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

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Message-ID: <20200129185842.GP24244@dhcp22.suse.cz>
Date:   Wed, 29 Jan 2020 19:58:42 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Yang Shi <yang.shi@...ux.alibaba.com>
Cc:     mtk.manpages@...il.com, akpm@...ux-foundation.org,
        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 Thu 30-01-20 02:30:32, 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.

Thanks a lot! Looks good to me. I would just add a note that it is
generally recommended to pre-initialize status array by -1 to have a
reliable way to check which pages have been migrated. The man page says
"The array contains valid values only if move_pages() did not return an
error."

I would just add. "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".

> Cc: Michal Hocko <mhocko@...e.com>
> Cc: Michael Kerrisk <mtk.manpages@...il.com>
> Signed-off-by: Yang Shi <yang.shi@...ux.alibaba.com>

Acked-by: Michal Hocko <mhocko@...e.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.
>  .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,
> -- 
> 1.8.3.1

-- 
Michal Hocko
SUSE Labs