From: Ilias Apalodimas <ilias.apalodimas@linaro.org>
To: Matthew Wilcox <willy@infradead.org>
Cc: Arnd Bergmann <arnd@kernel.org>,
Grygorii Strashko <grygorii.strashko@ti.com>,
netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
linux-mips@vger.kernel.org, linux-mm@kvack.org,
Jesper Dangaard Brouer <brouer@redhat.com>,
Matteo Croce <mcroce@linux.microsoft.com>,
linuxppc-dev@lists.ozlabs.org, Christoph Hellwig <hch@lst.de>,
linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH 1/1] mm: Fix struct page layout on 32-bit systems
Date: Wed, 14 Apr 2021 14:56:02 +0300 [thread overview]
Message-ID: <YHbYUvOYedojZAoB@apalos.home> (raw)
In-Reply-To: <20210414115052.GS2531743@casper.infradead.org>
On Wed, Apr 14, 2021 at 12:50:52PM +0100, Matthew Wilcox wrote:
> On Wed, Apr 14, 2021 at 10:10:44AM +0200, Jesper Dangaard Brouer wrote:
> > Yes, indeed! - And very frustrating. It's keeping me up at night.
> > I'm dreaming about 32 vs 64 bit data structures. My fitbit stats tell
> > me that I don't sleep well with these kind of dreams ;-)
>
> Then you're going to love this ... even with the latest patch, there's
> still a problem. Because dma_addr_t is still 64-bit aligned _as a type_,
> that forces the union to be 64-bit aligned (as we already knew and worked
> around), but what I'd forgotten is that forces the entirety of struct
> page to be 64-bit aligned. Which means ...
>
> /* size: 40, cachelines: 1, members: 4 */
> /* padding: 4 */
> /* forced alignments: 1 */
> /* last cacheline: 40 bytes */
> } __attribute__((__aligned__(8)));
>
> .. that we still have a hole! It's just moved from being at offset 4
> to being at offset 36.
>
> > That said, I think we need to have a quicker fix for the immediate
> > issue with 64-bit bit dma_addr on 32-bit arch and the misalignment hole
> > it leaves[3] in struct page. In[3] you mention ppc32, does it only
> > happens on certain 32-bit archs?
>
> AFAICT it happens on mips32, ppc32, arm32 and arc. It doesn't happen
> on x86-32 because dma_addr_t is 32-bit aligned.
>
> Doing this fixes it:
>
> +++ b/include/linux/types.h
> @@ -140,7 +140,7 @@ typedef u64 blkcnt_t;
> * so they don't care about the size of the actual bus addresses.
> */
> #ifdef CONFIG_ARCH_DMA_ADDR_T_64BIT
> -typedef u64 dma_addr_t;
> +typedef u64 __attribute__((aligned(sizeof(void *)))) dma_addr_t;
> #else
> typedef u32 dma_addr_t;
> #endif
>
> > I'm seriously considering removing page_pool's support for doing/keeping
> > DMA-mappings on 32-bit arch's. AFAIK only a single driver use this.
>
> ... if you're going to do that, then we don't need to do this.
FWIW I already proposed that to Matthew in private a few days ago...
II am not even sure the AM572x has that support. I'd much prefer getting rid
of it as well, instead of overcomplicating the struct for a device noone is
going to need.
Cheers
/Ilias
next prev parent reply other threads:[~2021-04-14 11:56 UTC|newest]
Thread overview: 29+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-04-10 20:52 [PATCH 0/1] Fix struct page layout on 32-bit systems Matthew Wilcox (Oracle)
2021-04-10 20:52 ` [PATCH 1/1] mm: " Matthew Wilcox (Oracle)
2021-04-11 9:43 ` Jesper Dangaard Brouer
2021-04-11 10:33 ` Matthew Wilcox
2021-04-12 1:15 ` Matthew Wilcox
2021-04-14 8:10 ` Jesper Dangaard Brouer
2021-04-14 11:50 ` Matthew Wilcox
2021-04-14 11:56 ` Ilias Apalodimas [this message]
2021-04-14 15:52 ` David Laight
2021-04-14 19:13 ` Jesper Dangaard Brouer
2021-04-14 21:35 ` Matthew Wilcox
2021-04-14 21:56 ` David Laight
2021-04-15 18:08 ` Jesper Dangaard Brouer
2021-04-15 18:21 ` Matthew Wilcox
2021-04-15 21:11 ` David Laight
2021-04-15 22:22 ` Matthew Wilcox
2021-04-16 7:32 ` David Laight
2021-04-16 11:05 ` Matthew Wilcox
2021-04-16 15:27 ` Matthew Wilcox
2021-04-16 17:08 ` Jesper Dangaard Brouer
2021-04-17 3:19 ` Matthew Wilcox
2021-04-17 10:31 ` Arnd Bergmann
2021-04-17 13:56 ` Matthew Wilcox
2021-04-17 17:30 ` Arnd Bergmann
2021-04-17 10:59 ` David Laight
2021-04-19 6:34 ` Christoph Hellwig
2021-04-19 7:15 ` Ilias Apalodimas
2021-04-12 18:23 ` Matthew Wilcox
2021-04-13 8:21 ` David Laight
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