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Subject: Re: [PATCH] move_pages.2: not return ENOENT if the page are already
 on the target nodes
To:     Michal Hocko <mhocko@kernel.org>,
        John Hubbard <jhubbard@nvidia.com>
Cc:     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
References: <1575596090-115377-1-git-send-email-yang.shi@linux.alibaba.com>
 <0dc96e40-5f2b-a2fe-6e5f-b6f3d5e9ebde@nvidia.com>
 <20191206094534.GL28317@dhcp22.suse.cz>
From:   Yang Shi <yang.shi@linux.alibaba.com>
Message-ID: <5384814f-c937-9622-adbe-c03e199e0267@linux.alibaba.com>
Date:   Fri, 6 Dec 2019 09:31:31 -0800
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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@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.

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.