linux-kernel - Re: [PATCH V4 1/2] vfio dma_map/unmap: optimized for hugetlbfs pages

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Date:   Wed, 9 Sep 2020 09:01:14 +0100
From:   Christoph Hellwig <hch@...radead.org>
To:     Ming Mao <maoming.maoming@...wei.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        linux-mm@...ck.org, alex.williamson@...hat.com,
        akpm@...ux-foundation.org, cohuck@...hat.com,
        jianjay.zhou@...wei.com, weidong.huang@...wei.com,
        peterx@...hat.com, aarcange@...hat.com, wangyunjian@...wei.com,
        willy@...radead.org, jhubbard@...dia.com
Subject: Re: [PATCH V4 1/2] vfio dma_map/unmap: optimized for hugetlbfs pages

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.