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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>
This commit is contained in:
Christian Borntraeger 2016-02-03 10:18:41 +01:00
parent 8a08b9c737
commit ab99a1cc7a
1 changed files with 0 additions and 2 deletions
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2
arch/s390/kvm/kvm-s390.c
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@ -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 */