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

From: [email-masked] (Santosh Shilimkar)
Date: Fri, 15 Nov 2013 09:55:16 -0500

On Friday 15 November 2013 06:43 AM, Marc Zyngier wrote:
> 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.
> 
Host boots bug guest fails. Patch needs small update as mentioned
with inline patch.

> From 01bc1c8eaebdd70b1ea044050144b9bfb3375f82 Mon Sep 17 00:00:00 2001
> From: Marc Zyngier <[email-masked]>
> Date: Fri, 15 Nov 2013 11:36:36 +0000
> Subject: [PATCH] arm/arm64: KVM: introduce new mapping API for percpu mappings
> 
> Using virt_to_phys on percpu mappings is horribly wrong (my own bad).
> Thankfully, the kernel offers a way to obtain the phisical address
> of such a mapping.
> 
> Add a new "create_hyp_percpu_mappings" to deal with those.
> 
> *Fully untested, don't merge*
> 
> Not-Even-Signed-off-by: Marc Zyngier <[email-masked]>
> ---
>  arch/arm/include/asm/kvm_mmu.h   |  1 +
>  arch/arm/kvm/arm.c               |  2 +-
>  arch/arm/kvm/mmu.c               | 20 ++++++++++++++++++++
>  arch/arm64/include/asm/kvm_mmu.h |  1 +
>  4 files changed, 23 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm/include/asm/kvm_mmu.h b/arch/arm/include/asm/kvm_mmu.h
> index 9b28c41..6dcb9ff 100644
> --- a/arch/arm/include/asm/kvm_mmu.h
> +++ b/arch/arm/include/asm/kvm_mmu.h
> @@ -43,6 +43,7 @@
>  #include <asm/pgalloc.h>
>  
>  int create_hyp_mappings(void *from, void *to);
> +int create_hyp_percpu_mappings(void *from, void *to);
>  int create_hyp_io_mappings(void *from, void *to, phys_addr_t);
>  void free_boot_hyp_pgd(void);
>  void free_hyp_pgds(void);
> diff --git a/arch/arm/kvm/arm.c b/arch/arm/kvm/arm.c
> index 9c697db..6191960 100644
> --- a/arch/arm/kvm/arm.c
> +++ b/arch/arm/kvm/arm.c
> @@ -911,7 +911,7 @@ static int init_hyp_mode(void)
>  		kvm_cpu_context_t *cpu_ctxt;
>  
>  		cpu_ctxt = per_cpu_ptr(kvm_host_cpu_state, cpu);
> -		err = create_hyp_mappings(cpu_ctxt, cpu_ctxt + 1);
> +		err = create_hyp_percpu_mappings(cpu_ctxt, cpu_ctxt + 1);
>  
>  		if (err) {
>  			kvm_err("Cannot map host CPU state: %d\n", err);
> diff --git a/arch/arm/kvm/mmu.c b/arch/arm/kvm/mmu.c
> index b0de86b..e509718 100644
> --- a/arch/arm/kvm/mmu.c
> +++ b/arch/arm/kvm/mmu.c
> @@ -331,6 +331,26 @@ int create_hyp_mappings(void *from, void *to)
>  }
>  
>  /**
> + * create_hyp_percpu_mappings - duplicate a percpu kernel virtual address
> + *				range in Hyp mode
> + * @from:	The virtual kernel start address of the range
> + * @to:		The virtual kernel end address of the range (exclusive)
> + *
> + * The same virtual address as the kernel virtual address is also used
> + * in Hyp-mode mapping (modulo HYP_PAGE_OFFSET) to the same underlying
> + * physical pages. It *has* to be a percpu pointer.
> + */
> +int create_hyp_percpu_mappings(void *from, void *to)
> +{
> +	unsigned long phys_addr = per_cpu_ptr_to_phys(from);
phys_addr_t phys_addr = per_cpu_ptr_to_phys(from);

With this change things work as expected without $subject patch.
Thanks for the patch.

Regards,
Ssantosh