Re: [v2 PATCH] move_pages.2: Returning positive value is a new error case

Yang Shi
2020-01-30T17:27:11Z

On 1/30/20 5:48 AM, Michal Hocko wrote:
> On Thu 30-01-20 13:56:20, Vlastimil Babka wrote:
>> On 1/30/20 1:02 PM, Michal Hocko wrote:
>>> On Thu 30-01-20 10:06:28, Vlastimil Babka wrote:
>>>> On 1/29/20 10:48 PM, Yang Shi wrote:
>>>>> Since commit a49bd4d71637 ("mm, numa: rework do_pages_move"),
>>>>> the semantic of move_pages() has changed to return the number of
>>>>> non-migrated pages if they were result of a non-fatal reasons (usually a
>>>>> busy page). This was an unintentional change that hasn't been noticed
>>>>> except for LTP tests which checked for the documented behavior.
>>>>>
>>>>> There are two ways to go around this change. We can even get back to the
>>>>> original behavior and return -EAGAIN whenever migrate_pages is not able
>>>> The manpage says EBUSY, not EAGAIN? And should its description be
>>>> updated too?
>>> The idea was that we _could_ return EAGAIN from the syscall if
>>> migrate_pages > 0.
>>>
>>>> I.e. that it's no longer returned since 4.17?
>>> I am pretty sure this will require a deeper consideration. Do we return
>>> EIO/EINVAL?
>> I thought the manpage says we return -EBUSY, but I misread it, this part
>> was not about errno, but the status array. So there's nothing to update
>> there, sorry about the noise.
>>
>> BTW, the suggestion to "Pre-initialization of the array to -1" means
>> effectively it's pre-initialized to -EPERM. That's fine now as -EPERM is
>> not one of the codes listed as possible to be returned via the array,
>> but perhaps it's not entirely future-proof?
> Hmm, I didn't realize EPERM is refering to 1. The wording however
> suggests also any other value that cannot represent a valid NUMA node.
> So maybe we should just drop the node about -1.

Or maybe we just say "any value which doesn't represent a valid NUMA
node or valid error of status array"?