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

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Message-ID: <20191206094534.GL28317@dhcp22.suse.cz>
Date:   Fri, 6 Dec 2019 10:45:34 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     Yang Shi <yang.shi@...ux.alibaba.com>, mtk.manpages@...il.com,
        cl@...ux.com, cai@....pw, akpm@...ux-foundation.org,
        linux-man@...r.kernel.org, linux-api@...r.kernel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] move_pages.2: not return ENOENT if the page are already
 on the target nodes

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@...il.com>
> > Cc: Christoph Lameter <cl@...ux.com>
> > Cc: John Hubbard <jhubbard@...dia.com>
> > Cc: Michal Hocko <mhocko@...e.com>
> > Cc: Qian Cai <cai@....pw>
> > Signed-off-by: Yang Shi <yang.shi@...ux.alibaba.com>
> > ---
> >   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.

Btw. Page states error codes could see some refinements as well.
-- 
Michal Hocko
SUSE Labs