linux-kernel - Re: [PATCH 1/3] vfio: Introduce vma ops registration and notifier

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Date:   Thu, 18 Feb 2021 19:04:04 -0400
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Alex Williamson <alex.williamson@...hat.com>
CC:     <cohuck@...hat.com>, <kvm@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <peterx@...hat.com>
Subject: Re: [PATCH 1/3] vfio: Introduce vma ops registration and notifier

On Thu, Feb 18, 2021 at 02:56:06PM -0700, Alex Williamson wrote:

> Looks pretty slick.  I won't claim it's fully gelled in my head yet,
> but AIUI you're creating these inodes on your new pseudo fs and
> associating it via the actual user fd via the f_mapping pointer, which
> allows multiple fds to associate and address space back to this inode
> when you want to call unmap_mapping_range().  

Yes, from what I can tell all the fs/inode stuff is just mandatory
overhead to get a unique address_space pointer, as that is the only
thing this is actually using.

I have to check the mmap flow more carefully, I recall pointing to a
existing race here with Daniel, but the general idea should hold.

> That clarifies from the previous email how we'd store the inode on
> the vfio_device without introducing yet another tracking list for
> device fds.

Right, you can tell from the vma what inode it is for, and the inode
can tell you if it is a VFIO VMA or not, so no tracking lists needed
at all.

Jason