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

[Yang Shi]

On 12/30/19 7:49 PM, Eric W. Biederman wrote:
> Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx> writes:
>
> > On 12/18/19 2:17 AM, Michal Hocko wrote:
> > > On Tue 17-12-19 23:36:09, John Hubbard wrote:
> > > [...]
> > > > diff --git a/man2/move_pages.2 b/man2/move_pages.2
> > > > index 2d96468fa..1bf1053f2 100644
> > > > --- a/man2/move_pages.2
> > > > +++ b/man2/move_pages.2
> > > > @@ -191,12 +191,6 @@ was specified or an attempt was made to migrate pages of a kernel thread.
> > > >    .B ENODEV
> > > >    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
> > > > -moved because they were mapped by multiple processes.
> > > > -.TP
> > > >    .B EPERM
> > > >    The caller specified
> > > >    .B MPOL_MF_MOVE_ALL
> > > >
> > > > ...But I'm not sure if we should change the implementation, instead, so
> > > > that it *can* return ENOENT. That's the main question to resolve before
> > > > creating any more patches, I think.
> > > I would start by dropping any note about ENOENT first. I am not really
> > > sure there is a reasonable usecase for it but maybe somebody comes up
> > > with something and only then we should consider it.
> > >
> > > Feel free to add
> > > Acked-by: Michal Hocko <mhocko@xxxxxxxx>
> > >
> > > ideally with a kernel commit which removed the ENOENT.
> > A quick audit doesn't show kernel code or comment notes about ENOENT
> > wrongly. The status could be set as ENOENT if the page is not present
> > (follow_page() returns NULL), and man page does match what kernel
> > does.
> Doesn't the function one layer up then consume the ENOENT?

No, it doesn't. The return value would be reset unconditionally by 
store_status(). This is what the man page patch tries to correct.

> Eric