[PATCH] btrfs: Remove unneeded cast to s64 for qgroup rfer state

[PATCH] btrfs: Remove unneeded cast to s64 for qgroup rfer state

[Alexandru Moise]

This patch reverts commit: b4fcd6be6bbd702ae1a6545c9b413681850a9814
Wang Shilong added those casts as a workaround for a bug reproduced
using the following steps:

Steps to reproduce:
>    	mkfs.btrfs <disk>
>    	mount <disk> <mnt>
>    	dd if=/dev/zero of=/<mnt>/data bs=1M count=10
>    	sync
>    	btrfs quota enable <mnt>
>    	btrfs qgroup create 0/5 <mnt>
>    	btrfs qgroup limit 5M 0/5 <mnt>
>    	rm -f /<mnt>/data
>    	sync
>    	btrfs qgroup show <mnt>
>    	dd if=/dev/zero of=data bs=1M count=1
>
>    >From the perspective of users, qgroup's referenced or exclusive
>    is negative,but user can not continue to write data! a workaround
>    way is to cast u64 to s64 when doing qgroup reservation

I am unable to reproduce this problem without his modification.
I could be wrong in reverting this, so I'm gonna CC Wang as well so
he is aware of this patch.

Signed-off-by: Alexandru Moise <00moses.alexander00@gmail.com>
---
 fs/btrfs/qgroup.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/fs/btrfs/qgroup.c b/fs/btrfs/qgroup.c
index 8a82029..9c75e86 100644
--- a/fs/btrfs/qgroup.c
+++ b/fs/btrfs/qgroup.c
@@ -2077,14 +2077,14 @@ int btrfs_qgroup_reserve(struct btrfs_root *root, u64 num_bytes)
 		qg = u64_to_ptr(unode->aux);
 
 		if ((qg->lim_flags & BTRFS_QGROUP_LIMIT_MAX_RFER) &&
-		    qg->reserved + (s64)qg->rfer + num_bytes >
+		    qg->reserved + qg->rfer + num_bytes >
 		    qg->max_rfer) {
 			ret = -EDQUOT;
 			goto out;
 		}
 
 		if ((qg->lim_flags & BTRFS_QGROUP_LIMIT_MAX_EXCL) &&
-		    qg->reserved + (s64)qg->excl + num_bytes >
+		    qg->reserved + qg->excl + num_bytes >
 		    qg->max_excl) {
 			ret = -EDQUOT;
 			goto out;
-- 
2.5.0

Re: [PATCH] btrfs: Remove unneeded cast to s64 for qgroup rfer state

[Qu Wenruo]

Alexandru Moise wrote on 2015/08/29 11:45 +0000:
> This patch reverts commit: b4fcd6be6bbd702ae1a6545c9b413681850a9814
> Wang Shilong added those casts as a workaround for a bug reproduced
> using the following steps:
>
> Steps to reproduce:
>>     	mkfs.btrfs <disk>
>>     	mount <disk> <mnt>
>>     	dd if=/dev/zero of=/<mnt>/data bs=1M count=10
>>     	sync
>>     	btrfs quota enable <mnt>
>>     	btrfs qgroup create 0/5 <mnt>
>>     	btrfs qgroup limit 5M 0/5 <mnt>
>>     	rm -f /<mnt>/data
>>     	sync
>>     	btrfs qgroup show <mnt>
>>     	dd if=/dev/zero of=data bs=1M count=1
>>
>>     >From the perspective of users, qgroup's referenced or exclusive
>>     is negative,but user can not continue to write data! a workaround
>>     way is to cast u64 to s64 when doing qgroup reservation
>
> I am unable to reproduce this problem without his modification.
> I could be wrong in reverting this, so I'm gonna CC Wang as well so
> he is aware of this patch.

The cast is a workaround for a quite old qgroup bug, which will cause 
excl/rfer overflow to minus.

The remove of cast rfer/exel now is OK, as qgroup keeps maturing, 
especially after 4.2-rc1 rfer/exel will keep sane under most case
(exception will be qgroup reassign and subvolume deletion, but will not 
case minus value even under than case).

But I'm not a fan to remove it now.
As qgroup still has a known huge bug for the qg->reserved part, we are 
aware of it and working on it actively.

So for such cleanup, I'd prefer to do it when we rework the accounting 
part of qgroup.

Thanks,
Qu

>
> Signed-off-by: Alexandru Moise <00moses.alexander00@gmail.com>
> ---
>   fs/btrfs/qgroup.c | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/fs/btrfs/qgroup.c b/fs/btrfs/qgroup.c
> index 8a82029..9c75e86 100644
> --- a/fs/btrfs/qgroup.c
> +++ b/fs/btrfs/qgroup.c
> @@ -2077,14 +2077,14 @@ int btrfs_qgroup_reserve(struct btrfs_root *root, u64 num_bytes)
>   		qg = u64_to_ptr(unode->aux);
>
>   		if ((qg->lim_flags & BTRFS_QGROUP_LIMIT_MAX_RFER) &&
> -		    qg->reserved + (s64)qg->rfer + num_bytes >
> +		    qg->reserved + qg->rfer + num_bytes >
>   		    qg->max_rfer) {
>   			ret = -EDQUOT;
>   			goto out;
>   		}
>
>   		if ((qg->lim_flags & BTRFS_QGROUP_LIMIT_MAX_EXCL) &&
> -		    qg->reserved + (s64)qg->excl + num_bytes >
> +		    qg->reserved + qg->excl + num_bytes >
>   		    qg->max_excl) {
>   			ret = -EDQUOT;
>   			goto out;
>

Re: [PATCH] btrfs: Remove unneeded cast to s64 for qgroup rfer state

[Alexandru Moise]

On Mon, Aug 31, 2015 at 09:44:49AM +0800, Qu Wenruo wrote:
> >>    >From the perspective of users, qgroup's referenced or exclusive
> >>    is negative,but user can not continue to write data! a workaround
> >>    way is to cast u64 to s64 when doing qgroup reservation
> >
> >I am unable to reproduce this problem without his modification.
> >I could be wrong in reverting this, so I'm gonna CC Wang as well so
> >he is aware of this patch.
> 
> The cast is a workaround for a quite old qgroup bug, which will
> cause excl/rfer overflow to minus.
> 
> The remove of cast rfer/exel now is OK, as qgroup keeps maturing,
> especially after 4.2-rc1 rfer/exel will keep sane under most case
> (exception will be qgroup reassign and subvolume deletion, but will
> not case minus value even under than case).

rfer/exel and reserved are all of type unsigned int, how exactly would
they overflow to minus?

