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Re: [PATCH] xfs: fix the extent count when allocating an new indirection

To: Dave Chinner <david@xxxxxxxxxxxxx>
Subject: Re: [PATCH] xfs: fix the extent count when allocating an new indirection array entry
From: Jeff Liu <jeff.liu@xxxxxxxxxx>
Date: Wed, 25 Sep 2013 15:38:11 +0800
Cc: "xfs@xxxxxxxxxxx" <xfs@xxxxxxxxxxx>
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <20130923002448.GL12541@dastard>
References: <523DAF67.9070206@xxxxxxxxxx> <20130923002448.GL12541@dastard>
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:11.0) Gecko/20120410 Thunderbird/11.0.1
On 09/23/2013 08:24 AM, Dave Chinner wrote:

> On Sat, Sep 21, 2013 at 10:38:31PM +0800, Jeff Liu wrote:
>> From: Jie Liu <jeff.liu@xxxxxxxxxx>
>>
>> At xfs_iext_add(), if extent(s) are being appended to the last
>> page in the indirection array and the new extent(s) don't fit
>> in the page, the number of extents(erp->er_extcount) in a new
>> allocated entry should be the minimum value between count and
>> XFS_LINEAR_EXTS, instead of count.
> 
> Definitely looks like a bug, but what are the symptoms of it and how
> did you find the problem? Is there any test case that demonstrates a
> problem with the er_extcount being set incorrectly here?

Sorry for the too late response, I found this problem while reading the code.
However, I can not figure out a test case to break the kernel until now. :(

> 
>> Signed-off-by: Jie Liu <jeff.liu@xxxxxxxxxx>
>> ---
>>  fs/xfs/xfs_inode_fork.c |    8 ++++----
>>  1 file changed, 4 insertions(+), 4 deletions(-)
>>
>> diff --git a/fs/xfs/xfs_inode_fork.c b/fs/xfs/xfs_inode_fork.c
>> index 02f1083..dfb4226 100644
>> --- a/fs/xfs/xfs_inode_fork.c
>> +++ b/fs/xfs/xfs_inode_fork.c
>> @@ -1035,11 +1035,11 @@ xfs_iext_add(
>>  
>>                      while (count) {
>>                              erp = xfs_iext_irec_new(ifp, erp_idx);
>> -                            erp->er_extcount = count;
>> -                            count -= MIN(count, (int)XFS_LINEAR_EXTS);
>> -                            if (count) {
>> +                            erp->er_extcount = MIN(count,
>> +                                                   (int)XFS_LINEAR_EXTS);
>> +                            count -= erp->er_extcount;
> 
> count is declared as an int, whereas XFS_LINEAR_EXTS probably ends
> up with a type of uint because of a cast in the macro. because we
> are decrementing to zero, the count can be declared as a uint, too,
> and the cast in the MIN() can go away. Indeed, MIN() should be
> converted to min() seeing as we are touching the code here, and if
> you want to retain the current types, the min_t() is appropriate,
> not min(x, (some cast)y)....

Ok, will fix it.

Thanks,
-Jeff

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