linux-kernel - Re: [PATCH 3/7] vfio/pci: Use vfio_device_unmap_mapping

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Date:   Tue, 10 Aug 2021 08:57:22 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Alex Williamson <alex.williamson@...hat.com>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        peterx@...hat.com
Subject: Re: [PATCH 3/7] vfio/pci: Use vfio_device_unmap_mapping_range()

On Tue, Aug 10, 2021 at 09:51:07AM +0100, Christoph Hellwig wrote:
> On Fri, Aug 06, 2021 at 02:17:45PM -0600, Alex Williamson wrote:
> > > Now that this is simplified so much, I wonder if we can drop the
> > > memory_lock and just use the dev_set->lock?
> > > 
> > > That avoids the whole down_write_trylock thing and makes it much more
> > > understandable?
> > 
> > Hmm, that would make this case a lot easier, but using a mutex,
> > potentially shared across multiple devices, taken on every non-mmap
> > read/write doesn't really feel like a good trade-off when we're
> > currently using a per device rwsem to retain concurrency here.  Thanks,
> 
> Using a per-set percpu_rw_semaphore might be a good plan here.  Probably
> makes sense to do that incrementally after this change, though.

I'm not sure there is a real performance win to chase here? Doesn't
this only protect mmap against reset? The mmap isn't performance
sensitive, right?

If this really needs extra optimization adding a rwsem to the devset
and using that across the whole set would surely be sufficient.

Jason