linux-kernel - Re: [PATCH] move_pages.2: Returning positive value is a new error case

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Date:   Fri, 31 Jan 2020 11:40:54 -0800
From:   Yang Shi <yang.shi@...ux.alibaba.com>
To:     David Hildenbrand <david@...hat.com>, mhocko@...e.com,
        mtk.manpages@...il.com, akpm@...ux-foundation.org
Cc:     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: Returning positive value is a new error
 case



On 1/31/20 2:49 AM, David Hildenbrand wrote:
> On 29.01.20 19:30, Yang Shi wrote:
>> Since commit a49bd4d71637 ("mm, numa: rework do_pages_move"),
>> the semantic of move_pages() has changed to return the number of
>> non-migrated pages if they were result of a non-fatal reasons (usually a
>> busy page).  This was an unintentional change that hasn't been noticed
>> except for LTP tests which checked for the documented behavior.
>>
>> There are two ways to go around this change.  We can even get back to the
>> original behavior and return -EAGAIN whenever migrate_pages is not able
>> to migrate pages due to non-fatal reasons.  Another option would be to
>> simply continue with the changed semantic and extend move_pages
>> documentation to clarify that -errno is returned on an invalid input or
>> when migration simply cannot succeed (e.g. -ENOMEM, -EBUSY) or the
>> number of pages that couldn't have been migrated due to ephemeral
>> reasons (e.g. page is pinned or locked for other reasons).
>>
>> We decided to keep the second option in kernel because this behavior is in
>> place for some time without anybody complaining and possibly new users
>> depending on it.  Also it allows to have a slightly easier error handling
>> as the caller knows that it is worth to retry when err > 0.
>>
>> Update man pages to reflect the new semantic.
>>
>> Cc: Michal Hocko <mhocko@...e.com>
>> Cc: Michael Kerrisk <mtk.manpages@...il.com>
>> Signed-off-by: Yang Shi <yang.shi@...ux.alibaba.com>
>> ---
>>   man2/move_pages.2 | 6 +++++-
>>   1 file changed, 5 insertions(+), 1 deletion(-)
>>
>> diff --git a/man2/move_pages.2 b/man2/move_pages.2
>> index 1bf1053..c6cf3f8 100644
>> --- a/man2/move_pages.2
>> +++ b/man2/move_pages.2
>> @@ -164,9 +164,13 @@ returns zero.
>>   .\" do the right thing?
>>   On error, it returns \-1, and sets
>>   .I errno
>> -to indicate the error.
>> +to indicate the error. Or positive value to report the number of
>> +non-migrated pages.
> "If a positive value is returned, it's the number of non-migrated pages".
>
>>   .SH ERRORS
>>   .TP
>> +.B Positive value
>> +The number of non-migrated pages if they were result of a non-fatal
>> +reasons since version 4.17.
> s/result/the result/ ?
>
> s/a reasons/reasons/ ?
>
> s/since version 4.17/(since 4.17)/ ?

Thanks. Will fix in new version.

>
>>   .B E2BIG
>>   Too many pages to move.
>>   Since Linux 2.6.29,
>>
>