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

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Message-ID: <Yv18eGBvWKi3l/eW@nvidia.com>
Date:   Wed, 17 Aug 2022 20:40:40 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Alistair Popple <apopple@...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()

On Thu, Aug 18, 2022 at 09:24:28AM +1000, Alistair Popple wrote:
> 	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.

That isn't "success oriented flow" :)

> > But why return 0 from the helper function in the first place?
> 
> To stick with the paradigm of 0 == success. Ie.

But it doesn't mean success if we squashed it to EAGAIN here.

-EAGAIN == retry
      0 == success, real success
everything else == -ERRNO, failure

Stick with one convection in all the functions in this file in this grouping.

Jason