Subject: [RFC PATCH 4/4] mm/vmalloc.c: Treat the entire kernel virtual space as vmalloc

From: [email-masked] (Dave Hansen)
Date: Thu, 14 Nov 2013 09:26:18 -0800

On 11/11/2013 03:26 PM, Laura Abbott wrote:
> With CONFIG_ENABLE_VMALLOC_SAVINGS, all lowmem is tracked in
> vmalloc. This means that all the kernel virtual address space
> can be treated as part of the vmalloc region. Allow vm areas
> to be allocated from the full kernel address range.
> 
> Signed-off-by: Laura Abbott <[email-masked]>
> Signed-off-by: Neeti Desai <[email-masked]>
> ---
>  mm/vmalloc.c |   11 +++++++++++
>  1 files changed, 11 insertions(+), 0 deletions(-)
> 
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index c7b138b..181247d 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -1385,16 +1385,27 @@ struct vm_struct *__get_vm_area_caller(unsigned long size, unsigned long flags,
>   */
>  struct vm_struct *get_vm_area(unsigned long size, unsigned long flags)
>  {
> +#ifdef CONFIG_ENABLE_VMALLOC_SAVING
> +	return __get_vm_area_node(size, 1, flags, PAGE_OFFSET, VMALLOC_END,
> +				  NUMA_NO_NODE, GFP_KERNEL,
> +				  __builtin_return_address(0));
> +#else
>  	return __get_vm_area_node(size, 1, flags, VMALLOC_START, VMALLOC_END,
>  				  NUMA_NO_NODE, GFP_KERNEL,
>  				  __builtin_return_address(0));
> +#endif
>  }
>  
>  struct vm_struct *get_vm_area_caller(unsigned long size, unsigned long flags,
>  				const void *caller)
>  {
> +#ifdef CONFIG_ENABLE_VMALLOC_SAVING
> +	return __get_vm_area_node(size, 1, flags, PAGE_OFFSET, VMALLOC_END,
> +				  NUMA_NO_NODE, GFP_KERNEL, caller);
> +#else
>  	return __get_vm_area_node(size, 1, flags, VMALLOC_START, VMALLOC_END,
>  				  NUMA_NO_NODE, GFP_KERNEL, caller);
> +#endif
>  }

Couple of nits: first of all, there's no reason to copy, paste, and
#ifdef this much code.  This just invites one of the copies to bitrot.
I'd much rather see this:

#ifdef CONFIG_ENABLE_VMALLOC_SAVING
#define LOWEST_VMALLOC_VADDR PAGE_OFFSET
#else
#define LOWEST_VMALLOC_VADDR VMALLOC_START
#endif

Then just replace the PAGE_OFFSET in the function arguments with
LOWEST_VMALLOC_VADDR.

Have you done any audits to make sure that the rest of the code that
deals with vmalloc addresses in the kernel is using is_vmalloc_addr()?
I'd be a bit worried that we might have picked up an assumption or two
that *all* vmalloc addresses are _above_ VMALLOC_START.

The percpu.c code looks like it might do this, and maybe the kcore code.
 The vmalloc.c code itself has this in get_vmalloc_info():

>                 /*
>                  * Some archs keep another range for modules in vmalloc space
>                  */
>                 if (addr < VMALLOC_START)
>                         continue;

Seems like that would break as well.

With this patch, VMALLOC_START loses enough of its meaning that I wonder
if we should even keep it around.  It's the start of the _dedicated_
vmalloc space, but it's mostly useless and obscure enough that maybe we
should get rid of its use in common code.