Re: swapping with MMU Notifiers V2

[Andrea Arcangeli <andrea-atKUWr5tajBWk0Htik3J/w@public.gmane.org>]

Ok, I think I found one first deadlock source during swapping with the
mmu notifiers and it's a KVM bug. I got a deadlock inversion between
PT lock and mmu_lock because of this bug. With PREEMPT=n it's not
enough to spin_lock(mmu_lock) to disable preempt and in turn the page
fault will go through and it'll take the PT lock _after_ the mmu_lock
which is a forbidden ordering (the VM often calls the mmu notifier
invalidate page with the PT lock held). For example FNAME(fetch) calls
kvm_read_guest_atomic with the mmu_lock held (taken by
FNAME(page_fault)). Not sure why I didn't trigger yet on my
workstation but only on the test system. This seem not enough to get
V2 stable yet (but I think it was enough to get my old codebase stable
on the test system). I'll now try to rollback kvm source to my last
stable status and to apply this fix and run it on V2/V3.

Signed-off-by: Andrea Arcangeli <andrea-atKUWr5tajBWk0Htik3J/w@public.gmane.org>

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 8fc12dc..48d9a11 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -553,7 +565,9 @@ int kvm_read_guest_atomic(struct kvm *kvm, gpa_t gpa, void *data,
 	addr = gfn_to_hva(kvm, gfn);
 	if (kvm_is_error_hva(addr))
 		return -EFAULT;
+	pagefault_disable();
 	r = __copy_from_user_inatomic(data, (void __user *)addr + offset, len);
+	pagefault_enable();
 	if (r)
 		return -EFAULT;
 	return 0;

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