linux-kernel - Re: [PATCH net-next v11 1/6] page_pool: fragment API support for 32-bit arch with 64-bit DMA

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Message-ID: <20231016183138.2c4366a7@kernel.org>
Date:   Mon, 16 Oct 2023 18:31:38 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Yunsheng Lin <linyunsheng@...wei.com>
Cc:     <davem@...emloft.net>, <pabeni@...hat.com>,
        <netdev@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        Alexander Lobakin <aleksander.lobakin@...el.com>,
        Lorenzo Bianconi <lorenzo@...nel.org>,
        Alexander Duyck <alexander.duyck@...il.com>,
        Liang Chen <liangchen.linux@...il.com>,
        Guillaume Tucker <guillaume.tucker@...labora.com>,
        Matthew Wilcox <willy@...radead.org>,
        Linux-MM <linux-mm@...ck.org>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        Ilias Apalodimas <ilias.apalodimas@...aro.org>,
        Eric Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH net-next v11 1/6] page_pool: fragment API support for
 32-bit arch with 64-bit DMA

On Fri, 13 Oct 2023 14:48:21 +0800 Yunsheng Lin wrote:
> Currently page_pool_alloc_frag() is not supported in 32-bit
> arch with 64-bit DMA because of the overlap issue between
> pp_frag_count and dma_addr_upper in 'struct page' for those
> arches, which seems to be quite common, see [1], which means
> driver may need to handle it when using fragment API.
> 
> It is assumed that the combination of the above arch with an
> address space >16TB does not exist, as all those arches have
> 64b equivalent, it seems logical to use the 64b version for a
> system with a large address space. It is also assumed that dma
> address is page aligned when we are dma mapping a page aligned
> buffer, see [2].
> 
> That means we're storing 12 bits of 0 at the lower end for a
> dma address, we can reuse those bits for the above arches to
> support 32b+12b, which is 16TB of memory.
> 
> If we make a wrong assumption, a warning is emitted so that
> user can report to us.

Let me apply this one already, I think it should be uncontroversial
from review perspective. And the more time it gets in linux-next
the better..