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

From: Michal Hocko <mhocko@kernel.org>
To: John Hubbard <jhubbard@nvidia.com>
Cc: Yang Shi <yang.shi@linux.alibaba.com>,
	mtk.manpages@gmail.com, cl@linux.com, cai@lca.pw,
	akpm@linux-foundation.org, linux-man@vger.kernel.org,
	linux-api@vger.kernel.org, linux-mm@kvack.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH] move_pages.2: not return ENOENT if the page are already on the target nodes
Date: Fri, 6 Dec 2019 10:45:34 +0100	[thread overview]
Message-ID: <20191206094534.GL28317@dhcp22.suse.cz> (raw)
In-Reply-To: <0dc96e40-5f2b-a2fe-6e5f-b6f3d5e9ebde@nvidia.com>

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@gmail.com>
> > Cc: Christoph Lameter <cl@linux.com>
> > Cc: John Hubbard <jhubbard@nvidia.com>
> > Cc: Michal Hocko <mhocko@suse.com>
> > Cc: Qian Cai <cai@lca.pw>
> > Signed-off-by: Yang Shi <yang.shi@linux.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