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Date: Mon, 19 Apr 2021 10:15:58 +0300
From: Ilias Apalodimas <ilias.apalodimas@linaro.org>
To: Christoph Hellwig <hch@lst.de>
Subject: Re: [PATCH 1/1] mm: Fix struct page layout on 32-bit systems
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Cc: Arnd Bergmann <arnd@kernel.org>,
 Grygorii Strashko <grygorii.strashko@ti.com>,
 "netdev@vger.kernel.org" <netdev@vger.kernel.org>,
 "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
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 "linux-mm@kvack.org" <linux-mm@kvack.org>,
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 <linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org>

Hi Christoph,

On Mon, Apr 19, 2021 at 08:34:41AM +0200, Christoph Hellwig wrote:
> On Fri, Apr 16, 2021 at 04:27:55PM +0100, Matthew Wilcox wrote:
> > On Thu, Apr 15, 2021 at 08:08:32PM +0200, Jesper Dangaard Brouer wrote:
> > > See below patch.  Where I swap32 the dma address to satisfy
> > > page->compound having bit zero cleared. (It is the simplest fix I could
> > > come up with).
> > 
> > I think this is slightly simpler, and as a bonus code that assumes the
> > old layout won't compile.
> 
> So, why do we even do this crappy overlay of a dma address?  This just
> all seems like a giant hack.  Random subsystems should not just steal
> a few struct page fields as that just turns into the desasters like the
> one we've seen here or probably something worse next time.

The page pool API was using page->private in the past to store these kind of
info. That caused a problem to begin with, since it would fail  on 32-bit
systems with 64bit DMA.  We had a similar discussion on the past but decided
struct page is the right place to store that [1].

Another advantage is that we can now use the information from the networking 
subsystem and enable recycling of SKBs and SKB fragments, by using the stored 
metadata of struct page [2].

[1] https://lore.kernel.org/netdev/20190207.132519.1698007650891404763.davem@davemloft.net/
[2] https://lore.kernel.org/netdev/20210409223801.104657-1-mcroce@linux.microsoft.com/

Cheers
/Ilias