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

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Message-ID: <3e96aef8009be69858a69d3e49a2bd7fc7d06f5f.camel@linux.intel.com>
Date: Tue, 04 Feb 2025 15:29:48 +0100
From: Thomas Hellström <thomas.hellstrom@...ux.intel.com>
To: Jason Gunthorpe <jgg@...pe.ca>
Cc: Yonatan Maman <ymaman@...dia.com>, kherbst@...hat.com, lyude@...hat.com,
 	dakr@...hat.com, airlied@...il.com, simona@...ll.ch, leon@...nel.org, 
	jglisse@...hat.com, akpm@...ux-foundation.org, GalShalom@...dia.com, 
	dri-devel@...ts.freedesktop.org, nouveau@...ts.freedesktop.org, 
	linux-kernel@...r.kernel.org, linux-rdma@...r.kernel.org,
 linux-mm@...ck.org, 	linux-tegra@...r.kernel.org
Subject: Re: [RFC 1/5] mm/hmm: HMM API to enable P2P DMA for device private
 pages

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