From: Jesper Dangaard Brouer <brouer@redhat.com>
To: Matthew Wilcox <willy@infradead.org>
Cc: Arnd Bergmann <arnd@kernel.org>,
Grygorii Strashko <grygorii.strashko@ti.com>,
netdev@vger.kernel.org,
Ilias Apalodimas <ilias.apalodimas@linaro.org>,
linux-mips@vger.kernel.org, linux-kernel@vger.kernel.org,
linux-mm@kvack.org, 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 10:10:44 +0200 [thread overview]
Message-ID: <20210414101044.19da09df@carbon> (raw)
In-Reply-To: <20210412011532.GG2531743@casper.infradead.org>
On Mon, 12 Apr 2021 02:15:32 +0100
Matthew Wilcox <willy@infradead.org> wrote:
> On Sun, Apr 11, 2021 at 11:33:18AM +0100, Matthew Wilcox wrote:
> > Basically, we have three aligned dwords here. We can either alias with
> > @flags and the first word of @lru, or the second word of @lru and @mapping,
> > or @index and @private. @flags is a non-starter. If we use @mapping,
> > then you have to set it to NULL before you free it, and I'm not sure
> > how easy that will be for you. If that's trivial, then we could use
> > the layout:
> >
> > unsigned long _pp_flags;
> > unsigned long pp_magic;
> > union {
> > dma_addr_t dma_addr; /* might be one or two words */
> > unsigned long _pp_align[2];
> > };
> > unsigned long pp_pfmemalloc;
> > unsigned long xmi;
>
> I forgot about the munmap path. That calls zap_page_range() which calls
> set_page_dirty() which calls page_mapping(). If we use page->mapping,
> that's going to get interpreted as an address_space pointer.
>
> *sigh*. Foiled at every turn.
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 ;-)
> I'm kind of inclined towards using two (or more) bits for PageSlab as
> we discussed here:
>
> https://lore.kernel.org/linux-mm/01000163efe179fe-d6270c58-eaba-482f-a6bd-334667250ef7-000000@email.amazonses.com/
>
> so we have PageKAlloc that's true for PageSlab, PagePool, PageDMAPool,
> PageVMalloc, PageFrag and maybe a few other kernel-internal allocations.
I actually like this idea a lot. I also think it will solve or remove
Matteo/Ilias'es[2] need to introduce the pp_magic signature. Ilias do
say[1] that page_pool pages could be used for TCP RX zerocopy, but I
don't think we should "allow" that (meaning page_pool should drop the
DMA-mapping and give up recycling). I should argue why in that thread.
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?
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.
> (see also here:)
> https://lore.kernel.org/linux-mm/20180518194519.3820-18-willy@infradead.org/
[1] https://lore.kernel.org/netdev/YHHuE7g73mZNrMV4@enceladus/
[2] https://patchwork.kernel.org/project/netdevbpf/patch/20210409223801.104657-3-mcroce@linux.microsoft.com/
[3] https://lore.kernel.org/linux-mm/20210410024313.GX2531743@casper.infradead.org/
--
Best regards,
Jesper Dangaard Brouer
MSc.CS, Principal Kernel Engineer at Red Hat
LinkedIn: http://www.linkedin.com/in/brouer
next prev parent reply other threads:[~2021-04-14 8:11 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 [this message]
2021-04-14 11:50 ` Matthew Wilcox
2021-04-14 11:56 ` Ilias Apalodimas
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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