> 
> But I'm not a fan to remove it now.
> As qgroup still has a known huge bug for the qg->reserved part, we
> are aware of it and working on it actively.

Can you tell me more about this known huge bug and how you can
reproduce it using the present implementation?

Re: [PATCH] btrfs: Remove unneeded cast to s64 for qgroup rfer state

[Qu Wenruo]

Alexandru Moise wrote on 2015/08/31 09:32 +0300:
> On Mon, Aug 31, 2015 at 09:44:49AM +0800, Qu Wenruo wrote:
>>>>     >From the perspective of users, qgroup's referenced or exclusive
>>>>     is negative,but user can not continue to write data! a workaround
>>>>     way is to cast u64 to s64 when doing qgroup reservation
>>>
>>> I am unable to reproduce this problem without his modification.
>>> I could be wrong in reverting this, so I'm gonna CC Wang as well so
>>> he is aware of this patch.
>>
>> The cast is a workaround for a quite old qgroup bug, which will
>> cause excl/rfer overflow to minus.
>>
>> The remove of cast rfer/exel now is OK, as qgroup keeps maturing,
>> especially after 4.2-rc1 rfer/exel will keep sane under most case
>> (exception will be qgroup reassign and subvolume deletion, but will
>> not case minus value even under than case).
>
> rfer/exel and reserved are all of type unsigned int, how exactly would
> they overflow to minus?

Due to qgroup bugs of course,
In old implement, btrfs_find_all_roots() will not always find the 
correct roots.

Causing quota to minus more bytes on existing qgroups.

For example qg->rfer is 16K, btrfs_find_all_roots() think the qg 
previously own a 32K extent but not now, and qgroup accounting decides 
to decrease qg->rfer by 32K, now you get -16K, which is a super huge 
number if used as u64.

>
>>
>> But I'm not a fan to remove it now.
>> As qgroup still has a known huge bug for the qg->reserved part, we
>> are aware of it and working on it actively.
>
> Can you tell me more about this known huge bug and how you can
> reproduce it using the present implementation?
>

Check the fstest patch I submitted:
https://patchwork.kernel.org/patch/7023301/

Btrfs qgroup has qgroup reserved space leak problem, and under some 
case, it can also overflow to minus.(I don't have a minus reproducer, 
but it already happened several times in my test environment)

That's what we are fixing now, trying to make it public before 4.3-rc1.

Thanks,
Qu

Re: [PATCH] btrfs: Remove unneeded cast to s64 for qgroup rfer state

[Alexandru Moise]

On Mon, Aug 31, 2015 at 02:51:08PM +0800, Qu Wenruo wrote:
> 
> 
> Alexandru Moise wrote on 2015/08/31 09:32 +0300:
> >On Mon, Aug 31, 2015 at 09:44:49AM +0800, Qu Wenruo wrote:
> >>>>    >From the perspective of users, qgroup's referenced or exclusive
> >>>>    is negative,but user can not continue to write data! a workaround
> >>>>    way is to cast u64 to s64 when doing qgroup reservation
> >>>
> >>>I am unable to reproduce this problem without his modification.
> >>>I could be wrong in reverting this, so I'm gonna CC Wang as well so
> >>>he is aware of this patch.
> >>
> >>The cast is a workaround for a quite old qgroup bug, which will
> >>cause excl/rfer overflow to minus.
> >>
> >>The remove of cast rfer/exel now is OK, as qgroup keeps maturing,
> >>especially after 4.2-rc1 rfer/exel will keep sane under most case
> >>(exception will be qgroup reassign and subvolume deletion, but will
> >>not case minus value even under than case).
> >
> >rfer/exel and reserved are all of type unsigned int, how exactly would
> >they overflow to minus?
> 
> Due to qgroup bugs of course,
> In old implement, btrfs_find_all_roots() will not always find the
> correct roots.
> 
> Causing quota to minus more bytes on existing qgroups.
> 
> For example qg->rfer is 16K, btrfs_find_all_roots() think the qg
> previously own a 32K extent but not now, and qgroup accounting
> decides to decrease qg->rfer by 32K, now you get -16K, which is a
> super huge number if used as u64.
> 
> >
> >>
> >>But I'm not a fan to remove it now.
> >>As qgroup still has a known huge bug for the qg->reserved part, we
> >>are aware of it and working on it actively.
> >
> >Can you tell me more about this known huge bug and how you can
> >reproduce it using the present implementation?
> >
> 
> Check the fstest patch I submitted:
> https://patchwork.kernel.org/patch/7023301/
> 
> Btrfs qgroup has qgroup reserved space leak problem, and under some
> case, it can also overflow to minus.(I don't have a minus
> reproducer, but it already happened several times in my test
> environment)
> 
> That's what we are fixing now, trying to make it public before 4.3-rc1.
> 
> Thanks,
> Qu

Thank you for the detailed explanation Qu, I will read more on your
changes and perhaps learn a thing or two.

All the respect,
    Alex