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From:   Alistair Popple <apopple@nvidia.com>
To:     Jason Gunthorpe <jgg@nvidia.com>
Cc:     linux-mm@kvack.org, akpm@linux-foundation.org,
        linux-kernel@vger.kernel.org,
        "Sierra Guiza, Alejandro (Alex)" <alex.sierra@amd.com>,
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        Muchun Song <songmuchun@bytedance.com>,
        Ralph Campbell <rcampbell@nvidia.com>,
        David Hildenbrand <david@redhat.com>,
        Matthew Wilcox <willy@infradead.org>
Subject: Re: [PATCH v4 2/2] mm/gup.c: Refactor
 check_and_migrate_movable_pages()
Date:   Wed, 17 Aug 2022 12:01:58 +1000
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Jason Gunthorpe <jgg@nvidia.com> writes:

> On Tue, Aug 16, 2022 at 03:29:20PM +1000, Alistair Popple wrote:
>
>> >> +static long check_and_migrate_movable_pages(unsigned long nr_pages,
>> >> +					    struct page **pages)
>> >> +{
>> >> +	int ret;
>> >> +	unsigned long collected;
>> >> +	LIST_HEAD(movable_page_list);
>> >> +
>> >> +	collected = collect_longterm_unpinnable_pages(&movable_page_list, nr_pages, pages);
>> >> +	if (!collected)
>> >> +		return 0;
>> >> +
>> >> +	ret = migrate_longterm_unpinnable_pages(&movable_page_list, nr_pages, pages);
>> >> +	if (!ret)
>> >> +		return -EAGAIN;
>> >> +	else
>> >> +		return ret;
>> >
>> > I would drop the else path and just return zero
>>
>> Unless I've misunderstood you I don't think that's correct though.
>> check_and_migrate_movable_pages() needs to return one of 3 conditions:
>>
>>  - 0: All pages are in the correct zone and are still pinned (ie. "success").
>>  - -EAGAIN: Some pages were migrated, all pages need re-pinning.
>>  - errno: Migration failed, pins were dropped and PUP should fail.
>>
>> John's suggested comment update spells this out more clearly.
>
> But that is not what it does..
>
> if (!ret) == if (ret != 0)

Huh? Unless I'm misinterpretting you or am really behind on coffee
if (!ret) is equivalent to if (ret == 0), not if (ret != 0).

Ie:

if (ret == 0)
    return -EAGAIN;
else
    return !0;

Thus we will never return 0 from the above snippet.

>         return -EAGAIN
> thus ret == 0
> return ret == return 0
>
> Jason