Re: [PATCH] move_pages.2: not return ENOENT if the page are already on the target nodes

[Yang Shi]

On 12/6/19 1:45 AM, Michal Hocko wrote:
> On Fri 06-12-19 00:25:53, John Hubbard wrote:
> > On 12/5/19 5:34 PM, Yang Shi wrote:
> > > Since commit e78bbfa82624 ("mm: stop returning -ENOENT
> > > from sys_move_pages() if nothing got migrated"), move_pages doesn't
> > > return -ENOENT anymore if the pages are already on the target nodes, but
> > > this change is never reflected in manpage.
> > >
> > > Cc: Michael Kerrisk <mtk.manpages@xxxxxxxxx>
> > > Cc: Christoph Lameter <cl@xxxxxxxxx>
> > > Cc: John Hubbard <jhubbard@xxxxxxxxxx>
> > > Cc: Michal Hocko <mhocko@xxxxxxxx>
> > > Cc: Qian Cai <cai@xxxxxx>
> > > Signed-off-by: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>
> > > ---
> > >    man2/move_pages.2 | 5 ++---
> > >    1 file changed, 2 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/man2/move_pages.2 b/man2/move_pages.2
> > > index 2d96468..2a2f3cd 100644
> > > --- a/man2/move_pages.2
> > > +++ b/man2/move_pages.2
> > > @@ -192,9 +192,8 @@ was specified or an attempt was made to migrate pages of a kernel thread.
> > >    One of the target nodes is not online.
> > >    .TP
> > >    .B ENOENT
> > > -No pages were found that require moving.
> > > -All pages are either already
> > > -on the target node, not present, had an invalid address or could not be
> > > +No pages were found.
> > > +All pages are either not present, had an invalid address or could not be
> > >    moved because they were mapped by multiple processes.
> > >    .TP
> > >    .B EPERM
> > whoa, hold on. If I'm reading through the various error paths correctly, then this
> > code is *never* going to return ENOENT for the whole function. It can fill in that
> > value per-page, in the status array, but that's all. Did I get that right?
> You are right. Both store_status and do_move_pages_to_node do overwrite
> the error code. So you are right that ENOENT return value is not
> possible. I haven't checked since when this is the case. This whole
> syscall is a disaster from the API and documentation POV.

It looks since commit e78bbfa82624 ("mm: stop returning -ENOENT from 
sys_move_pages() if nothing got migrated") too, which reset err to 0 
unconditionally. It seems it is on purpose by that commit the syscall 
caller should check status for the details according to the commit log.

> Btw. Page states error codes could see some refinements as well.