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

[Wei Yang]

On Fri, Jan 24, 2020 at 03:46:43PM +0100, Michal Hocko wrote:
>On Fri 24-01-20 22:15:38, Wei Yang wrote:
>> 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.
>
>And this is the expected logic. The unexpected behavior is the one you
>have fixed by this patch because err = 1 wouldn't get overriden and that
>should have been.

Ok, if the sentence cover this case, the wording looks good to me.

Thanks :-)

>-- 
>Michal Hocko
>SUSE Labs

-- 
Wei Yang
Help you, Help me