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

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Date:   Wed, 17 Aug 2022 16:05:47 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     Alistair Popple <apopple@...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 8/17/22 15:50, Jason Gunthorpe wrote:
> 
> But why return 0 from the helper function in the first place?
> 

The caller is looking for three distinct return code cases: 0, -EAGAIN,
or some other -errno.  Unless you restructure more, you need all three
of those cases. The 0 case leads to the caller, __gup_longterm_locked,
returning nr_pages.

Here is the comment block that I recommended earlier in this thread, that
documents those cases:

    https://lore.kernel.org/r/dc8fd102-ba30-d980-bdbb-11f39326103d@nvidia.com




thanks,
-- 
John Hubbard
NVIDIA