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

[Michal Hocko <mhocko@kernel.org>]

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.
-- 
Michal Hocko
SUSE Labs