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

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Message-ID: <cce2e784-8092-00f5-32bf-d23ab7a53476@suse.cz>
Date:   Thu, 30 Jan 2020 13:56:20 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Yang Shi <yang.shi@...ux.alibaba.com>, 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: [v2 PATCH] move_pages.2: Returning positive value is a new error
 case

On 1/30/20 1:02 PM, Michal Hocko wrote:
> On Thu 30-01-20 10:06:28, Vlastimil Babka wrote:
>> 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?
> 
> The idea was that we _could_ return EAGAIN from the syscall if
> migrate_pages > 0.
> 
>> I.e. that it's no longer returned since 4.17?
> 
> I am pretty sure this will require a deeper consideration. Do we return
> EIO/EINVAL?

I thought the manpage says we return -EBUSY, but I misread it, this part
was not about errno, but the status array. So there's nothing to update
there, sorry about the noise.

BTW, the suggestion to "Pre-initialization of the array to -1" means
effectively it's pre-initialized to -EPERM. That's fine now as -EPERM is
not one of the codes listed as possible to be returned via the array,
but perhaps it's not entirely future-proof?