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Subject: Re: [PATCH] move_pages.2: not return ENOENT if the page are already
 on the target nodes
To:     "Eric W. Biederman" <ebiederm@xmission.com>
Cc:     Michal Hocko <mhocko@kernel.org>,
        John Hubbard <jhubbard@nvidia.com>,
        "Michael Kerrisk (man-pages)" <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
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From:   Yang Shi <yang.shi@linux.alibaba.com>
Message-ID: <d081b674-b360-4a0a-eec3-cf7434003cc5@linux.alibaba.com>
Date:   Thu, 2 Jan 2020 14:15:12 -0800
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On 12/30/19 7:49 PM, Eric W. Biederman wrote:
> Yang Shi <yang.shi@linux.alibaba.com> 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@suse.com>
>>>
>>> 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