Re: [PATCH 2 of 3] mremap: avoid sending one IPI per page

[Mel Gorman <mel@csn.ul.ie>]

On Sat, Aug 06, 2011 at 06:58:04PM +0200, aarcange@redhat.com wrote:
> From: Andrea Arcangeli <aarcange@redhat.com>
> 
> This replaces ptep_clear_flush() with ptep_get_and_clear() and a single
> flush_tlb_range() at the end of the loop, to avoid sending one IPI for each
> page.
> 
> The mmu_notifier_invalidate_range_start/end section is enlarged accordingly but
> this is not going to fundamentally change things. It was more by accident that
> the region under mremap was for the most part still available for secondary
> MMUs: the primary MMU was never allowed to reliably access that region for the
> duration of the mremap (modulo trapping SIGSEGV on the old address range which
> sounds unpractical and flakey). If users wants secondary MMUs not to lose
> access to a large region under mremap they should reduce the mremap size
> accordingly in userland and run multiple calls. Overall this will run faster so
> it's actually going to reduce the time the region is under mremap for the
> primary MMU which should provide a net benefit to apps.
> 
> For KVM this is a noop because the guest physical memory is never mremapped,
> there's just no point it ever moving it while guest runs. One target of this
> optimization is JVM GC (so unrelated to the mmu notifier logic).
> 
> Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>

Acked-by: Mel Gorman <mgorman@suse.de>

-- 
Mel Gorman
SUSE Labs

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