Re: [PATCH v12 84/84] KVM: Don't grab reference on VM_MIXEDMAP pfns that have a "struct page"

Paolo Bonzini
2024-07-31T09:51:04Z

On 7/30/24 22:21, Sean Christopherson wrote:
> On Tue, Jul 30, 2024, Paolo Bonzini wrote:
>> On 7/27/24 01:52, Sean Christopherson wrote:
>>> Now that KVM no longer relies on an ugly heuristic to find its struct page
>>> references, i.e. now that KVM can't get false positives on VM_MIXEDMAP
>>> pfns, remove KVM's hack to elevate the refcount for pfns that happen to
>>> have a valid struct page. In addition to removing a long-standing wart
>>> in KVM, this allows KVM to map non-refcounted struct page memory into the
>>> guest, e.g. for exposing GPU TTM buffers to KVM guests.
>>
>> Feel free to leave it to me for later, but there are more cleanups that
>> can be made, given how simple kvm_resolve_pfn() is now:
>
> I'll revisit kvm_resolve_pfn(), Maxim also wasn't a fan of a similar helper that
> existed in v11.

FWIW kvm_resolve_pfn() is totally fine as an intermediate step. Just
food for thought for possible follow-ups.

>> Also, check_user_page_hwpoison() should not be needed anymore, probably
>> not since commit 234b239bea39 ("kvm: Faults which trigger IO release the
>> mmap_sem", 2014-09-24) removed get_user_pages_fast() from hva_to_pfn_slow().
>
> Ha, I *knew* this sounded familiar. Past me apparently came to the same
> conclusion[*], though I wrongly suspected a memory leak and promptly forgot to
> ever send a patch. I'll tack one on this time around.

As you prefer.

Paolo