Re: [PATCH v1 4/4] mm/hugetlb: fix excessive IPI broadcasts when unsharing PMD tables using mmu_gather

["David Hildenbrand (Red Hat)" <david@kernel.org>]

>>>
>>> Isn't really the correct comment here that ranges that previously mapped the
>>> shared pages might no longer, so we must clear the TLB? I may be missing
>>> something :)
>>
>> There are cases where we defer flushing the TLB until we dropped all (exclusive) locks.
>> In particular, MADV_DONTNEED does that in some cases, essentially deferring the flush
>> to the tlb_finish_mmu().
>>
>> free_pgtables() will also defer the flush, performing the TLB flush during tlb_finish_mmu(),
>> before
>>
>> The point is (as I tried to make clear in the comment), for unsharing we have no control
>> whenn the page table gets freed after we drop the lock.
>>
>> So we must flush the TLB now and cannot defer it like we do in the other cases.
> 
> Yeah I guess because of the above - that is - other users may unshare for their
> CPUs but not unshare for ours?
Yes :)

It's all very complicated, therefore I decided to rather add more 
comments describing what we depend on.

-- 
Cheers

David