linux-kernel - Re: [PATCH v5 3/5] vduse: Add function to get/free the pages for reconnection

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Message-ID: <20240425062507-mutt-send-email-mst@kernel.org>
Date: Thu, 25 Apr 2024 06:26:54 -0400
From: "Michael S. Tsirkin" <mst@...hat.com>
To: Jason Wang <jasowang@...hat.com>
Cc: Cindy Lu <lulu@...hat.com>, linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH v5 3/5] vduse: Add function to get/free the pages for
 reconnection

On Thu, Apr 25, 2024 at 09:35:58AM +0800, Jason Wang wrote:
> On Wed, Apr 24, 2024 at 5:51 PM Michael S. Tsirkin <mst@...hat.com> wrote:
> >
> > On Wed, Apr 24, 2024 at 08:44:10AM +0800, Jason Wang wrote:
> > > On Tue, Apr 23, 2024 at 4:42 PM Michael S. Tsirkin <mst@...hat.com> wrote:
> > > >
> > > > On Tue, Apr 23, 2024 at 11:09:59AM +0800, Jason Wang wrote:
> > > > > On Tue, Apr 23, 2024 at 4:05 AM Michael S. Tsirkin <mst@...hat.com> wrote:
> > > > > >
> > > > > > On Thu, Apr 18, 2024 at 08:57:51AM +0800, Jason Wang wrote:
> > > > > > > On Wed, Apr 17, 2024 at 5:29 PM Michael S. Tsirkin <mst@...hat.com> wrote:
> > > > > > > >
> > > > > > > > On Fri, Apr 12, 2024 at 09:28:23PM +0800, Cindy Lu wrote:
> > > > > > > > > Add the function vduse_alloc_reconnnect_info_mem
> > > > > > > > > and vduse_alloc_reconnnect_info_mem
> > > > > > > > > These functions allow vduse to allocate and free memory for reconnection
> > > > > > > > > information. The amount of memory allocated is vq_num pages.
> > > > > > > > > Each VQS will map its own page where the reconnection information will be saved
> > > > > > > > >
> > > > > > > > > Signed-off-by: Cindy Lu <lulu@...hat.com>
> > > > > > > > > ---
> > > > > > > > >  drivers/vdpa/vdpa_user/vduse_dev.c | 40 ++++++++++++++++++++++++++++++
> > > > > > > > >  1 file changed, 40 insertions(+)
> > > > > > > > >
> > > > > > > > > diff --git a/drivers/vdpa/vdpa_user/vduse_dev.c b/drivers/vdpa/vdpa_user/vduse_dev.c
> > > > > > > > > index ef3c9681941e..2da659d5f4a8 100644
> > > > > > > > > --- a/drivers/vdpa/vdpa_user/vduse_dev.c
> > > > > > > > > +++ b/drivers/vdpa/vdpa_user/vduse_dev.c
> > > > > > > > > @@ -65,6 +65,7 @@ struct vduse_virtqueue {
> > > > > > > > >       int irq_effective_cpu;
> > > > > > > > >       struct cpumask irq_affinity;
> > > > > > > > >       struct kobject kobj;
> > > > > > > > > +     unsigned long vdpa_reconnect_vaddr;
> > > > > > > > >  };
> > > > > > > > >
> > > > > > > > >  struct vduse_dev;
> > > > > > > > > @@ -1105,6 +1106,38 @@ static void vduse_vq_update_effective_cpu(struct vduse_virtqueue *vq)
> > > > > > > > >
> > > > > > > > >       vq->irq_effective_cpu = curr_cpu;
> > > > > > > > >  }
> > > > > > > > > +static int vduse_alloc_reconnnect_info_mem(struct vduse_dev *dev)
> > > > > > > > > +{
> > > > > > > > > +     unsigned long vaddr = 0;
> > > > > > > > > +     struct vduse_virtqueue *vq;
> > > > > > > > > +
> > > > > > > > > +     for (int i = 0; i < dev->vq_num; i++) {
> > > > > > > > > +             /*page 0~ vq_num save the reconnect info for vq*/
> > > > > > > > > +             vq = dev->vqs[i];
> > > > > > > > > +             vaddr = get_zeroed_page(GFP_KERNEL);
> > > > > > > >
> > > > > > > >
> > > > > > > > I don't get why you insist on stealing kernel memory for something
> > > > > > > > that is just used by userspace to store data for its own use.
> > > > > > > > Userspace does not lack ways to persist data, for example,
> > > > > > > > create a regular file anywhere in the filesystem.
> > > > > > >
> > > > > > > Good point. So the motivation here is to:
> > > > > > >
> > > > > > > 1) be self contained, no dependency for high speed persist data
> > > > > > > storage like tmpfs
> > > > > >
> > > > > > No idea what this means.
> > > > >
> > > > > I mean a regular file may slow down the datapath performance, so
> > > > > usually the application will try to use tmpfs and other which is a
> > > > > dependency for implementing the reconnection.
> > > >
> > > > Are we worried about systems without tmpfs now?
> > >
> > > Yes.
> >
> > Why? Who ships these?
> 
> Not sure, but it could be disabled or unmounted. I'm not sure make
> VDUSE depends on TMPFS is a good idea.
> 
> Thanks

Don't disable or unmount it then?
The use-case needs to be much clearer if we are adding a way for
userspace to pin kernel memory for unlimited time.

-- 
MST