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:

quoted

From: Jie Liu <redacted>

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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