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

[Vlastimil Babka <vbabka@suse.cz>]

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?