Re: [RFC 1/5] mm/hmm: HMM API to enable P2P DMA for device private pages

[Jason Gunthorpe <jgg@ziepe.ca>]

On Thu, Jan 30, 2025 at 05:09:44PM +0100, Simona Vetter wrote:

> > You could also use an integer instead of a pointer to indicate the
> > cluster of interconnect, I think there are many options..
> 
> Hm yeah I guess an integer allocater of the atomic_inc kind plus "surely
> 32bit is enough" could work. But I don't think it's needed, if we can
> reliable just unregister the entire dev_pagemap and then just set up a new
> one. Plus that avoids thinking about which barriers we might need where
> exactly all over mm code that looks at the owner field.

IMHO that is the best answer if it works for the driver.
> > ? It is supposed to work, it blocks until all the pages are freed, but
> > AFAIK ther is no fundamental life time issue. The driver is
> > responsible to free all its usage.
> 
> Hm I looked at it again, and I guess with the fixes to make migration to
> system memory work reliable in Matt Brost's latest series it should indeed
> work reliable. The devm_ version still freaks me out because of how easily
> people misuse these for things that are memory allocations.

I also don't like the devm stuff, especially in costly places like
this. Oh well.

> > > An optional callback is a lot less scary to me here (or redoing
> > > hmm_range_fault or whacking the migration helpers a few times) looks a lot
> > > less scary than making pgmap->owner mutable in some fashion.
> > 
> > It extra for every single 4k page on every user :\
> > 
> > And what are you going to do better inside this callback?
> 
> Having more comfy illusions :-P

Exactly!

> Slightly more seriously, I can grab some locks and make life easier,

Yes, but then see my concern about performance again. Now you are
locking/unlocking every 4k? And then it still races since it can
change after hmm_range_fault returns. That's not small, and not really
better.

> whereas sprinkling locking or even barriers over pgmap->owner in core mm
> is not going to fly. Plus more flexibility, e.g. when the interconnect
> doesn't work for atomics or some other funny reason it only works for some
> of the traffic, where you need to more dynamically decide where memory is
> ok to sit.

Sure, an asymmetric mess could be problematic, and we might need more
later, but lets get to that first..

> Or checking p2pdma connectivity and all that stuff.

Should be done in the core code, don't want drivers open coding this
stuff.

> Also note that fundamentally you can't protect against the hotunplug or
> driver unload case for hardware access. So some access will go to nowhere
> when that happens, until we've torn down all the mappings and migrated
> memory out.

I think a literal (safe!) hot unplug must always use the page map
revoke, and that should be safely locked between hmm_range_fault and
the notifier.

If the underlying fabric is loosing operations during an unplanned hot
unplug I expect things will need resets to recover..

Jason