Re: [PATCH v6 09/12] KVM: arm64: Split huge pages when dirty logging is enabled

[Ricardo Koller <ricarkol@google.com>]

On Sun, Mar 12, 2023 at 12:54:17PM +0000, Marc Zyngier wrote:
> On Tue, 07 Mar 2023 03:45:52 +0000,
> Ricardo Koller <ricarkol@google.com> wrote:
> > 
> > Split huge pages eagerly when enabling dirty logging. The goal is to
> > avoid doing it while faulting on write-protected pages, which
> > negatively impacts guest performance.
> > 
> > A memslot marked for dirty logging is split in 1GB pieces at a time.
> > This is in order to release the mmu_lock and give other kernel threads
> > the opportunity to run, and also in order to allocate enough pages to
> > split a 1GB range worth of huge pages (or a single 1GB huge page).
> > Note that these page allocations can fail, so eager page splitting is
> > best-effort.  This is not a correctness issue though, as huge pages
> > can still be split on write-faults.
> > 
> > The benefits of eager page splitting are the same as in x86, added
> > with commit a3fe5dbda0a4 ("KVM: x86/mmu: Split huge pages mapped by
> > the TDP MMU when dirty logging is enabled"). For example, when running
> > dirty_log_perf_test with 64 virtual CPUs (Ampere Altra), 1GB per vCPU,
> > 50% reads, and 2MB HugeTLB memory, the time it takes vCPUs to access
> > all of their memory after dirty logging is enabled decreased by 44%
> > from 2.58s to 1.42s.
> > 
> > Signed-off-by: Ricardo Koller <ricarkol@google.com>
> > Reviewed-by: Shaoqin Huang <shahuang@redhat.com>
> > ---
> >  arch/arm64/kvm/mmu.c | 118 ++++++++++++++++++++++++++++++++++++++++++-
> >  1 file changed, 116 insertions(+), 2 deletions(-)
> > 
> > diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> > index 898985b09321..b1b8da5f8b6c 100644
> > --- a/arch/arm64/kvm/mmu.c
> > +++ b/arch/arm64/kvm/mmu.c
> > @@ -31,14 +31,21 @@ static phys_addr_t __ro_after_init hyp_idmap_vector;
> >  
> >  static unsigned long __ro_after_init io_map_base;
> >  
> > -static phys_addr_t stage2_range_addr_end(phys_addr_t addr, phys_addr_t end)
> > +static phys_addr_t __stage2_range_addr_end(phys_addr_t addr, phys_addr_t end,
> > +					   phys_addr_t size)
> >  {
> > -	phys_addr_t size = kvm_granule_size(KVM_PGTABLE_MIN_BLOCK_LEVEL);
> >  	phys_addr_t boundary = ALIGN_DOWN(addr + size, size);
> >  
> >  	return (boundary - 1 < end - 1) ? boundary : end;
> >  }
> >  
> > +static phys_addr_t stage2_range_addr_end(phys_addr_t addr, phys_addr_t end)
> > +{
> > +	phys_addr_t size = kvm_granule_size(KVM_PGTABLE_MIN_BLOCK_LEVEL);
> > +
> > +	return __stage2_range_addr_end(addr, end, size);
> > +}
> > +
> >  /*
> >   * Release kvm_mmu_lock periodically if the memory region is large. Otherwise,
> >   * we may see kernel panics with CONFIG_DETECT_HUNG_TASK,
> > @@ -75,6 +82,77 @@ static int stage2_apply_range(struct kvm_s2_mmu *mmu, phys_addr_t addr,
> >  #define stage2_apply_range_resched(mmu, addr, end, fn)			\
> >  	stage2_apply_range(mmu, addr, end, fn, true)
> >  
> > +static bool need_topup_split_page_cache_or_resched(struct kvm *kvm, uint64_t min)
> 
> Please don't use the words "page cache", it triggers a painful
> Pavlovian reflex. Something like "need_split_memcache_topup_or_reched"
> makes me feel less anxious.
>

fixed

> > +{
> > +	struct kvm_mmu_memory_cache *cache;
> > +
> > +	if (need_resched() || rwlock_needbreak(&kvm->mmu_lock))
> > +		return true;
> > +
> > +	cache = &kvm->arch.mmu.split_page_cache;
> > +	return kvm_mmu_memory_cache_nr_free_objects(cache) < min;
> > +}
> > +
> > +/*
> > + * Get the maximum number of page-tables needed to split a range of
> 
> nit: page-table pages.
>

fixed

> > + * blocks into PAGE_SIZE PTEs. It assumes the range is already mapped
> > + * at the PMD level, or at the PUD level if allowed.
> > + */
> > +static int kvm_mmu_split_nr_page_tables(u64 range)
> > +{
> > +	int n = 0;
> > +
> > +	if (KVM_PGTABLE_MIN_BLOCK_LEVEL < 2)
> > +		n += DIV_ROUND_UP_ULL(range, PUD_SIZE);
> > +	n += DIV_ROUND_UP_ULL(range, PMD_SIZE);
> > +	return n;
> > +}
> > +
> > +static int kvm_mmu_split_huge_pages(struct kvm *kvm, phys_addr_t addr,
> > +				    phys_addr_t end)
> > +{
> > +	struct kvm_mmu_memory_cache *cache;
> > +	struct kvm_pgtable *pgt;
> > +	int ret;
> > +	u64 next;
> > +	u64 chunk_size = kvm->arch.mmu.split_page_chunk_size;
> > +	int cache_capacity = kvm_mmu_split_nr_page_tables(chunk_size);
> > +
> > +	if (chunk_size == 0)
> > +		return 0;
> > +
> > +	lockdep_assert_held_write(&kvm->mmu_lock);
> 
> Please check for the lock being held early, even in the 0-sized chunk
> condition.
> 

