linux-kernel - Re: [PATCH v5 3/4] mm/gup: allow FOLL_LONGTERM & FOLL_PCI

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Message-ID: <66bb91fbcbe66_1c18294fe@dwillia2-mobl3.amr.corp.intel.com.notmuch>
Date: Tue, 13 Aug 2024 10:03:55 -0700
From: Dan Williams <dan.j.williams@...el.com>
To: Jason Gunthorpe <jgg@...pe.ca>, Christoph Hellwig <hch@...radead.org>
CC: Martin Oliveira <martin.oliveira@...eticom.com>,
	<linux-rdma@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<linux-mm@...ck.org>, Andrew Morton <akpm@...ux-foundation.org>, "Artemy
 Kovalyov" <artemyko@...dia.com>, Greg Kroah-Hartman
	<gregkh@...uxfoundation.org>, Leon Romanovsky <leon@...nel.org>, "Logan
 Gunthorpe" <logang@...tatee.com>, Michael Guralnik <michaelgur@...dia.com>,
	Mike Marciniszyn <mike.marciniszyn@...el.com>, Shiraz Saleem
	<shiraz.saleem@...el.com>, Tejun Heo <tj@...nel.org>, John Hubbard
	<jhubbard@...dia.com>, Dan Williams <dan.j.williams@...el.com>, David Sloan
	<david.sloan@...eticom.com>
Subject: Re: [PATCH v5 3/4] mm/gup: allow FOLL_LONGTERM & FOLL_PCI_P2PDMA

Jason Gunthorpe wrote:
> On Mon, Aug 12, 2024 at 10:41:20PM -0700, Christoph Hellwig wrote:
> > On Mon, Aug 12, 2024 at 08:12:49PM -0300, Jason Gunthorpe wrote:
> > > > This is unfortunately not really minor unless we have a well documented
> > > > way to force this :(
> > > 
> > > It is not that different from blocking driver unbind while FDs are
> > > open which a lot of places do in various ways?
> > 
> > Where do we block driver unbind with an open resource?  
> 
> I keep seeing it in different subsystems, safe driver unbind is really
> hard. :\ eg I think VFIO has some waits in it
> 
> > The whole concept is that open resources will pin the in-memory
> > object (and modulo for a modular driver), but never an unbind or
> > hardware unplug, of which unbind really just is a simulation.
> 
> Yes, ideally, but not every part of the kernel hits that ideal in my
> experience. It is alot of work and some places don't have any good
> solutions, like here.

...but there is a distinction between transient and permanent waits,
right? The difficult aspect of FOLL_LONGTERM is the holder has no idea
someone is trying to cleanup and may never drop its pin.