linux-kernel - Re: [v2 PATCH] move_pages.2: Returning positive value is a new error case

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Date:   Thu, 30 Jan 2020 10:06:28 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Yang Shi <yang.shi@...ux.alibaba.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: [v2 PATCH] move_pages.2: Returning positive value is a new error
 case

On 1/29/20 10:48 PM, 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

The manpage says EBUSY, not EAGAIN? And should its description be
updated too? I.e. that it's no longer returned since 4.17?

> 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.
> 
> Acked-by: Michal Hocko <mhocko@...e.com>
> Cc: Michael Kerrisk <mtk.manpages@...il.com>
> Signed-off-by: Yang Shi <yang.shi@...ux.alibaba.com>
> ---
> v2: * Added notes about status array per Michal.
>     * Added Michal's Acked-by.
> 
>  man2/move_pages.2 | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/man2/move_pages.2 b/man2/move_pages.2
> index 1bf1053..83d5c81 100644
> --- a/man2/move_pages.2
> +++ b/man2/move_pages.2
> @@ -104,7 +104,9 @@ pages that need to be moved.
>  is an array of integers that return the status of each page.
>  The array contains valid values only if
>  .BR move_pages ()
> -did not return an error.
> +did not return an error.  Pre-initialization of the array to -1 or
> +similar value which cannot represent a real numa node could help to
> +identify pages that have been migrated
>  .PP
>  .I flags
>  specify what types of pages to move.
> @@ -164,9 +166,13 @@ 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.
>  .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.
>  .B E2BIG
>  Too many pages to move.
>  Since Linux 2.6.29,
>