linux-kernel - Re: [PATCH 04/10] mm/migrate: make migrate_pages() return nr

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Date:   Fri, 9 Apr 2021 07:06:43 +0200
From:   Oscar Salvador <osalvador@...e.de>
To:     Yang Shi <shy828301@...il.com>
Cc:     Dave Hansen <dave.hansen@...ux.intel.com>,
        Linux MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Yang Shi <yang.shi@...ux.alibaba.com>, weixugc@...gle.com,
        Huang Ying <ying.huang@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        David Hildenbrand <david@...hat.com>
Subject: Re: [PATCH 04/10] mm/migrate: make migrate_pages() return
 nr_succeeded

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