linux-kernel - Re: [PATCH] vfio/pci: take mmap write lock for io_remap_pfn

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Message-ID: <ZFwBYtjL1V0r5WW3@nvidia.com>
Date:   Wed, 10 May 2023 17:41:06 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Alex Williamson <alex.williamson@...hat.com>
Cc:     Yan Zhao <yan.y.zhao@...el.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, kevin.tian@...el.com,
        yishaih@...dia.com, shameerali.kolothum.thodi@...wei.com,
        Cédric Le Goater <clg@...hat.com>
Subject: Re: [PATCH] vfio/pci: take mmap write lock for io_remap_pfn_range

On Mon, May 08, 2023 at 02:57:15PM -0600, Alex Williamson wrote:

> We already try to set the flags in advance, but there are some
> architectural flags like VM_PAT that make that tricky.  Cedric has been
> looking at inserting individual pages with vmf_insert_pfn(), but that
> incurs a lot more faults and therefore latency vs remapping the entire
> vma on fault.  I'm not convinced that we shouldn't just attempt to
> remove the fault handler entirely, but I haven't tried it yet to know
> what gotchas are down that path.  Thanks,

I thought we did it like this because there were races otherwise with
PTE insertion and zapping? I don't remember well anymore.

I vaugely remember the address_space conversion might help remove the
fault handler?

Jason