Re: [Hypervisor Live Update] Notes from March 10, 2025

[Pratyush Yadav <pratyush@kernel.org>]

On Thu, Mar 20 2025, Jason Gunthorpe wrote:

>> I didn't mean the exact flags value, but the ability to have
>> per-folio flags. The exact bits and their meaning would of course
>> need to be part of the ABI. Shmem uses the dirty and uptodate flags
>> to track some state on the folios, and the flags can affect it's
>> behavior (lazily zeroing out falloc-ed pages for example). I am
>> assuming other FD types or drivers might also want to store
>> per-folio information. Having KHO core provide this facility can
>> avoid duplicating the logic in each subsystem.
>
> For something simple like shmem I'd probably just suggest a side car bitmap
> array or something?
>
> The trouble with trying to feed flags through the xarray thing is that
> the memory holding that pfn data across the kexec is not itself
> preserved memory so it is all blown away once the allocator starts.
>
> Any data that needs to be preserved further has to be copied into the
> frozen struct page, which is pretty limiting in terms of what you
> could preserve. A few bits could maybe work out but not alot of data.

Right, that makes sense. I can live with a sidecar bitmap then.

[...]

-- 
Regards,
Pratyush Yadav