Re: [PATCH 1/6] powerpc/mm/32: Use pfn_valid to check if pointer is in RAM

From: "Jonathan Neuschäfer" <j.neuschaefer@gmx.net>
To: christophe leroy <christophe.leroy@c-s.fr>
Cc: "Jonathan Neuschäfer" <j.neuschaefer@gmx.net>,
	linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org,
	"Michael Ellerman" <mpe@ellerman.id.au>,
	linux-mm@kvack.org, "Joel Stanley" <joel@jms.id.au>,
	"Benjamin Herrenschmidt" <benh@kernel.crashing.org>,
	"Paul Mackerras" <paulus@samba.org>,
	"Balbir Singh" <bsingharora@gmail.com>,
	"Guenter Roeck" <linux@roeck-us.net>
Subject: Re: [PATCH 1/6] powerpc/mm/32: Use pfn_valid to check if pointer is in RAM
Date: Wed, 21 Feb 2018 14:51:19 +0100	[thread overview]
Message-ID: <20180221135119.d3qgvdck5yruomi7@latitude> (raw)
In-Reply-To: <0d14cb2c-dd00-d258-cb15-302b2a9d684f@c-s.fr>

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Hello Christophe,

On Tue, Feb 20, 2018 at 06:45:09PM +0100, christophe leroy wrote:
[...]
> > -	if (slab_is_available() && (p < virt_to_phys(high_memory)) &&
> > +	if (slab_is_available() && pfn_valid(__phys_to_pfn(p)) &&
> 
> I'm not sure this is equivalent:
> 
> high_memory = (void *) __va(max_low_pfn * PAGE_SIZE);
> #define ARCH_PFN_OFFSET		((unsigned long)(MEMORY_START >> PAGE_SHIFT))
> #define pfn_valid(pfn)		((pfn) >= ARCH_PFN_OFFSET && (pfn) < max_mapnr)
> set_max_mapnr(max_pfn);
> 
> So in the current implementation it checks against max_low_pfn while your
> patch checks against max_pfn
> 
> 	max_low_pfn = max_pfn = memblock_end_of_DRAM() >> PAGE_SHIFT;
> #ifdef CONFIG_HIGHMEM
> 	max_low_pfn = lowmem_end_addr >> PAGE_SHIFT;
> #endif

Good point, I haven't considered CONFIG_HIGHMEM before.

As far as I understand it, in the non-CONFIG_HIGHMEM case:

  - max_low_pfn is set to the same value as max_pfn, so the ioremap
    check should detect the same PFNs as RAM.

and with CONFIG_HIGHMEM:

  - max_low_pfn is set to lowmem_end_addr >> PAGE_SHIFT
  - but max_pfn isn't

So, I think you're right.

While looking through arch/powerpc/mm, I noticed that there's a
page_is_ram function, which simply uses the memblocks directly, on
PPC32. It seems like a good candidate for the RAM check in
__ioremap_caller, except that there's this code, which apparently
trashes memblock 0 completely on non-CONFIG_NEED_MULTIPLE_NODES:

  https://elixir.bootlin.com/linux/v4.16-rc2/source/arch/powerpc/mm/mem.c#L223

Thanks,
Jonathan Neuschäfer

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