linux-kernel - Re: [PATCH] ARM: fix bug which lowmem size is limited to 760MB

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Date:	Mon, 7 Sep 2015 22:01:41 -0400 (EDT)
From:	Nicolas Pitre <nicolas.pitre@...aro.org>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
cc:	Arnd Bergmann <arnd@...db.de>,
	linux-arm-kernel@...ts.infradead.org,
	Yongtaek Lee <ytk.lee@...sung.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ARM: fix bug which lowmem size is limited to 760MB

On Tue, 8 Sep 2015, Russell King - ARM Linux wrote:

> On Mon, Sep 07, 2015 at 03:40:36PM -0400, Nicolas Pitre wrote:
> > On Mon, 7 Sep 2015, Arnd Bergmann wrote:
> > 
> > > On Thursday 03 September 2015 21:24:00 Nicolas Pitre wrote:
> > > > If 768MB targets were common place then it could be worth changing the 
> > > > default vmalloc size to accommodate this memory size and testing all the 
> > > > other targets to make sure no regressions are introduced.  But given it 
> > > > is easy to change the default via the kernel cmdline, and that you lose 
> > > > only 8 MB otherwise, I don't think it is worth the trouble and/or the 
> > > > risk.
> > > 
> > > Agreed.
> > 
> > Well... I think there is a better solution.
> 
> Doesn't this clash with things like:
> 
> #define UNCACHEABLE_ADDR        0xff000000      /* IRQ_STAT */
> 
> ?

It looks like the move might actually "fix" it.  That UNCACHEABLE_ADDR 
is mapped with:

static struct map_desc ebsa110_io_desc[] __initdata = {
        /*
         * sparse external-decode ISAIO space
         */
        {       /* IRQ_STAT/IRQ_MCLR */
                .virtual        = (unsigned long)IRQ_STAT,
                .pfn            = __phys_to_pfn(TRICK4_PHYS),
                .length         = TRICK4_SIZE,
                .type           = MT_DEVICE
        },
[...]
};

This is passed to iotable_init(), then to create_mapping(). There you 
have:

        if ((md->type == MT_DEVICE || md->type == MT_ROM) &&
            md->virtual >= PAGE_OFFSET &&
            (md->virtual < VMALLOC_START || md->virtual >= VMALLOC_END)) {
                pr_warn("BUG: mapping for 0x%08llx at 0x%08lx out of vmalloc space\n",
                        (long long)__pfn_to_phys((u64)md->pfn), md->virtual);
        }

So you must have hit the above warning somehow. Incidentally, this 
IRQ_STAT entry is the only one that happened to be outside the vmalloc 
area. By moving VMALLOC_END from 0xff000000 to 0xff800000 the warning 
will be gone.


Nicolas
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