Subject: [PATCH 3/6] ARM: mm: Drop the lowmem watermark check from virt_addr_valid()

From: [email-masked] (Marc Zyngier)
Date: Fri, 15 Nov 2013 11:43:37 +0000

On 15/11/13 00:22, Christoffer Dall wrote:
> On Thu, Nov 14, 2013 at 02:37:43PM -0500, Santosh Shilimkar wrote:
>> Slab allocator can allocate memory beyond the lowmem watermark
>> which can lead to false failure of virt_addr_valid().
>>
>> So drop the check. The issue was seen with percpu_alloc()
>> in KVM code which was allocating memory beyond lowmem watermark.
>>
>> Am not completly sure whether this is the right fix and if it could
>> impact any other user of virt_addr_valid(). Without this fix as
>> pointed out the KVM init was failing in my testing.
>>
>> Cc: Christoffer Dall <[email-masked]>
>> Cc: Marc Zyngier <[email-masked]>
>> Cc: Russell King <[email-masked]>
>> Cc: Catalin Marinas <[email-masked]>
>> Cc: Will Deacon <[email-masked]>
>>
>> Signed-off-by: Santosh Shilimkar <[email-masked]>
>> ---
>>  arch/arm/include/asm/memory.h |    3 +--
>>  1 file changed, 1 insertion(+), 2 deletions(-)
>>
>> diff --git a/arch/arm/include/asm/memory.h b/arch/arm/include/asm/memory.h
>> index 4dd2145..412da47 100644
>> --- a/arch/arm/include/asm/memory.h
>> +++ b/arch/arm/include/asm/memory.h
>> @@ -343,8 +343,7 @@ static inline __deprecated void *bus_to_virt(unsigned long x)
>>  #define ARCH_PFN_OFFSET		PHYS_PFN_OFFSET
>>  
>>  #define virt_to_page(kaddr)	pfn_to_page(__pa(kaddr) >> PAGE_SHIFT)
>> -#define virt_addr_valid(kaddr)	((unsigned long)(kaddr) >= PAGE_OFFSET && (unsigned long)(kaddr) < (unsigned long)high_memory)
>> -
>> +#define virt_addr_valid(kaddr)	((unsigned long)(kaddr) >= PAGE_OFFSET)
>>  #endif
>>  
>>  #include <asm-generic/memory_model.h>
>> -- 
>> 1.7.9.5
>>
> 
> This looks wrong to me.  Check Documentation/arm/memory.txt, this would
> return true for the VMALLOC region, which would cause virt_to_phys to
> give you something invalid, which would be bad.
> 
> We use the check in create_hyp_mappings to be sure that the physical
> address returned by virt_to_phys is valid and that if we're mapping more
> than one page that those pages are physically contiguous.
> 
> So if you want to get rid of this check, you need to change the mapping
> functionality to obtain the physical address by walking the page table
> mappings for each page that you are mapping instead.  Or limit each call
> to a single page in size and take the physical address as input and use
> per_cpu_ptr_to_phys at the caller side instead.
> 
> Alternatively, we need to get rid of alloc_percpu and use regular
> kmallocs instead, unless anyone else knows of an even better way.

alloc_percpu has nice properties (cache locality, mostly).

One way out of it would be to give percpu stuff a special treatment. Can
you try the attach patch as a first approximation? It needs more
refinements (allocations straddling two pages?), but I think that's the
right sort of things.

Let me know how it works for you.

	M.
-- 
Jazz is not dead. It just smells funny...
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