linux-kernel - Re: [PATCH 00/20] virtiofs: Add DAX support

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Message-ID: <20200311130945.GA90606@redhat.com>
Date:   Wed, 11 Mar 2020 09:09:45 -0400
From:   Vivek Goyal <vgoyal@...hat.com>
To:     Amir Goldstein <amir73il@...il.com>
Cc:     linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>, virtio-fs@...hat.com,
        Miklos Szeredi <miklos@...redi.hu>,
        Stefan Hajnoczi <stefanha@...hat.com>,
        "Dr. David Alan Gilbert" <dgilbert@...hat.com>, mst@...hat.com
Subject: Re: [PATCH 00/20] virtiofs: Add DAX support

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@...hat.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.

Lot of this code is coming from logic to reclaim dax memory range
assigned to inode. I will look into moving some of it to a separate
file.

Vivek