Re: Calling vmalloc_to_page() on ioremap memory?

[Alexander Potapenko <glider@google.com>]

On Mon, Jun 25, 2018 at 6:00 PM Mark Rutland <mark.rutland@arm.com> wrote:
>
> On Mon, Jun 25, 2018 at 04:59:23PM +0200, Alexander Potapenko wrote:
> > Hi Ard, Mark, Andrew and others,
> >
> > AFAIU, commit 029c54b09599573015a5c18dbe59cbdf42742237 ("mm/vmalloc.c:
> > huge-vmap: fail gracefully on unexpected huge vmap mappings") was
> > supposed to make vmalloc_to_page() return NULL for pointers not
> > returned by vmalloc().
>
> It's a little more subtle than that -- avoiding an edge case where we
> unexpectedly hit huge mappings, rather than determining whether an
> address same from vmalloc().
Ok, but anyway, acpi_os_ioremap() creates a huge page mapping via
__ioremap_caller() (see
https://elixir.bootlin.com/linux/latest/source/arch/x86/mm/ioremap.c#L133)
Shouldn't these checks detect that as well?

> > For memory error detection purposes I'm trying to map the addresses
> > from the vmalloc range to valid struct pages, or at least make sure
> > there's no struct page for a given address.
> > Looking up the vmap_area_root rbtree isn't an option, as this must be
> > done from instrumented code, including interrupt handlers.
>
> I'm not sure how you can do this without looking at VMAs.
>
> In general, the vmalloc area can contain addresses which are not memory,
> and this cannot be detremined from the address alone.
I thought this was exactly what vmalloc_to_page() did, but apparently no.

> You *might* be able to get away with pfn_valid(vmalloc_to_pfn(x)), but
> IIRC there's some disagreement on the precise meaning of pfn_valid(), so
> that might just tell you that the address happens to fall close to some
> valid memory.
This appears to work, at least for ACPI mappings. I'll check other cases though.
Thank you!
> Thanks,
> Mark.



-- 
Alexander Potapenko
Software Engineer

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