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Date: Mon, 16 Oct 2023 18:31:38 -0700
From: Jakub Kicinski <kuba@kernel.org>
To: Yunsheng Lin <linyunsheng@huawei.com>
Cc: <davem@davemloft.net>, <pabeni@redhat.com>, <netdev@vger.kernel.org>,
 <linux-kernel@vger.kernel.org>, Alexander Lobakin
 <aleksander.lobakin@intel.com>, Lorenzo Bianconi <lorenzo@kernel.org>,
 Alexander Duyck <alexander.duyck@gmail.com>, Liang Chen
 <liangchen.linux@gmail.com>, Guillaume Tucker
 <guillaume.tucker@collabora.com>, Matthew Wilcox <willy@infradead.org>,
 Linux-MM <linux-mm@kvack.org>, Jesper Dangaard Brouer <hawk@kernel.org>,
 Ilias Apalodimas <ilias.apalodimas@linaro.org>, Eric Dumazet
 <edumazet@google.com>
Subject: Re: [PATCH net-next v11 1/6] page_pool: fragment API support for
 32-bit arch with 64-bit DMA
Message-ID: <20231016183138.2c4366a7@kernel.org>
In-Reply-To: <20231013064827.61135-2-linyunsheng@huawei.com>
References: <20231013064827.61135-1-linyunsheng@huawei.com>
	<20231013064827.61135-2-linyunsheng@huawei.com>
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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..