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[94.65.92.88]) by smtp.gmail.com with ESMTPSA id q10sm8586361eds.26.2021.04.19.00.16.00 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 19 Apr 2021 00:16:01 -0700 (PDT) Date: Mon, 19 Apr 2021 10:15:58 +0300 From: Ilias Apalodimas To: Christoph Hellwig Cc: Matthew Wilcox , Jesper Dangaard Brouer , David Laight , "linux-kernel@vger.kernel.org" , "linux-mm@kvack.org" , "netdev@vger.kernel.org" , "linuxppc-dev@lists.ozlabs.org" , "linux-arm-kernel@lists.infradead.org" , "linux-mips@vger.kernel.org" , Matteo Croce , Grygorii Strashko , Arnd Bergmann Subject: Re: [PATCH 1/1] mm: Fix struct page layout on 32-bit systems Message-ID: References: <20210411103318.GC2531743@casper.infradead.org> <20210412011532.GG2531743@casper.infradead.org> <20210414101044.19da09df@carbon> <20210414115052.GS2531743@casper.infradead.org> <20210414211322.3799afd4@carbon> <20210414213556.GY2531743@casper.infradead.org> <20210415200832.32796445@carbon> <20210416152755.GL2531743@casper.infradead.org> <20210419063441.GA18787@lst.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210419063441.GA18787@lst.de> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.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