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

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Date:   Tue, 10 Aug 2021 09:51:07 +0100
From:   Christoph Hellwig <hch@...radead.org>
To:     Alex Williamson <alex.williamson@...hat.com>
Cc:     Jason Gunthorpe <jgg@...dia.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 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.