linux-kernel - Re: [PATCH v4 2/2] mm/gup.c: Refactor check_and_migrate_movable

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Message-ID: <87k076tpd1.fsf@nvdebian.thelocal>
Date:   Thu, 18 Aug 2022 09:24:28 +1000
From:   Alistair Popple <apopple@...dia.com>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     John Hubbard <jhubbard@...dia.com>, linux-mm@...ck.org,
        akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
        "Sierra Guiza, Alejandro (Alex)" <alex.sierra@....com>,
        Chaitanya Kulkarni <kch@...dia.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Felix Kuehling <Felix.Kuehling@....com>,
        Logan Gunthorpe <logang@...tatee.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Muchun Song <songmuchun@...edance.com>,
        Ralph Campbell <rcampbell@...dia.com>,
        David Hildenbrand <david@...hat.com>,
        Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH v4 2/2] mm/gup.c: Refactor
 check_and_migrate_movable_pages()


Jason Gunthorpe <jgg@...dia.com> writes:

> On Wed, Aug 17, 2022 at 01:35:12PM -0700, John Hubbard wrote:
>> How's this look to you:

I agree, I think all the refactoring left this written in a weird way. I
was going to suggest this though:

	collected = collect_longterm_unpinnable_pages(&movable_page_list,
						      nr_pages, pages);
	if (collected == 0)
		return 0;

	ret = migrate_longterm_unpinnable_pages(&movable_page_list, nr_pages,
						pages);
	if (ret)
		return ret;

	return -EAGAIN;

Which IMHO looks at lot more normal and sane than what I had.

>> 	collected = collect_longterm_unpinnable_pages(&movable_page_list,
>> 						      nr_pages, pages);
>> 	if (collected == 0)
>> 		return 0;
>>
>> 	ret = migrate_longterm_unpinnable_pages(&movable_page_list, nr_pages,
>> 						pages);
>>
>> 	/* If we got here, we have some unpinnable pages... */
>>
>> 	if (ret == 0) {
>> 		/*
>> 		 * ...and we successfully migrated those pages. Which means that
>> 		 * the caller should retry the operation now.
>> 		 */
>> 		ret = -EAGAIN;
>
> return -EAGAIN
>
>> 	}
>>
>> 	return ret;
>
> But why return 0 from the helper function in the first place?

To stick with the paradigm of 0 == success. Ie.
migrate_longterm_unpinnable_pages() successfully migrated everything
requested. I don't feel particularly strongly about this though - happy
to return -EAGAIN directly from migrate_longterm_unpinnable_pages() and
just pass that return code up the stack if others think it's clearer.

> Jason