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

[Gao Xiang]

On 2019/3/7 23:21, Greg KH wrote:
> On Thu, Mar 07, 2019 at 09:04:06PM +0800, Gao Xiang wrote:
>> Hi Greg,
>>
>> On 2019/3/7 20:25, Greg KH wrote:
>>> 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 at vger.kernel.org> # 4.19+
>>>> Signed-off-by: Gao Xiang <gaoxiang25 at huawei.com>
>>>> Reviewed-by: Chao Yu <yuchao0 at huawei.com>
>>>> ---
>>>>  drivers/staging/erofs/unzip_vle.c | 69 +++++++++++++++++++++++++--------------
>>>>  1 file changed, 44 insertions(+), 25 deletions(-)
>>>>
>>>
>>> thanks for the backport, now applied.
>>
>> Yes, it seems that Sasha's bot cannot cherry-pick it directly,
>> therefore I made this patch for 4.19 by hand...
>>
>> BTW, there is another patch called "staging: erofs: compressed_pages should not be accessed again after freed"
>> in the same condition... And I also send 4.19 and 4.20 version to stable mailing list,
>> could you also kindly check them out?
> 
> Yes, please give me a chance to catch up on stable pending patches to
> process:
> 
> 	$ mdfrm -c ~/mail/stable/
> 	585 messages in /home/gregkh/mail/stable/
> 
> You are in good company :)

Thanks for taking time.
I received more reports which shows some 5.1-rc patches are failed to be applied,
I will backport them later. :)

Thanks,
Gao Xiang

> 
> greg k-h
>