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

[Thomas Hellström]

On Tue, 2025-02-04 at 09:26 -0400, Jason Gunthorpe wrote:
> On Tue, Feb 04, 2025 at 10:32:32AM +0100, Thomas Hellström wrote:
> > 
> 
> > 1) Existing users would never use the callback. They can still rely
> > on
> > the owner check, only if that fails we check for callback
> > existence.
> > 2) By simply caching the result from the last checked dev_pagemap,
> > most
> > callback calls could typically be eliminated.
> 
> But then you are not in the locked region so your cache is racy and
> invalid.

I'm not sure I follow? If a device private pfn handed back to the
caller is dependent on dev_pagemap A having a fast interconnect to the
client, then subsequent pfns in the same hmm_range_fault() call must be
able to make the same assumption (pagemap A having a fast
interconnect), else the whole result is invalid?

> 
> > 3) As mentioned before, a callback call would typically always be
> > followed by either migration to ram or a page-table update.
> > Compared to
> > these, the callback overhead would IMO be unnoticeable.
> 
> Why? Surely the normal case should be a callback saying the memory
> can
> be accessed?

Sure, but at least on the xe driver, that means page-table repopulation
since the hmm_range_fault() typically originated from a page-fault.


> 
> > 4) pcie_p2p is already planning a dev_pagemap callback?
> 
> Yes, but it is not a racy validation callback, and it already is
> creating a complicated lifecycle problem inside the exporting the
> driver.

Yeah, I bet there are various reasons against a callback. I just don't
see the performance argument being a main concern. 

> 
> Jason

/Thomas