linux-kernel - RE: [PATCH] memremap: Fix NULL pointer BUG in get_zone_device

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Date:   Wed, 24 Aug 2016 04:28:54 +0000
From:   "Kani, Toshimitsu" <toshi.kani@....com>
To:     Dan Williams <dan.j.williams@...el.com>
CC:     "Mulumudi, Abhilash Kumar" <m.abhilash-kumar@....com>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        "ard.biesheuvel@...aro.org" <ard.biesheuvel@...aro.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "brian.starkey@....com" <brian.starkey@....com>
Subject: RE: [PATCH] memremap: Fix NULL pointer BUG in get_zone_device_page()

> On Tue, Aug 23, 2016 at 7:53 PM, Dan Williams <dan.j.williams@...el.com>
> wrote:
> > On Tue, Aug 23, 2016 at 6:29 PM, Kani, Toshimitsu <toshi.kani@....com>
> wrote:
> >>> On Tue, Aug 23, 2016 at 4:47 PM, Kani, Toshimitsu
> <toshi.kani@....com>
> >>> wrote:
 :
> >>
> >> crash> p {struct vmem_altmap} 0xffff88046d045410
> >> $6 = {
> >>   base_pfn = 0x480000,
> >>   reserve = 0x2,        // PHYS_PFN(SZ_8K)
> >>   free = 0x101fe,
> >>   align = 0x1fe,
> >>   alloc = 0x10000
> >> }
> >
> > Ah, so, on second look the 0x490200000 data offset looks correct.  The
> > total size of the address range is 16GB which equates to 256MB needed
> > for struct page, plus 2MB more to re-align the data on the next 2MB
> > boundary.
> >
> > The question now is why is the guest faulting on an access to an
> > address less than 0x490200000?
> 
> Does the attached patch fix this for you?

Yeah, that makes sense.  I will test it tomorrow.

BTW, why does devm_memremap_pages() put a whole range to pgmap_radix
as device memory, but only initialize page->pgmap for its data range?  Is there
particular reason for this inconsistency?

Thanks,
-Toshi