Commit ab99a1cc authored Feb 03, 2016 by Christian Borntraeger

KVM: s390: do not take mmap_sem on dirty log query



Dirty log query can take a long time for huge guests.
Holding the mmap_sem for very long times  can cause some unwanted
latencies.
Turns out that we do not need to hold the mmap semaphore.
We hold the slots_lock for gfn->hva translation and walk the page
tables with that address, so no need to look at the VMAs. KVM also
holds a reference to the mm, which should prevent other things
going away. During the walk we take the necessary ptl locks.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Reviewed-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>

parent 8a08b9c7

arch/s390/kvm/kvm-s390.c

+0 −2

Original line number	Diff line number	Diff line
		@@ -274,7 +274,6 @@ static void kvm_s390_sync_dirty_log(struct kvm *kvm,
		unsigned long address;
		struct gmap *gmap = kvm->arch.gmap;

		down_read(&gmap->mm->mmap_sem);
		/* Loop over all guest pages */
		last_gfn = memslot->base_gfn + memslot->npages;
		for (cur_gfn = memslot->base_gfn; cur_gfn <= last_gfn; cur_gfn++) {
		@@ -283,7 +282,6 @@ static void kvm_s390_sync_dirty_log(struct kvm *kvm,
		if (gmap_test_and_clear_dirty(address, gmap))
		mark_page_dirty(kvm, cur_gfn);
		}
		up_read(&gmap->mm->mmap_sem);
		}

		/* Section: vm related */

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