[RFC][PATCH] arm: highmem: Add support for flushing kmap_atomic mappings

[Russell King - ARM Linux]

On Fri, Apr 05, 2013 at 02:42:51PM -0700, Laura Abbott wrote:
> The highmem code provides kmap_flush_unused to ensure all kmap
> mappings are really removed if they are used. This code does not
> handle kmap_atomic mappings since they are managed separately.
> This prevents an issue for any code which relies on having absolutely
> no mappings for a particular page. Rather than pay the penalty of
> having CONFIG_DEBUG_HIGHMEM on all the time, add functionality
> to remove the kmap_atomic mappings in a similar way to kmap_flush_unused.

I get the feeling that there's something behind this which is pushing
for there only being no mappings for a particular page.  In a SMP
environment that sounds like a dodgy assumption.  Maybe you could
provide the background behind this patch?

> This is intended to be an RFC to make sure this approach is
> reasonable. The goal is to have kmap_atomic_flush_unused be a public
> API.

That implies that there's some callers which need it, which aren't
part of this patch - and a motivation for this outside of what we
can see here.  That needs to be explained.  Plus, unused APIs in
the mainline kernel don't tend ot stick around for long, so it really
needs a use case to be demonstrated before it can be merged.

> 
> Signed-off-by: Laura Abbott <lauraa at codeaurora.org>
> ---
>  arch/arm/mm/highmem.c |   51 +++++++++++++++++++++++++++++++++++++++++++++++++
>  1 files changed, 51 insertions(+), 0 deletions(-)
> 
> diff --git a/arch/arm/mm/highmem.c b/arch/arm/mm/highmem.c
> index 21b9e1b..f4c0466 100644
> --- a/arch/arm/mm/highmem.c
> +++ b/arch/arm/mm/highmem.c
> @@ -10,6 +10,7 @@
>   * published by the Free Software Foundation.
>   */
>  
> +#include <linux/cpu.h>
>  #include <linux/module.h>
>  #include <linux/highmem.h>
>  #include <linux/interrupt.h>
> @@ -135,3 +136,53 @@ struct page *kmap_atomic_to_page(const void *ptr)
>  
>  	return pte_page(get_top_pte(vaddr));
>  }
> +
> +static void kmap_remove_unused_cpu(int cpu)
> +{
> +	int start_idx, idx, type;
> +
> +	pagefault_disable();
> +	type = kmap_atomic_idx();
> +	start_idx = type + 1 + KM_TYPE_NR * cpu;
> +
> +	for (idx = start_idx; idx < KM_TYPE_NR + KM_TYPE_NR * cpu; idx++) {
> +		unsigned long vaddr = __fix_to_virt(FIX_KMAP_BEGIN + idx);
> +		set_top_pte(vaddr, __pte(0));
> +	}
> +	pagefault_enable();

Hmm.  Why are you using pagefault_disable() around this?  This code itself
should not cause any page faults.  Maybe you mean to disable preemption,
but that should already be disabled (if not the above is buggy.)  Maybe
interrupts - that would make more sense, but as soon as interrupts are
re-enabled, it's possible for drivers to start create kmap_atomic()
mappings again.

So... the question is... why this... for what use case?

At the moment it just feels like it's rather buggy and racy.

> +static int hotplug_kmap_atomic_callback(struct notifier_block *nfb,
> +                                unsigned long action, void *hcpu)
> +{
> +        switch (action & (~CPU_TASKS_FROZEN)) {

Parens around ~CPU_TASKS_FROZEN is not required.

> +        case CPU_DYING:
> +		kmap_remove_unused_cpu((int)hcpu);
> +                break;
> +        default:
> +                break;
> +        }
> +
> +        return NOTIFY_OK;

Please watch your indentation...

> +}
> +
> +static struct notifier_block hotplug_kmap_atomic_notifier = {
> +        .notifier_call = hotplug_kmap_atomic_callback,
> +};
> +
> +static int __init init_kmap_atomic(void)
> +{
> +

And this blank line isn't needed.