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Date:   Sat, 17 Apr 2021 04:19:06 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     Jesper Dangaard Brouer <brouer@redhat.com>
Cc:     David Laight <David.Laight@aculab.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        "netdev@vger.kernel.org" <netdev@vger.kernel.org>,
        "linuxppc-dev@lists.ozlabs.org" <linuxppc-dev@lists.ozlabs.org>,
        "linux-arm-kernel@lists.infradead.org" 
        <linux-arm-kernel@lists.infradead.org>,
        "linux-mips@vger.kernel.org" <linux-mips@vger.kernel.org>,
        Ilias Apalodimas <ilias.apalodimas@linaro.org>,
        Matteo Croce <mcroce@linux.microsoft.com>,
        Grygorii Strashko <grygorii.strashko@ti.com>,
        Arnd Bergmann <arnd@kernel.org>, Christoph Hellwig <hch@lst.de>
Subject: Re: [PATCH 1/1] mm: Fix struct page layout on 32-bit systems
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On Fri, Apr 16, 2021 at 07:08:23PM +0200, Jesper Dangaard Brouer wrote:
> On Fri, 16 Apr 2021 16:27:55 +0100
> Matthew Wilcox <willy@infradead.org> 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.
> 
> This is clever, I like it!  When reading the code one just have to
> remember 'unsigned long' size difference between 64-bit vs 32-bit.
> And I assume compiler can optimize the sizeof check out then doable.

I checked before/after with the replacement patch that doesn't
have compiler warnings.  On x86, there is zero codegen difference
(objdump -dr before/after matches exactly) for both x86-32 with 32-bit
dma_addr_t and x86-64.  For x86-32 with 64-bit dma_addr_t, the compiler
makes some different inlining decisions in page_pool_empty_ring(),
page_pool_put_page() and page_pool_put_page_bulk(), but it's not clear
to me that they're wrong.

Function                                     old     new   delta
page_pool_empty_ring                         387     307     -80
page_pool_put_page                           604     516     -88
page_pool_put_page_bulk                      690     517    -173