Re: [PATCH v3 kvm/queue 05/16] KVM: Maintain ofs_tree for fast memslot lookup by file offset

[Sean Christopherson]

On Fri, Dec 31, 2021, Chao Peng wrote:
> On Tue, Dec 28, 2021 at 09:48:08PM +0000, Sean Christopherson wrote:
> >KVM handles
> > reverse engineering the memslot to get the offset and whatever else it needs.
> > notify_fallocate() and other callbacks are unchanged, though they probably can
> > drop the inode.
> > 
> > E.g. likely with bad math and handwaving on the overlap detection:
> > 
> > int kvm_private_fd_fallocate_range(void *owner, pgoff_t start, pgoff_t end)
> > {
> > 	struct kvm_memory_slot *slot = owner;
> > 	struct kvm_gfn_range gfn_range = {
> > 		.slot	   = slot,
> > 		.start	   = (start - slot->private_offset) >> PAGE_SHIFT,
> > 		.end	   = (end - slot->private_offset) >> PAGE_SHIFT,
> > 		.may_block = true,
> > 	};
> > 
> > 	if (!has_overlap(slot, start, end))
> > 		return 0;
> > 
> > 	gfn_range.end = min(gfn_range.end, slot->base_gfn + slot->npages);
> > 
> > 	kvm_unmap_gfn_range(slot->kvm, &gfn_range);
> > 	return 0;
> > }
> 
> I understand this KVM side handling, but again one fd can have multiple
> memslots. How shmem decides to notify which memslot from a list of
> memslots when it invokes the notify_fallocate()? Or just notify all
> the possible memslots then let KVM to check? 

Heh, yeah, those are the two choices.  :-)

Either the backing store needs to support registering callbacks for specific,
arbitrary ranges, or it needs to invoke all registered callbacks.  Invoking all
callbacks has my vote; it's much simpler to implement and is unlikely to incur
meaningful overhead.  _Something_ has to find the overlapping ranges, that cost
doesn't magically go away if it's pushed into the backing store.

Note, invoking all notifiers is also aligned with the mmu_notifier behavior.