fixed

> > +
> > +	cache = &kvm->arch.mmu.split_page_cache;
> > +
> > +	do {
> > +		if (need_topup_split_page_cache_or_resched(kvm,
> > +							   cache_capacity)) {
> 
> Since cache_capacity is stored in the kvm struct, why not just passing
> it to the helper function and let it deal with it?
>

removed the cache_capacity arg.

> > +			write_unlock(&kvm->mmu_lock);
> > +			cond_resched();
> > +			/* Eager page splitting is best-effort. */
> > +			ret = __kvm_mmu_topup_memory_cache(cache,
> > +							   cache_capacity,
> > +							   cache_capacity);
> > +			write_lock(&kvm->mmu_lock);
> > +			if (ret)
> > +				break;
> > +		}
> > +
> > +		pgt = kvm->arch.mmu.pgt;
> > +		if (!pgt)
> > +			return -EINVAL;
> > +
> > +		next = __stage2_range_addr_end(addr, end, chunk_size);
> > +		ret = kvm_pgtable_stage2_split(pgt, addr, next - addr,
> > +					       cache, cache_capacity);
> > +		if (ret)
> > +			break;
> > +	} while (addr = next, addr != end);
> > +
> > +	return ret;
> > +}
> > +
> >  static bool memslot_is_logging(struct kvm_memory_slot *memslot)
> >  {
> >  	return memslot->dirty_bitmap && !(memslot->flags & KVM_MEM_READONLY);
> > @@ -773,6 +851,7 @@ int kvm_init_stage2_mmu(struct kvm *kvm, struct kvm_s2_mmu *mmu, unsigned long t
> >  void kvm_uninit_stage2_mmu(struct kvm *kvm)
> >  {
> >  	kvm_free_stage2_pgd(&kvm->arch.mmu);
> > +	kvm_mmu_free_memory_cache(&kvm->arch.mmu.split_page_cache);
> >  }
> >  
> >  static void stage2_unmap_memslot(struct kvm *kvm,
> > @@ -999,6 +1078,31 @@ static void kvm_mmu_write_protect_pt_masked(struct kvm *kvm,
> >  	stage2_wp_range(&kvm->arch.mmu, start, end);
> >  }
> >  
> > +/**
> > + * kvm_mmu_split_memory_region() - split the stage 2 blocks into PAGE_SIZE
> > + *				   pages for memory slot
> > + * @kvm:	The KVM pointer
> > + * @slot:	The memory slot to split
> > + *
> > + * Acquires kvm->mmu_lock. Called with kvm->slots_lock mutex acquired,
> > + * serializing operations for VM memory regions.
> > + */
> > +static void kvm_mmu_split_memory_region(struct kvm *kvm, int slot)
> > +{
> > +	struct kvm_memslots *slots = kvm_memslots(kvm);
> > +	struct kvm_memory_slot *memslot = id_to_memslot(slots, slot);
> > +	phys_addr_t start, end;
> > +
> > +	lockdep_assert_held(&kvm->slots_lock);
> 
> You have already accessed the memslots by the time you check for the
> lock. Not great.
> 

fixed

> > +
> > +	start = memslot->base_gfn << PAGE_SHIFT;
> > +	end = (memslot->base_gfn + memslot->npages) << PAGE_SHIFT;
> > +
> > +	write_lock(&kvm->mmu_lock);
> > +	kvm_mmu_split_huge_pages(kvm, start, end);
> > +	write_unlock(&kvm->mmu_lock);
> > +}
> > +
> >  /*
> >   * kvm_arch_mmu_enable_log_dirty_pt_masked - enable dirty logging for selected
> >   * dirty pages.
> > @@ -1790,6 +1894,16 @@ void kvm_arch_commit_memory_region(struct kvm *kvm,
> >  			return;
> >  
> >  		kvm_mmu_wp_memory_region(kvm, new->id);
> > +		kvm_mmu_split_memory_region(kvm, new->id);
> 
> Would there be an advantage in merging these two operations somehow?
>

I guess we could. The only issue is that it could be useful to
write-protect a memslot without splitting huge pages.

> > +	} else {
> > +		/*
> > +		 * Free any leftovers from the eager page splitting cache. Do
> > +		 * this when deleting, moving, disabling dirty logging, or
> > +		 * creating the memslot (a nop). Doing it for deletes makes
> > +		 * sure we don't leak memory, and there's no need to keep the
> > +		 * cache around for any of the other cases.
> > +		 */
> > +		kvm_mmu_free_memory_cache(&kvm->arch.mmu.split_page_cache);
> >  	}
> >  }
> >  
> 
> Thanks,
> 
> 	M.
> 
> -- 
> Without deviation from the norm, progress is not possible.