Re: [PATCHv7 00/29] THP-enabled tmpfs/shmem using compound pages

[Andrea Arcangeli <aarcange@redhat.com>]

Hello,

On Mon, Apr 18, 2016 at 03:55:44PM -0700, Shi, Yang wrote:
> Hi Kirill,
> 
> Finally, I got some time to look into and try yours and Hugh's patches, 
> got two problems.

One thing that come to mind to test is this: qemu with -machine
accel=kvm -mem-path=/dev/shm/,share=on .

The THP Compound approach in tmpfs may just happen to work already
with KVM (or at worst it'd require minor adjustments) because it uses
the exact same model KVM is already aware about from THP in anonymous
memory, example from arch/x86/kvm/mmu.c:

static void transparent_hugepage_adjust(struct kvm_vcpu *vcpu,
					gfn_t *gfnp, kvm_pfn_t *pfnp,
					int *levelp)
{
	kvm_pfn_t pfn = *pfnp;
	gfn_t gfn = *gfnp;
	int level = *levelp;

	/*
	 * Check if it's a transparent hugepage. If this would be an
	 * hugetlbfs page, level wouldn't be set to
	 * PT_PAGE_TABLE_LEVEL and there would be no adjustment done
	 * here.
	 */
	if (!is_error_noslot_pfn(pfn) && !kvm_is_reserved_pfn(pfn) &&
	    level == PT_PAGE_TABLE_LEVEL &&
	    PageTransCompound(pfn_to_page(pfn)) &&
	    !mmu_gfn_lpage_is_disallowed(vcpu, gfn, PT_DIRECTORY_LEVEL)) {

Not using two different models between THP in tmpfs and THP in anon is
essential not just to significantly reduce the size of the kernel
code, but also because THP knowledge can't be self contained in the
mm/shmem.c file. Having to support two different models would
complicate things for secondary MMU drivers (i.e. mmu notifer users)
like KVM who also need to create huge mapping in the shadow pagetable
layer in arch/x86/kvm if the primary MMU allows for it.

> x86-64 and ARM64 with yours and Hugh's patches (linux-next tree), I got 
> the program execution time reduced by ~12% on x86-64, it looks very 
> impressive.

Agreed, both patchset are impressive works and achieving amazing
results!

My view is that in terms of long-lived computation from userland point
of view, both models are malleable enough and could achieve everything
we need in the end, but as far as the overall kernel efficiency is
concerned the compound model will always retain a slight advantage in
performance by leveraging a native THP compound refcounting that
requires just one atomic_inc/dec per THP mapcount instead of 512 of
them. Other advantages of the compound model is that it's half in code
size despite already including khugepaged (i.e. the same
split_huge_page works for both tmpfs and anon) and like said above it
won't introduce much complications for drivers like KVM as the model
didn't change.

Thanks,
Andrea

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