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

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Date:   Fri, 19 Feb 2021 15:02:19 -0700
From:   Alex Williamson <alex.williamson@...hat.com>
To:     Jason Gunthorpe <jgg@...dia.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, 18 Feb 2021 19:04:04 -0400
Jason Gunthorpe <jgg@...dia.com> wrote:

> 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.

Seems to be a nice cleanup for vfio as well, more testing and analysis
required, but here are a few (4) wip commits that re-implement the
current vma zapping scheme following your example: 

https://github.com/awilliam/linux-vfio/commits/vfio-unmap-mapping-range

Thanks,
Alex