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Date:   Wed, 9 Sep 2020 14:41:36 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     Christoph Hellwig <hch@infradead.org>
Cc:     Ming Mao <maoming.maoming@huawei.com>,
        linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
        linux-mm@kvack.org, alex.williamson@redhat.com,
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        peterx@redhat.com, aarcange@redhat.com, wangyunjian@huawei.com,
        jhubbard@nvidia.com
Subject: Re: [PATCH V4 1/2] vfio dma_map/unmap: optimized for hugetlbfs pages
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 <20200909080114.GA8321@infradead.org>
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On Wed, Sep 09, 2020 at 09:01:14AM +0100, Christoph Hellwig wrote:
> 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.

This seems to be similar to a flag I added last week to
pagecache_get_page() called FGP_HEAD:

+ * * %FGP_HEAD - If the page is present and a THP, return the head page
+ *   rather than the exact page specified by the index.

I think "return the head page" is probably what we want from what I
understand of this patch.  The caller can figure out the appropriate
bv_offset / bv_len for a bio_vec, if that's what they want to do with it.

http://git.infradead.org/users/willy/pagecache.git/commitdiff/ee88eeeb6b0f35e95ef82b11dfc24dc04c3dcad8 is the exact commit where I added that, but it depends on a number of other patches in this series:
http://git.infradead.org/users/willy/pagecache.git/shortlog

I'm going to send out a subset of patches later today which will include
that one and some others.  I haven't touched the GUP paths at all in
that series, but it's certainly going to make THPs (of various sizes)
much more present in the system.