Re: [PATCH] xfs: fix the wrong new_size/rnew_size at xfs_iext_realloc_direct()

[Dave Chinner <david@fromorbit.com>]

On Sun, Sep 22, 2013 at 04:25:15PM +0800, Jeff Liu wrote:
> From: Jie Liu <jeff.liu@oracle.com>
> 
> 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...

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com

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