linux-kernel - Re: [PATCH 1/2] x86/mm: maintain a percpu "in get_user_pages

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Date:	Sat, 28 Mar 2009 06:48:53 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
CC:	Nick Piggin <npiggin@...e.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linux Memory Management List <linux-mm@...ck.org>,
	the arch/x86 maintainers <x86@...nel.org>
Subject: Re: [PATCH 1/2] x86/mm: maintain a percpu "in get_user_pages_fast"
 flag

Jeremy Fitzhardinge wrote:
> get_user_pages_fast() relies on cross-cpu tlb flushes being a barrier
> between clearing and setting a pte, and before freeing a pagetable page.
> It usually does this by disabling interrupts to hold off IPIs, but
> some tlb flush implementations don't use IPIs for tlb flushes, and
> must use another mechanism.
>
> In this change, add in_gup_cpumask, which is a cpumask of cpus currently
> performing a get_user_pages_fast traversal of a pagetable.  A cross-cpu
> tlb flush function can use this to determine whether it should hold-off
> on the flush until the gup_fast has finished.
>
> @@ -255,6 +260,10 @@ int get_user_pages_fast(unsigned long start, int 
> nr_pages, int write,
>      * address down to the the page and take a ref on it.
>      */
>     local_irq_disable();
> +
> +    cpu = smp_processor_id();
> +    cpumask_set_cpu(cpu, in_gup_cpumask);
> +

This will bounce a cacheline, every time.  Please wrap in CONFIG_XEN and 
skip at runtime if Xen is not enabled.


-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.

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