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

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Date:   Tue, 9 Mar 2021 13:47:39 -0500
From:   Peter Xu <peterx@...hat.com>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     Alex Williamson <alex.williamson@...hat.com>,
        "Zengtao (B)" <prime.zeng@...ilicon.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 12:40:04PM -0400, Jason Gunthorpe wrote:
> 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..

It seems to me that searching vma list is still the simplest way to fix the
problem for the current code base.  I see io_remap_pfn_range() is also used in
the new series - maybe that'll need to be moved to where PCI_COMMAND_MEMORY got
turned on/off in the new series (I just noticed remap_pfn_range modifies vma
flags..), as you suggested in the other email.

Thanks,

-- 
Peter Xu