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From:   Amir Goldstein <amir73il@gmail.com>
Date:   Wed, 11 Mar 2020 21:32:17 +0200
Message-ID: <CAOQ4uxjja3cReO28qOd-YGmhU-_KrLxOCaBeqZYydxPAte9_pg@mail.gmail.com>
Subject: Re: [PATCH 00/20] virtiofs: Add DAX support
To:     Vivek Goyal <vgoyal@redhat.com>
Cc:     linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        linux-nvdimm <linux-nvdimm@lists.01.org>, virtio-fs@redhat.com,
        Miklos Szeredi <miklos@szeredi.hu>,
        Stefan Hajnoczi <stefanha@redhat.com>,
        "Dr. David Alan Gilbert" <dgilbert@redhat.com>, mst@redhat.com
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On Wed, Mar 11, 2020 at 8:48 PM Vivek Goyal <vgoyal@redhat.com> wrote:
>
> On Wed, Mar 11, 2020 at 07:22:51AM +0200, Amir Goldstein wrote:
> > On Wed, Mar 4, 2020 at 7:01 PM Vivek Goyal <vgoyal@redhat.com> wrote:
> > >
> > > Hi,
> > >
> > > This patch series adds DAX support to virtiofs filesystem. This allows
> > > bypassing guest page cache and allows mapping host page cache directly
> > > in guest address space.
> > >
> > > When a page of file is needed, guest sends a request to map that page
> > > (in host page cache) in qemu address space. Inside guest this is
> > > a physical memory range controlled by virtiofs device. And guest
> > > directly maps this physical address range using DAX and hence gets
> > > access to file data on host.
> > >
> > > This can speed up things considerably in many situations. Also this
> > > can result in substantial memory savings as file data does not have
> > > to be copied in guest and it is directly accessed from host page
> > > cache.
> > >
> > > Most of the changes are limited to fuse/virtiofs. There are couple
> > > of changes needed in generic dax infrastructure and couple of changes
> > > in virtio to be able to access shared memory region.
> > >
> > > These patches apply on top of 5.6-rc4 and are also available here.
> > >
> > > https://github.com/rhvgoyal/linux/commits/vivek-04-march-2020
> > >
> > > Any review or feedback is welcome.
> > >
> > [...]
> > >  drivers/dax/super.c                |    3 +-
> > >  drivers/virtio/virtio_mmio.c       |   32 +
> > >  drivers/virtio/virtio_pci_modern.c |  107 +++
> > >  fs/dax.c                           |   66 +-
> > >  fs/fuse/dir.c                      |    2 +
> > >  fs/fuse/file.c                     | 1162 +++++++++++++++++++++++++++-
> >
> > That's a big addition to already big file.c.
> > Maybe split dax specific code to dax.c?
> > Can be a post series cleanup too.
>
> How about fs/fuse/iomap.c instead. This will have all the iomap related logic
> as well as all the dax range allocation/free logic which is required
> by iomap logic. That moves about 900 lines of code from file.c to iomap.c
>

Fine by me. I didn't take time to study the code in file.c
I just noticed is has grown a lot bigger and wasn't sure that
it made sense. Up to you. Only if you think the result would be nicer
to maintain.

Thanks,
Amir.