Re: [PATCH] vmap: add flag to allow lazy unmap to be disabled at runtime

[Nick Piggin <npiggin@suse.de>]

On Mon, Jul 26, 2010 at 01:24:51PM -0700, Jeremy Fitzhardinge wrote:
> 
> 
> [ Nick, I forget if I sent this to you before.  Could you Ack it if it looks OK? Thanks, J ]
> 
> Add a flag to force lazy_max_pages() to zero to prevent any outstanding
> mapped pages.  We'll need this for Xen.

You have sent this to me before, probably several times, and I always
forget about it right as you send it again.

It's no problem merging something like this for Xen, although as you
know I would love to see an approach where Xen would benefit from
delayed flushing as well :)

You will need to disable lazy flushing from the per-cpu allocator
(vm_map_ram/vm_unmap_ram, which are used by XFS now). That's not
tied to the lazy_max stuff (which it should be, arguably)

That code basically allocates per-cpu chunks of va from the global
allocator, uses them, then frees them back to the global allocator
all without doing any TLB flushing.

If you have to do global TLB flushing there, then it's probably not
much value in per-cpu locking of the address allocator anyway, so
you could just add a test for vmap_lazy_unmap in these branches:

  if (likely(count <= VMAP_MAX_ALLOC) && !vmap_lazy_unmap)

Thanks,
Nick

> 
> Signed-off-by: Jeremy Fitzhardinge<jeremy.fitzhardinge@citrix.com>
> 
> diff --git a/include/linux/vmalloc.h b/include/linux/vmalloc.h
> index 227c2a5..b840fda 100644
> --- a/include/linux/vmalloc.h
> +++ b/include/linux/vmalloc.h
> @@ -7,6 +7,8 @@
> 
>  struct vm_area_struct;		/* vma defining user mapping in mm_types.h */
> 
> +extern bool vmap_lazy_unmap;
> +
>  /* bits in flags of vmalloc's vm_struct below */
>  #define VM_IOREMAP	0x00000001	/* ioremap() and friends */
>  #define VM_ALLOC	0x00000002	/* vmalloc() */
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index ae00746..7f35fe2 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -31,6 +31,7 @@
>  #include<asm/tlbflush.h>
>  #include<asm/shmparam.h>
> 
> +bool vmap_lazy_unmap __read_mostly = true;
> 
>  /*** Page table manipulation functions ***/
> 
> @@ -502,6 +503,9 @@ static unsigned long lazy_max_pages(void)
>  {
>  	unsigned int log;
> 
> +	if (!vmap_lazy_unmap)
> +		return 0;
> +
>  	log = fls(num_online_cpus());
> 
>  	return log * (32UL * 1024 * 1024 / PAGE_SIZE);
>