(+), 0 deletions(-)
>>
>> diff --git a/fs/namespace.c b/fs/namespace.c
>> index e608199..8823ba8 100644
>> --- a/fs/namespace.c
>> +++ b/fs/namespace.c
>> @@ -2633,3 +2633,20 @@ bool our_mnt(struct vfsmount *mnt)
>>   {
>>       return check_mnt(real_mount(mnt));
>>   }
>> +
>> +/*
>> + * Lock the mount namespace when performing a particular container
>> + * disk quotactl operation, especially to protect removing it.
>> + */
>> +void lock_mnt_ns(struct mnt_namespace *ns)
>> +{
>> +    down_write(&namespace_sem);
>> +}
>> +
>> +/*
>> + * Unlock the mount namespace after quotactl on that was done.
>> + */
>> +void unlock_mnt_ns(struct mnt_namespace *ns)
>> +{
>> +    up_write(&namespace_sem);
>> +}
>> diff --git a/include/linux/mnt_namespace.h
>> b/include/linux/mnt_namespace.h
>> index 5a8e390..ff72ed5 100644
>> --- a/include/linux/mnt_namespace.h
>> +++ b/include/linux/mnt_namespace.h
>> @@ -9,6 +9,19 @@ extern struct mnt_namespace *copy_mnt_ns(unsigned
>> long, struct mnt_namespace *,
>>           struct fs_struct *);
>>   extern void put_mnt_ns(struct mnt_namespace *ns);
>>
>> +#ifdef CONFIG_NS_QUOTA
>> +extern void lock_mnt_ns(struct mnt_namespace *);
>> +extern void unlock_mnt_ns(struct mnt_namespace *);
>> +#else
>> +void lock_mnt_ns(struct mnt_namespace *)
>> +{
>> +}
>> +
>> +void unlock_mnt_ns(struct mnt_namespace *)
>> +{
>> +}
>> +#endif
>> +
> 
> Seems weird. There is nothing ns-quota specific in locking a namespace.
> Also, just grabbing a lock from an outer subsystem is quite ugly. If you
> are grabbing a namespace lock, this suggests you are tweaking namespace
> data. So you should have that file provide you with a function that does
> the full operation.

Maybe this can be safely removed, it is presented like a place holder,
which is suggests to protect changing data inner quota info of a mount
namespace.
However, both quota info initialization and destroy business are all
projected at mount init and put mount namespace stage, so that the
lock/unlock are make less sense here.

Thanks,
-Jeff

> 
> Of course there are exceptions to that, but it rings a bell.


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