On 2015/11/14 18:20, Marcelo Tosatti wrote:
The actual issue is this: a higher level page that had, under its children,
no out of sync pages, now, due to your addition, a child that is unsync:
initial state:
level1
final state:
level1 -x-> level2 -x-> level3
Where -x-> are the links created by this pagefault fixing round.
If _any_ page under you is unsync (not necessarily the ones this
pagefault is accessing), you have to mark parents unsync.
I understand this, but I don't think my patch will break this.
What kvm_mmu_mark_parents_unsync() does is:
for each p_i in sp->parent_ptes rmap chain
mark_unsync(p_i);
Then, mark_unsync() finds the parent sp including that p_i to
set ->unsync_child_bitmap and increment ->unsync_children if
necessary. It may also call kvm_mmu_mark_parents_unsync()
recursively.
I understand we need to tell the parents "you have an unsync
child/descendant" until this information reaches the top level
by that recursive calls.
But since these recursive calls cannot come back to the starting sp,
the child->parent graph has no loop, each mark_unsync(p_i) will not
be affected by other parents in that sp->parent_ptes rmap chain,
from which we started the recursive calls.
As the following code shows, my patch does mark_unsync(parent_pte)
separately, and then mmu_page_add_parent_pte(vcpu, sp, parent_pte):
- } else if (sp->unsync)
+ if (parent_pte)
+ mark_unsync(parent_pte);
+ } else if (sp->unsync) {
kvm_mmu_mark_parents_unsync(sp);
+ if (parent_pte)
+ mark_unsync(parent_pte);
+ }
+ mmu_page_add_parent_pte(vcpu, sp, parent_pte);
So, as you worried, during each mark_unsync(p_i) is processed,
this parent_pte does not exist in that sp->parent_ptes rmap chain.
But as I explained above, this does not change anything about what
each mark_unsync(p_i) call does, so keeps the original behaviour.
By the way, I think "kvm_mmu_mark_parents_unsync" and "mark_unsync"
do not tell what they actually do well. When I first saw the names,
I thought they would just set the parents' sp->unsync.
To reflect the following meaning better, it should be
propagate_unsync(_to_parents) or something:
Tell the parents "you have an unsync child/descendant"
until this unsync information reaches the top level
Thanks,
Takuya
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