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Date: Wed, 8 May 2024 21:49:39 -0700
From: Christoph Hellwig <hch@infradead.org>
To: Pavel Begunkov <asml.silence@gmail.com>
Cc: Christoph Hellwig <hch@infradead.org>, Jason Gunthorpe <jgg@ziepe.ca>,
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Subject: Re: [RFC PATCH net-next v8 02/14] net: page_pool: create hooks for
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On Wed, May 08, 2024 at 06:02:14PM +0100, Pavel Begunkov wrote:
> Well, the example fell flat, but you don't use dmabuf when there are
> no upsides from using it. For instance, when you already have pinned
> pages, you're going to use pages, and there are no other refcounting
> concerns.

Sure.

> Unless there is an advantage of dmabufs over FOLL_LONGTERM
> that I don't know about when used with normal user pages.

The advantages of using a dma-buf over FOLL_LONGTERM are:

 a) you pre-dma map, which is a significant performance advantage for
    IOMMU-based setups
 b) you support any dma-buf exported and not just user memory.  This
    is primarily important for PCIe P2P, but there might be other
    useful exporters as well

> > wish io_uring would have just implemented them from the start instead of
> > the current fixed buffers that are not quite as useful by not
> > pre-mapping DMA and not supporting P2P.
> 
> fdget(dmabuf) would be horrible, I assume that's not the suggestion.

I'm not even sure what you mean with that.