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

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Message-ID: <20180221135119.d3qgvdck5yruomi7@latitude>
Date:   Wed, 21 Feb 2018 14:51:19 +0100
From:   Jonathan Neuschäfer <j.neuschaefer@....net>
To:     christophe leroy <christophe.leroy@....fr>
Cc:     Jonathan Neuschäfer <j.neuschaefer@....net>,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
        Michael Ellerman <mpe@...erman.id.au>, linux-mm@...ck.org,
        Joel Stanley <joel@....id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Balbir Singh <bsingharora@...il.com>,
        Guenter Roeck <linux@...ck-us.net>
Subject: Re: [PATCH 1/6] powerpc/mm/32: Use pfn_valid to check if pointer is
 in RAM

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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