linux-kernel - Re: [PATCH] vfio/pci: make the vfio_pci_mmap

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Date:   Tue, 9 Mar 2021 12:40:04 -0400
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Alex Williamson <alex.williamson@...hat.com>
Cc:     "Zengtao (B)" <prime.zeng@...ilicon.com>,
        Peter Xu <peterx@...hat.com>,
        Cornelia Huck <cohuck@...hat.com>,
        Kevin Tian <kevin.tian@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Giovanni Cabiddu <giovanni.cabiddu@...el.com>,
        Michel Lespinasse <walken@...gle.com>,
        Jann Horn <jannh@...gle.com>,
        Max Gurtovoy <mgurtovoy@...dia.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Linuxarm <linuxarm@...wei.com>
Subject: Re: [PATCH] vfio/pci: make the vfio_pci_mmap_fault reentrant

On Tue, Mar 09, 2021 at 08:29:51AM -0700, Alex Williamson wrote:
> On Tue, 9 Mar 2021 08:46:09 -0400
> Jason Gunthorpe <jgg@...dia.com> wrote:
> 
> > On Tue, Mar 09, 2021 at 03:49:09AM +0000, Zengtao (B) wrote:
> > > Hi guys:
> > > 
> > > Thanks for the helpful comments, after rethinking the issue, I have proposed
> > >  the following change: 
> > > 1. follow_pte instead of follow_pfn.  
> > 
> > Still no on follow_pfn, you don't need it once you use vmf_insert_pfn
> 
> vmf_insert_pfn() only solves the BUG_ON, follow_pte() is being used
> here to determine whether the translation is already present to avoid
> both duplicate work in inserting the translation and allocating a
> duplicate vma tracking structure.
 
Oh.. Doing something stateful in fault is not nice at all

I would rather see __vfio_pci_add_vma() search the vma_list for dups
than call follow_pfn/pte..

> For the vma tracking and testing whether the fault is already
> populated.  Once we get rid of the vma list, maybe it makes sense to
> only insert the faulting page rather than the entire vma, at which
> point I think we'd have no reason to serialize.  Thanks,

Yes, the address_space stuff is a much better solution to all of this.

Jason