netdev - Re: [RFC PATCH] net/mlx4: Get rid of page operation after dma_alloc

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Message-Id: <20181218.105134.1658119814567562312.davem@davemloft.net>
Date:   Tue, 18 Dec 2018 10:51:34 -0800 (PST)
From:   David Miller <davem@...emloft.net>
To:     hch@....de
Cc:     jgg@...pe.ca, swarren@...dotorg.org, tariqt@...lanox.com,
        xavier.huwei@...wei.com, netdev@...r.kernel.org,
        linux-rdma@...r.kernel.org, dledford@...hat.com
Subject: Re: [RFC PATCH] net/mlx4: Get rid of page operation after
 dma_alloc_coherent

From: Christoph Hellwig <hch@....de>
Date: Tue, 18 Dec 2018 19:50:12 +0100

> On Tue, Dec 18, 2018 at 11:43:08AM -0700, Jason Gunthorpe wrote:
>> So the problem here is that on some arches
>> 
>>  sg_set_buf(sg, dma_coherent_buf, size)
>>  p = sg_virt(sg);
>>  assert(p == dma_coherent_buf);
> 
> dma allocations purely return a virtual address, you must never
> call virt_to_page or virt_to_phys on them, which sg_set_buf
> will do.  On many architectures this will give your the wrong
> result as the coherent DMA address is a vmap or ioremap address.

Correct.

And I don't think it makes sense to create fake pages for these
remapped areas to resolve to.