Re: [RFC PATCH 1/2] arm64: mm: Make virt_addr_valid to check for pfn_valid again

[Mike Rapoport <rppt@kernel.org>]

Hi,

On Mon, Aug 02, 2021 at 01:19:48PM +0100, Catalin Marinas wrote:
> Adding Mike and Anshuman,
> 
> On Wed, Jul 28, 2021 at 07:44:15PM +0300, Oleksandr Tyshchenko wrote:
> > From: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
> > 
> > The problem is that Arm's implementation of virt_addr_valid()
> > leads to memblock_is_map_memory() check, which will fail for
> > ZONE_DEVICE based addresses. But, the pfn_valid() check in turn
> > is able to cope with ZONE_DEVICE based memory.
> > 
> > You can find a good explanation of that problem at:
> > https://lore.kernel.org/lkml/1614921898-4099-2-git-send-email-anshuman.khandual@arm.com
> > 
> > Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
> > ---
> > I am not quite sure whether it is a "correct" place and
> > the change itself, I just partially restored a behaviour before:
> > https://lore.kernel.org/lkml/20210511100550.28178-4-rppt@kernel.org
> > So, the target of this patch is to get a feedback how to resolve
> > this properly if, of course, this really needs to be resolved
> > (I might miss important bits here).
> > 
> > It is worth mentioning that patch doesn't fix the current code base
> > (if I am not mistaken, no one calls virt_addr_valid() on Arm64 for
> > ZONE_DEVICE based addresses at the moment, so it seems that nothing
> > is broken), the fix is intended for the subsequent patch in this
> > series that will try to enable Xen's "unpopulated-alloc" usage
> > on Arm (it was enabled on x86 so far).
> > Please see:
> > [RFC PATCH 2/2] xen/unpopulated-alloc: Query hypervisor to provide
> > unallocated space
> > 
> > The subsequent patch will enable the code where virt_addr_valid()
> > is used in drivers/xen/unpopulated-alloc.c:fill_list() to check that
> > a virtual address returned by memremap_pages() is valid.
 
> I wonder what the point of calling virt_addr_valid() in fill_list() is?
> If memremap_pages() succeeded, the pages were mapped at the returned
> vaddr, there's no need for an additional virt_addr_valid() check.

The virt_addr_valid() check in fill_list() looks bogus to me as well. If
memremap_pages() succeeds the range is guaranteed to have proper page
table.

I believe the first patch should be rather removal of the virt_addr_valid()
check in fill_list().
 
> > ---
> >  arch/arm64/include/asm/memory.h | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/arch/arm64/include/asm/memory.h b/arch/arm64/include/asm/memory.h
> > index 824a365..1a35a44 100644
> > --- a/arch/arm64/include/asm/memory.h
> > +++ b/arch/arm64/include/asm/memory.h
> > @@ -351,7 +351,7 @@ static inline void *phys_to_virt(phys_addr_t x)
> >  
> >  #define virt_addr_valid(addr)	({					\
> >  	__typeof__(addr) __addr = __tag_reset(addr);			\
> > -	__is_lm_address(__addr) && pfn_is_map_memory(virt_to_pfn(__addr));	\
> > +	__is_lm_address(__addr) && pfn_valid(virt_to_pfn(__addr));	\
> >  })
> 
> pfn_valid() only guarantees the presence of a struct page but not
> necessarily that the virtual address is accessible (valid). So this
> change would break the NOMAP ranges case.

+1

-- 
Sincerely yours,
Mike.