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Message-ID: <20370ebb-8e63-2065-307a-81fa874eced2@nvidia.com>
Date:   Wed, 17 Aug 2022 16:05:47 -0700
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Subject: Re: [PATCH v4 2/2] mm/gup.c: Refactor
 check_and_migrate_movable_pages()
Content-Language: en-US
To:     Jason Gunthorpe <jgg@nvidia.com>
Cc:     Alistair Popple <apopple@nvidia.com>, linux-mm@kvack.org,
        akpm@linux-foundation.org, linux-kernel@vger.kernel.org,
        "Sierra Guiza, Alejandro (Alex)" <alex.sierra@amd.com>,
        Chaitanya Kulkarni <kch@nvidia.com>,
        Dan Williams <dan.j.williams@intel.com>,
        Felix Kuehling <Felix.Kuehling@amd.com>,
        Logan Gunthorpe <logang@deltatee.com>,
        Miaohe Lin <linmiaohe@huawei.com>,
        Muchun Song <songmuchun@bytedance.com>,
        Ralph Campbell <rcampbell@nvidia.com>,
        David Hildenbrand <david@redhat.com>,
        Matthew Wilcox <willy@infradead.org>
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From:   John Hubbard <jhubbard@nvidia.com>
In-Reply-To: <Yv1wntznpzSXuTYh@nvidia.com>
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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