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Date:   Wed, 9 Sep 2020 09:01:14 +0100
From:   Christoph Hellwig <hch@infradead.org>
To:     Ming Mao <maoming.maoming@huawei.com>
Cc:     linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
        linux-mm@kvack.org, alex.williamson@redhat.com,
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        jianjay.zhou@huawei.com, weidong.huang@huawei.com,
        peterx@redhat.com, aarcange@redhat.com, wangyunjian@huawei.com,
        willy@infradead.org, jhubbard@nvidia.com
Subject: Re: [PATCH V4 1/2] vfio dma_map/unmap: optimized for hugetlbfs pages
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 <20200908133204.1338-2-maoming.maoming@huawei.com>
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I really don't think this approach is any good.  You workaround
a deficiency in the pin_user_pages API in one particular caller for
one particular use case.

I think you'd rather want either:

 (1) a FOLL_HUGEPAGE flag for the pin_user_pages API family that returns
     a single struct page for any kind of huge page, which would also
     benefit all kinds of other users rather than adding these kinds of
     hacks to vfio.
 (2) add a bvec version of the API that returns a variable size
     "extent"

I had started on (2) a while ago, and here is branch with my code (which
is broken and fails test, but might be a start):

http://git.infradead.org/users/hch/misc.git/shortlog/refs/heads/gup-bvec

But for now I wonder if (1) is the better start, which could still be
reused to (2) later.