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

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Message-ID: <20250204191601.GK2296753@ziepe.ca>
Date: Tue, 4 Feb 2025 15:16:01 -0400
From: Jason Gunthorpe <jgg@...pe.ca>
To: Thomas Hellström <thomas.hellstrom@...ux.intel.com>
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, Feb 04, 2025 at 03:29:48PM +0100, Thomas Hellström wrote:
> 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?

But what is the receiver going to do with this device private page?
Relock it again and check again if it is actually OK? Yuk.

> > > 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.

Yes, I expect all hmm_range_fault()'s to be on page fault paths, and
we'd like it to be as fast as we can in the CPU present case..

Jason