Commit 443a8133 authored Feb 24, 2016 by Martin Schwidefsky

s390/kvm: simplify set_guest_storage_key



Git commit ab3f285f
"KVM: s390/mm: try a cow on read only pages for key ops"
added a fixup_user_fault to set_guest_storage_key force a copy on
write if the page is mapped read-only. This is supposed to fix the
problem of differing storage keys for shared mappings, e.g. the
empty_zero_page.
But if the storage key is set before the pte is mapped the storage
key update is done on the pgste. A later fault will happily map the
shared page with the key from the pgste.

Eventually git commit 2faee8ff
"s390/mm: prevent and break zero page mappings in case of storage keys"
fixed this problem for the empty_zero_page. The commit makes sure that
guests enabled for storage keys will not use the empty_zero_page at all.

As the call to fixup_user_fault in set_guest_storage_key depends on the
order of the storage key operation vs. the fault that maps the pte
it does not really fix anything. Just remove it.

Reviewed-by: Dominik Dingel <dingel@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

parent 993e0681

arch/s390/mm/pgtable.c

+0 −17

Original line number	Diff line number	Diff line
		@@ -809,30 +809,13 @@ int set_guest_storage_key(struct mm_struct *mm, unsigned long addr,
		spinlock_t *ptl;
		pgste_t old, new;
		pte_t *ptep;
		bool unlocked;

		down_read(&mm->mmap_sem);
		retry:
		unlocked = false;
		ptep = get_locked_pte(mm, addr, &ptl);
		if (unlikely(!ptep)) {
		up_read(&mm->mmap_sem);
		return -EFAULT;
		}
		if (!(pte_val(*ptep) & _PAGE_INVALID) &&
		(pte_val(*ptep) & _PAGE_PROTECT)) {
		pte_unmap_unlock(ptep, ptl);
		/*
		* We do not really care about unlocked. We will retry either
		* way. But this allows fixup_user_fault to enable userfaultfd.
		*/
		if (fixup_user_fault(current, mm, addr, FAULT_FLAG_WRITE,
		&unlocked)) {
		up_read(&mm->mmap_sem);
		return -EFAULT;
		}
		goto retry;
		}

		new = old = pgste_get_lock(ptep);
		pgste_val(new) &= ~(PGSTE_GR_BIT \| PGSTE_GC_BIT \|