Re: [Patch v2] mm/migrate.c: also overwrite error when it is bigger than zero

[Wei Yang]

On Fri, Jan 24, 2020 at 08:21:27AM +0100, Michal Hocko wrote:
>[Sorry I have missed this patch previously]
>

No problem, thanks for your comment.

>On Sun 19-01-20 14:57:53, Wei Yang wrote:
>> If we get here after successfully adding page to list, err would be
>> 1 to indicate the page is queued in the list.
>> 
>> Current code has two problems:
>> 
>>   * on success, 0 is not returned
>>   * on error, if add_page_for_migratioin() return 1, and the following err1
>>     from do_move_pages_to_node() is set, the err1 is not returned since err
>>     is 1
>
>This made my really scratch my head to grasp. So essentially err > 0
>will happen when we reach the end of the loop and rely on the
>out_flush flushing to migrate the batch. Then err contains the
>add_page_for_migratioin return value. And that would leak to the
>userspace.
>
>What would you say about the following wording instead?
>"
>out_flush part of do_pages_move is responsible for migrating the last
>batch that accumulated while processing the input in the loop.
>do_move_pages_to_node return value is supposed to override any
>preexisting error (e.g. when the user input is garbage) but the current

I am afraid I have a different understanding here.

If we jump to out_flush on the test of node_isset(), err is -EACCESS. Current
logic would return this instead of the error from do_move_pages_to_node().
Seems we don't override -EACCESS.

Is my understanding correct?

>logic is wrong because add_page_for_migration returns 1 when
>successfully adding a page into the batch and therefore this will be the
>last err value after the loop is processed without any actual error.
>We want to override that value of course because do_pages_move would
>return 1 to the userspace even without any errors.
>"
>
>> And these behaviors break the user interface.
>> 
>> Fixes: e0153fc2c760 ("mm: move_pages: return valid node id in status if the
>> page is already on the target node").
>> Signed-off-by: Wei Yang <richardw.yang@xxxxxxxxxxxxxxx>
>
>Acked-by: Michal Hocko <mhocko@xxxxxxxx>
>
>> 
>> ---
>> v2:
>>   * put more words to explain the error case
>> ---
>>  mm/migrate.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>> 
>> diff --git a/mm/migrate.c b/mm/migrate.c
>> index 86873b6f38a7..430fdccc733e 100644
>> --- a/mm/migrate.c
>> +++ b/mm/migrate.c
>> @@ -1676,7 +1676,7 @@ static int do_pages_move(struct mm_struct *mm, nodemask_t task_nodes,
>>  	err1 = do_move_pages_to_node(mm, &pagelist, current_node);
>>  	if (!err1)
>>  		err1 = store_status(status, start, current_node, i - start);
>> -	if (!err)
>> +	if (err >= 0)
>>  		err = err1;
>>  out:
>>  	return err;
>> -- 
>> 2.17.1
>> 
>
>-- 
>Michal Hocko
>SUSE Labs

-- 
Wei Yang
Help you, Help me