Re: [PATCH 02/10] migration/rdma: Remove unregister code

On 2026/8/18 21:01, Peter Xu wrote:
> On Tue, Aug 18, 2026 at 07:57:06PM +0800, Yanfei Xu wrote:
>> Hi Peter,
> Hi, Yanfei,
>
>> No objection to removing the dead code — it clearly never worked
>>
>> I do have one question about the direction, though. The removed logic
>> was the only in-tree attempt at MR unregistration for the non-pin-all
>> path. Without it, registered MRs grow monotonically over a migration,
>> and with large, widely-spread dirty memory over chunks the accumulated
>> MR metadata (user + kernel) can cost more than pin-all and even perform
>> worse — which rather defeats the purpose of not pinning everything.
>>
>> do we still intend to keep and improve the non-pin-all path going
>> forward? If so, some form of dynamic MR unregistration will eventually
>> be needed and it might be worth keeping this code，or at least leaving
>> a TODO to mark the gap?
> Thanks for taking a look.  This is a valid question to ask.
>
> Though it was there for 13 years without being "enhanced", it means the
> possibility we leverage it in the next couple of years is low.
>
> You also discussed the other side of things: I am not a frequent RDMA user,
> but my understanding is frequent MR reg operations already slow down
> migration quite a bit.  It means dynamic management including unregisters
> will be even worse.  AFAICT, it'll be a challenging task if we want to keep
> the performance in bar and add a hard throttle to pinned memory.

One advantage of non-pin-all is that it neither sends the all-zero chunk
nor registers the corresponding MRs. For guests with a low dirty-page
workload and a large number of zero pages, this lets it migrate faster
than pin-all and pin less guest memory during the migration.

>
> Obviously, RDMA migration users care a bunch on performance.
>
> So I see no good reason to not drop it.  Even if someone will work out that
> problem, it's still not much code to add, and it'll likely be easier we
> design it from scratch with the demand, and without worry of breaking
> anyone.

Got it, thanks for the explanation.

Regards,
Yanfei

>
> When that happens (if it ever will..), someone should also send an update
> to MAINTAINER file on RDMA migration to change it out of Odd Fixes stage..
>
> Thanks,
>