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

To: Jeff Liu <jeff.liu@xxxxxxxxxx>
Subject: Re: [PATCH] xfs: fix the wrong new_size/rnew_size at xfs_iext_realloc_direct()
From: Dave Chinner <david@xxxxxxxxxxxxx>
Date: Mon, 23 Sep 2013 10:56:57 +1000
Cc: "xfs@xxxxxxxxxxx" <xfs@xxxxxxxxxxx>
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <523EA96B.3040904@xxxxxxxxxx>
References: <523EA96B.3040904@xxxxxxxxxx>
User-agent: Mutt/1.5.21 (2010-09-15)
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...


Dave Chinner

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