Re: [RFC PATCH 09/17] KVM: arm64: Tear down unlinked page tables in parallel walk

[Quentin Perret <qperret@google.com>]

Hi Oliver,

On Friday 15 Apr 2022 at 21:58:53 (+0000), Oliver Upton wrote:
> Breaking a table pte is insufficient to guarantee ownership of an
> unlinked subtree. Parallel software walkers could be traversing
> substructures and changing their mappings.
> 
> Recurse through the unlinked subtree and lock all descendent ptes
> to take ownership of the subtree. Since the ptes are actually being
> evicted, return table ptes back to the table walker to ensure child
> tables are also traversed. Note that this is done both in both the
> pre-order and leaf visitors as the underlying pte remains volatile until
> it is unlinked.

Still trying to get the full picture of the series so bear with me. IIUC
the case you're dealing with here is when we're coallescing a table into
a block with concurrent walkers making changes in the sub-tree. I
believe this should happen when turning dirty logging off?

Why do we need to recursively lock the entire sub-tree at all in this
case? As long as the table is turned into a locked invalid PTE, what
concurrent walkers are doing in the sub-tree should be irrelevant no?
None of the changes they do will be made visible to the hardware anyway.
So as long as the sub-tree isn't freed under their feet (which should be
the point of the RCU protection) this should be all fine? Is there a
case where this is not actually true?

Thanks,
Quentin
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