On 4/9/23 2:29 PM, Ricardo Koller 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@xxxxxxxxxx>
Reviewed-by: Shaoqin Huang <shahuang@xxxxxxxxxx>
---
arch/arm64/kvm/mmu.c | 123 ++++++++++++++++++++++++++++++++++++++++++-
1 file changed, 121 insertions(+), 2 deletions(-)
In the changelog, it may be worthy to be mentioned:
The eager page splitting only takes effect when the huge page mapping has
been existing in the stage-2 page table. Otherwise, the huge page will be
mapped to multiple base pages on handling page fault.
The code changes look good to me:
Reviewed-by: Gavin Shan <gshan@xxxxxxxxxx>
diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
index 898985b09321a..aaefabd8de89d 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 +82,79 @@ 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)
+/*
+ * Get the maximum number of page-tables pages needed to split a range
+ * of blocks into PAGE_SIZE PTEs. It assumes the range is already
+ * mapped at level 2, or at level 1 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 bool need_split_memcache_topup_or_resched(struct kvm *kvm)
+{
+ struct kvm_mmu_memory_cache *cache;
+ u64 chunk_size, min;
+
+ if (need_resched() || rwlock_needbreak(&kvm->mmu_lock))
+ return true;
+
+ chunk_size = kvm->arch.mmu.split_page_chunk_size;
+ min = kvm_mmu_split_nr_page_tables(chunk_size);
+ cache = &kvm->arch.mmu.split_page_cache;
+ return kvm_mmu_memory_cache_nr_free_objects(cache) < min;
+}
+
+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, cache_capacity;
+ u64 next, chunk_size;
+
+ lockdep_assert_held_write(&kvm->mmu_lock);
+
+ chunk_size = kvm->arch.mmu.split_page_chunk_size;
+ cache_capacity = kvm_mmu_split_nr_page_tables(chunk_size);
+
+ if (chunk_size == 0)
+ return 0;
+
+ cache = &kvm->arch.mmu.split_page_cache;
+
+ do {
+ if (need_split_memcache_topup_or_resched(kvm)) {
+ 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);
+ 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 +853,7 @@ 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 +1080,34 @@ 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;
+ struct kvm_memory_slot *memslot;
+ phys_addr_t start, end;
+
+ lockdep_assert_held(&kvm->slots_lock);
+
+ slots = kvm_memslots(kvm);
+ memslot = id_to_memslot(slots, slot);
+
+ 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 +1899,16 @@ 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);
+ } 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,
Gavin
