Re: [PATCH for-4.19] staging: erofs: fix mis-acted TAIL merging behavior

[Greg KH <greg@kroah.com>]

On Thu, Mar 07, 2019 at 01:34:04PM +0800, Gao Xiang wrote:
> commit a112152f6f3a2a88caa6f414d540bd49e406af60 upstream.
> 
> EROFS has an optimized path called TAIL merging, which is designed
> to merge multiple reads and the corresponding decompressions into
> one if these requests read continuous pages almost at the same time.
> 
> In general, it behaves as follows:
>  ________________________________________________________________
>   ... |  TAIL  .  HEAD  |  PAGE  |  PAGE  |  TAIL    . HEAD | ...
>  _____|_combined page A_|________|________|_combined page B_|____
>         1  ]  ->  [  2                          ]  ->  [ 3
> If the above three reads are requested in the order 1-2-3, it will
> generate a large work chain rather than 3 individual work chains
> to reduce scheduling overhead and boost up sequential read.
> 
> However, if Read 2 is processed slightly earlier than Read 1,
> currently it still generates 2 individual work chains (chain 1, 2)
> but it does in-place decompression for combined page A, moreover,
> if chain 2 decompresses ahead of chain 1, it will be a race and
> lead to corrupted decompressed page. This patch fixes it.
> 
> Fixes: 3883a79abd02 ("staging: erofs: introduce VLE decompression support")
> Cc: <stable@vger.kernel.org> # 4.19+
> Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
> Reviewed-by: Chao Yu <yuchao0@huawei.com>
> ---
>  drivers/staging/erofs/unzip_vle.c | 69 +++++++++++++++++++++++++--------------
>  1 file changed, 44 insertions(+), 25 deletions(-)
> 

thanks for the backport, now applied.

greg k-h