Re: [PATCH 04/10] mm/migrate: make migrate_pages() return nr_succeeded

[Oscar Salvador <osalvador@suse.de>]

On Thu, Apr 08, 2021 at 01:40:33PM -0700, Yang Shi wrote:
> Thanks a lot for the example code. You didn't miss anything. At first
> glance, I thought your suggestion seemed neater. Actually I
> misunderstood what Dave said about "That could really have caused some
> interesting problems." with multiple calls to migrate_pages(). I was
> thinking about:
> 
> unsigned long foo()
> {
>     unsigned long *ret_succeeded;
> 
>     migrate_pages(..., ret_succeeded);
> 
>     migrate_pages(..., ret_succeeded);
> 
>     return *ret_succeeded;
> }

But that would not be a problem as well. I mean I am not sure what is
foo() supposed to do.
I assume is supposed to return the *total* number of pages that were
migrated?

Then could do something like:

 unsigned long foo()
 {
     unsigned long ret_succeeded;
     unsigned long total_succeeded = 0;

     migrate_pages(..., &ret_succeeded);
     total_succeeded += ret_succeeded;

     migrate_pages(..., &ret_succeeded);
     total_succeeded += ret_succeeded;

     return *total_succeeded;
 }

 But AFAICS, you would have to do that with Wei Xu's version and with
 mine, no difference there.

IIUC, Dave's concern was that nr_succeeded was only set to 0 at the beginning
of the function, and never reset back, which means, we would carry the
sum of previous nr_succeeded instead of the nr_succeeded in that round.
That would be misleading for e.g: reclaim in case we were to call
migrate_pages() several times, as instead of a delta value, nr_succeeded
would accumulate.

But that won't happen neither with Wei Xu's version nor with mine. 

-- 
Oscar Salvador
SUSE L3