Re: [PATCH v6 3/3] mm/gup: disallow FOLL_LONGTERM GUP-fast writing to file-backed mappings

From: Lorenzo Stoakes <lstoakes@gmail.com>
To: David Hildenbrand <david@redhat.com>
Cc: Matthew Rosato <mjrosato@linux.ibm.com>,
	Jason Gunthorpe <jgg@nvidia.com>,
	Christian Borntraeger <borntraeger@linux.ibm.com>,
	linux-mm@kvack.org, linux-kernel@vger.kernel.org,
	Andrew Morton <akpm@linux-foundation.org>,
	Jens Axboe <axboe@kernel.dk>,
	Matthew Wilcox <willy@infradead.org>,
	Dennis Dalessandro <dennis.dalessandro@cornelisnetworks.com>,
	Leon Romanovsky <leon@kernel.org>,
	Christian Benvenuti <benve@cisco.com>,
	Nelson Escobar <neescoba@cisco.com>,
	Bernard Metzler <bmt@zurich.ibm.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@redhat.com>,
	Arnaldo Carvalho de Melo <acme@kernel.org>,
	Mark Rutland <mark.rutland@arm.com>,
	Alexander Shishkin <alexander.shishkin@linux.intel.com>,
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	Dave Chinner <david@fromorbit.com>, Theodore Ts'o <tytso@mit.edu>,
	Peter Xu <peterx@redhat.com>
Subject: Re: [PATCH v6 3/3] mm/gup: disallow FOLL_LONGTERM GUP-fast writing to file-backed mappings
Date: Tue, 2 May 2023 16:19:10 +0100	[thread overview]
Message-ID: <25b7aa40-fad0-4886-90b2-c5d68d75d28b@lucifer.local> (raw)
In-Reply-To: <1f3231c0-34b2-1e78-0bf0-f32d5b67811d@redhat.com>

On Tue, May 02, 2023 at 05:09:06PM +0200, David Hildenbrand wrote:
> On 02.05.23 15:56, Matthew Rosato wrote:
> > On 5/2/23 9:50 AM, Jason Gunthorpe wrote:
> > > On Tue, May 02, 2023 at 03:47:43PM +0200, David Hildenbrand wrote:
> > > > > Eventually we want to implement a mechanism where we can dynamically pin in response to RPCIT.
> > > >
> > > > Okay, so IIRC we'll fail starting the domain early, that's good. And if we
> > > > pin all guest memory (instead of small pieces dynamically), there is little
> > > > existing use for file-backed RAM in such zPCI configurations (because memory
> > > > cannot be reclaimed either way if it's all pinned), so likely there are no
> > > > real existing users.
> > >
> > > Right, this is VFIO, the physical HW can't tolerate not having pinned
> > > memory, so something somewhere is always pinning it.
> >
> > I might have mis-explained above.
> >
> > With iommufd nesting, we will pin everything upfront as a starting point.
> >
> > The current usage of vfio type1 iommu for s390 does not pin the entirety of guest memory upfront, it happens as guest RPCITs occur / type1 mappings are made.
>
> ... so, after the domain started successfully on the libvirt/QEMU side ? :/
>
> It would be great to confirm that. There might be a BUG in patch #2 (see my
> reply to patch #2) that might not allow you to reproduce it right now.
>

Yes apologies - thank you VERY much for doing this Matthew, but apologies, I
made rather a clanger in patch 2 which would mean fast patch degrading to slow
path would pass even for file-backed.

Will respin a v7 + cc you on that, if you could be so kind as to test that?

> --
> Thanks,
>
> David / dhildenb
>