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
