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

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Message-ID: <20240813160502.GH1985367@ziepe.ca>
Date: Tue, 13 Aug 2024 13:05:02 -0300
From: Jason Gunthorpe <jgg@...pe.ca>
To: 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

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.

Jason