Re: [v2 PATCH] mm: move_pages: report the number of non-attempted pages

[Wei Yang <richardw.yang@linux.intel.com>]

On Thu, Jan 23, 2020 at 09:55:26AM +0100, Michal Hocko wrote:
>On Thu 23-01-20 11:27:36, Wei Yang wrote:
>> On Thu, Jan 23, 2020 at 07:38:51AM +0800, Yang Shi wrote:
>> >Since commit a49bd4d71637 ("mm, numa: rework do_pages_move"),
>> >the semantic of move_pages() was changed to return the number of
>> >non-migrated pages (failed to migration) and the call would be aborted
>> >immediately if migrate_pages() returns positive value.  But it didn't
>> >report the number of pages that we even haven't attempted to migrate.
>> >So, fix it by including non-attempted pages in the return value.
>> >
>> 
>> First, we want to change the semantic of move_pages(2). The return value
>> indicates the number of pages we didn't managed to migrate?
>> 
>> Second, the return value from migrate_pages() doesn't mean the number of pages
>> we failed to migrate. For example, one -ENOMEM is returned on the first page,
>> migrate_pages() would return 1. But actually, no page successfully migrated.
>
>ENOMEM is considered a permanent failure and as such it is returned by
>migrate pages (see goto out).
>
>> Third, even the migrate_pages() return the exact non-migrate page, we are not
>> sure those non-migrated pages are at the tail of the list. Because in the last
>> case in migrate_pages(), it just remove the page from list. It could be a page
>> in the middle of the list. Then, in userspace, how the return value be
>> leveraged to determine the valid status? Any page in the list could be the
>> victim.
>
>Yes, I was wrong when stating that the caller would know better which
>status to check. I misremembered the original patch as it was quite some
>time ago. While storing the error code would be possible after some
>massaging of migrate_pages is this really something we deeply care
>about. The caller can achieve the same by initializing the status array
>to a non-node number - e.g. -1 - and check based on that.
>

So for a user, the best practice is to initialize the status array to -1 and
check each status to see whether the page is migrated successfully?

Then do we need to return the number of non-migrated page? What benefit could
user get from the number. How about just return an error code to indicate the
failure? I may miss some point, would you mind giving me a hint?

>This system call has quite a complex semantic and I am not 100% sure
>what is the right thing to do here. Maybe we do want to continue and try
>to migrate as much as possible on non-fatal migration failures and
>accumulate the number of failed pages while doing so.
>
>The main problem is that we can have an academic discussion but
>the primary question is what do actual users want. A lack of real
>bug reports suggests that nobody has actually noticed this. So I
>would rather keep returning the correct number of non-migrated
>pages. Why? Because new users could have started depending on it. It
>is not all that unlikely that the current implementation would just
>work for them because they are migrating a set of pages on to the same
>node so the batch would be a single list throughout the whole given
>page set.
>-- 
>Michal Hocko
>SUSE Labs

-- 
Wei Yang
Help you, Help me