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Re: [PATCH] xfs: fix the wrong new_size/rnew_size at xfs_iext_realloc_di

To: Dave Chinner <david@xxxxxxxxxxxxx>
Subject: Re: [PATCH] xfs: fix the wrong new_size/rnew_size at xfs_iext_realloc_direct()
From: Jeff Liu <jeff.liu@xxxxxxxxxx>
Date: Mon, 23 Sep 2013 12:47:23 +0800
Cc: "xfs@xxxxxxxxxxx" <xfs@xxxxxxxxxxx>
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <20130923005657.GN12541@dastard>
References: <523EA96B.3040904@xxxxxxxxxx> <20130923005657.GN12541@dastard>
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:11.0) Gecko/20120410 Thunderbird/11.0.1
Hi Dave,

On 09/23/2013 08:56 AM, Dave Chinner wrote:

> On Sun, Sep 22, 2013 at 04:25:15PM +0800, Jeff Liu wrote:
>> From: Jie Liu <jeff.liu@xxxxxxxxxx>
>>
>> At xfs_iext_realloc_direct(), the new_size is changed by adding
>> if_bytes if originally the extent records are stored at the inline
>> extent buffer, and we have to switch from it to a direct extent
>> list for those new allocated extents, this is wrong. e.g,
>>
>> Create a file with three extents which was showing as following,
>>
>> xfs_io -f -c "truncate 100m" /xfs/testme
>>
>> for i in $(seq 0 5 10); do
>>      offset=$(($i * $((1 << 20))))
>>      xfs_io -c "pwrite $offset 1m" /xfs/testme
>> done
>>
>> Inline
>> ------
>> irec:        if_bytes        bytes_diff      new_size
>> 1st  0               16              16
>> 2nd  16              16              32
>>
>> Switching
>> ---------                                            rnew_size
>> 3rd  32              16              48 + 32 = 80    roundup=128
>>
>> In this case, the desired value of new_size should be 48, and then
>> it will be roundup to 64 and be assigned to rnew_size.
> 
> Ok, so it allocates 128 bytes instead of 64 bytes. It tracks that
> allocation size correctly ifp->if_real_bytes, and all it means is
> that there are 4 empty extra slots in the extent array. The code
> already handles having empty slots in the direct extent array, so
> what impact is there as a result of the oversized initial allocation
> that is currently happening?
> 
> i.e. if fixing the oversized results in more memory allocations due
> to resizing more regularly, then is there a benefit to changing this
> code given that the rewrite of the ifp->if_bytes value in the case
> where we do inline->direct conversion prevents this over-allocation
> from being a problem...

I guess my current patch subject/description mislead you.  The result
of the oversized can be ignored since this can be handled in the direct
extent array as empty slots.

Actually, what I want to say is that we don't need to perform "new_size += 
ifp->if_bytes;"
again at xfs_iext_realloc_direct() because the new_size at xfs_iext_add()
already be the size of extents after adding, just as the variable comments
is mentioned.

Thanks,
-Jeff

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