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

[Yang Shi]

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"?